TECHNICAL SUPPORT DOCUMENT 

Vulnerability Evaluation Framework 

for Geologic Sequestration of Carbon 

Dioxide 

July 10, 2008 

U.S. Environmental Protection Agency 

EPA430-R-08-009 
For Further Information:

Anhar Karimjee

Climate Change Division, Offi ce of Atmospheric Programs

U.S. Environmental Protection Agency

202-343-9260

Karimjee.anhar@epa.gov

Lisa Bacanskas

Climate Change Division, Offi ce of Atmospheric Programs

U.S. Environmental Protection Agency

202-343-9758

Bacanskas.lisa@epa.gov

Peer Reviewed Document

Th is report has undergone an external peer review consistent with the guidelines of the 

U.S. EPA Peer Review Policy. See the Acknowledgements section for a list of reviewers. 

A copy of the EPA Peer Review guidelines may be downloaded from the following web 

page at http://epa.gov/osa/spc/2peerrev.htm
CONTENTS

Acknowledgments............................................................................ iv

Acronyms...........................................................................................v

1. Introduction ................................................................................. 1

1.1 Background on Climate Change and Geologic Sequestration..............................1

1.2 Vulnerability Assessment Approach .....................................................................2

1.3 Report Organization ...........................................................................................3

2. Background on Geologic Sequestration ........................................ 5

2.1 Geologic Settings under Consideration for Sequestration ....................................5

2.2 Subsurface CO2 Movement and Storage Mechanisms .........................................8

 2.2.1 Factors that Control the Rate of CO2 Movement in the Subsurface .....8

 2.2.2 Physical and Geochemical Trapping Mechanisms .................................8

2.3 Practical Experience Relevant to Geologic Sequestration ...................................10

 2.3.1 Existing Projects.................................................................................10

 2.3.2 Natural and Industrial Storage Analogs ..............................................11

 2.3.3 Regulatory Experience........................................................................11

2.4 Chapter Summary.............................................................................................12

3. Vulnerability Evaluation Framework for Geologic Sequestration: 

Geologic System and Attributes ...................................................... 13

3.1 Geologic Sequestration System and Geologic Attributes....................................15

 3.1.1 Confi ning System and Related Geologic Attributes ............................15

 3.1.2 Injection Zone and Related Geologic Attributes.................................19

 3.1.3 Carbon Dioxide Stream......................................................................22

3.2 Spatial Area of Evaluation: Geologic Sequestration Footprint ............................23

3.3 Chapter Summary.............................................................................................26

4. Vulnerability Evaluation Framework for Geologic Sequestration: 

Impacts and Receptors.................................................................... 27

4.1 Potential Human Health and Welfare Impacts...................................................27

 4.1.1 Human Populations ...........................................................................28

 4.1.2 Populations Covered by Executive Orders ..........................................30

 4.1.3 Cultural and Recreational Resources...................................................30

 4.1.4 Economic Resources (Surface and Subsurface)....................................31

4.2 Potential Atmospheric Impacts..........................................................................31

4.3 Potential Ecosystems Impacts ............................................................................32

 4.3.1 Sensitive Species .................................................................................32

 4.3.2 Legislatively Protected Species ............................................................35

i
4.4 Potential Groundwater and Surface Water Impacts............................................35

 4.4.1 Water Quality.....................................................................................36

 4.4.2 Regional Groundwater Flow...............................................................38

 4.4.3 Protected and Sensitive Drinking Water Supplies ...............................38

4.5 Potential Geosphere Impacts .............................................................................38

4.6 Spatial Area of Evaluation: Infl uence of Receptors.............................................38

 4.6.1 Carbon Dioxide Spatial Area of Evaluation ........................................39

 4.6.2 Pressure Spatial Area of Evaluation .....................................................40

4.7 Chapter Summary.............................................................................................42

5. Vulnerability Evaluation Framework for Geologic 

Sequestration: Key Considerations .......................................... 43

5.1 Holistic Approach to Evaluating Vulnerability...................................................43

 5.1.1 Interplay of Geologic Attributes and Receptors ..................................43

 5.1.2 Evaluation of Vulnerability at Diff erent Temporal Scales ....................44

5.2 Key Attributes in Evaluating Geologic Sequestration System Vulnerability........45

 5.2.1 Wells as Fluid-Conducting Pathways..................................................46

 5.2.2 Faults and Fractures as Fluid-Conducting Pathways ...........................48

 5.2.3 Pressure-Induced Physical Eff ects .......................................................49

5.3 Chapter Summary.............................................................................................50

6. Monitoring and Mitigation.................................................. 51

6.1 Monitoring .......................................................................................................51

 6.1.1 Purposes of Monitoring......................................................................51

 6.1.2 Monitoring Technologies....................................................................52

 6.1.3 Timeframe Implications for Monitoring.............................................53

6.2 Mitigation.........................................................................................................54

6.3 Chapter Summary.............................................................................................55

7. Summary and Next Steps ..................................................... 56

Bibliography............................................................................ 57

Glossary................................................................................... 69

Appendix A. Properties of Carbon Dioxide (CO2)................... 76

Appendix B. Comparison of Attributes: VEF, 

IPIECA, and IPCC ..............................................................78

ii
Figures and Table

Figure 2.1. Examples of Geologic Formations Being Considered for GS....................6 

Figure 2.2. Eff ectiveness of Trapping Mechanisms Over Time ...................................9

Figure 3.1. VEF Conceptual Model.........................................................................13

Figure 3.2. Confi ning System Evaluation ................................................................18

Figure 3.3. Injection Zone (IZ) Evaluation..............................................................21

Figure 3.4. Carbon Dioxide Stream Evaluation .......................................................23

Figure 3.5. Geologic Sequestration (GS) Footprint Delineation ..............................24

Figure 4.1. Human Health and Welfare Evaluation .................................................28

Figure 4.2. Atmosphere Evaluation..........................................................................32

Figure 4.3. Ecological Receptors Evaluation ............................................................33

Figure 4.4. Groundwater and Surface Water Evaluation ..........................................36

Figure 4.5. Carbon Dioxide Spatial Area of Evaluation............................................40

Figure 4.6. Pressure Spatial Area of Evaluation ........................................................41

Figure 5.1. Risk Profi le for CO2 Storage..................................................................44

Figure 5.2. Wells Evaluation....................................................................................47

Figure 5.3. Faults Evaluation ...................................................................................49

Figure A.1. Phase Diagram for CO2 ........................................................................76

Table B.1. Comparison of Attributes/Pathways Identifi ed as Being Likely to 

Aff ect Unanticipated Migration or Leakage from GS Projects..................................78

iii
ACKNOWLEDGMENTS

Th e U.S. Environmental Protection Agency (EPA) would like to acknowledge those individuals 

whose eff orts helped bring this report to fruition. 

Th e Vulnerability Evaluation Framework (VEF) was developed by a multi-disciplinary team of 

physical and social scientists who provided research, analysis, and technical skills. Th e eff ort was 

led by EPA’s Climate Change Division (CCD) in the Offi ce of Atmospheric Programs, with critical 

support from EPA’s Drinking Water Protection Division (DWPD) in the Offi ce of Ground Water 

and Drinking Water, and Stratus Consulting Inc. 

Th e following reviewers provided useful input: the members of the internal EPA Workgroup 

on Geologic Sequestration, James J. Dooley and Casie Davidson (Pacifi c Northwest National 

Laboratory), Neeraj Gupta (Battelle), Jens Birkholzer (Lawrence Berkeley National Laboratory), 

Rajesh Pawar (Los Alamos National Laboratory), Grant Bromhal and John Litynski (National 

Energy Technology Laboratory), Jean-Philippe Nicot (University of Texas), Scott Anderson 

(Environmental Defense), George Peridas (National Resource Defense Council), John Venezia 

(World Resources Institute), and Kurt Waltzer (Clean Air Task Force), and the U.S. Department of 

Energy’s (DOE’s) Carbon Sequestration Regional Partnership Program Outreach Group.

Th e International Energy Agency Greenhouse Gas R&D Programme provided an invaluable service 

by facilitating a formal peer review of this report. Special thanks to the co-facilitators, Brendan Beck 

and Tim Dixon, and the expert reviewers: Jason Anderson, (Institute for European Environmental 

Policy), Rick Chalaturnyk (University of Alberta), Sevket Durucan (Imperial College London), Tony 

Espie (BP), Todd Flach (Det Norske Veritas), Wolfgang Heidug (Shell), John Kaldi (CO2CRC), 

Jonathan Pearce (British Geological Survey), and Malcolm Wilson (University of Regina).

iv
ACRONYMS

Ar Argon

CCD Climate Change Division

CCS Carbon Dioxide Capture and Storage

CH4 Methane

Cn Cyanide

CO Carbon Monoxide

CO2 Carbon Dioxide

DOE U.S. Department of Energy

DWPD Drinking Water Protection Division

EOR Enhanced Oil Recovery

EPA U.S. Environmental Protection Agency

Fe Iron

GHG Greenhouse Gas

GS Geologic Sequestration

H2 Hydrogen

H2S Hydrogen Sulfi de

Hg Mercury

IEA International Energy Agency

IPCC Intergovernmental Panel on Climate Change

IZ Injection Zone

mg/L Milligrams per Liter

N2 Nitrogen

NETL National Energy Technology Laboratory

NO Nitrogen Oxide

NO3 Nitrate

NOAA National Oceanic and Atmospheric Administration

O2 Oxygen

ppm Parts Per Million

SDWA Safe Drinking Water Act

SO2 Sulfur Dioxide

SO4 Sulfate

TDS Total Dissolved Solids

UIC Underground Injection Control

USDW Underground Source of Drinking Water

USGS U.S. Geological Survey

VEF Vulnerability Evaluation Framework

v
1

1

CHAPTER 1

INTRODUCTION

Geologic sequestration1 (GS) of carbon dioxide 

(CO2), a greenhouse gas (GHG), has been identifi ed 

as one of several approaches to reduce atmospheric 

concentrations of CO2, thereby contributing to 

the mitigation of climate change (IPCC, 2005). A 

large body of literature indicates that GS is a viable 

technology that can be conducted in a safe manner 

when coupled with a comprehensive approach to ensure 

protection of human health and the environment (see 

text box). Nonetheless, there are potential risks and 

uncertainties associated with GS. To systematically 

identify those conditions that could increase the 

potential for adverse impacts from GS, the U.S. 

Environmental Protection Agency (EPA) has developed 

a Vulnerability Evaluation Framework (VEF).

“ With appropriate site selection 

based on available subsurface 

information, a monitoring 

program to detect problems, 

a regulator y system, and the 

appropriate use of remediation 

methods to stop or control CO2

releases if they arise, the local 

health, safety and environment 

risks of geologic storage would 

be comparable to risks of current

activities such as natural gas 

storage, [enhanced oil recover y], 

and deep underground disposal 

of acid gas.” –IPCC (2005) 1.1 Background on Climate Change 

and Geologic Sequestration 

In its 2007 scientifi c assessment, the Intergovernmental Panel on Climate Change (IPCC) concluded 

that “warming of the climate system is unequivocal” (IPCC, 2007). Th is warming trend has been 

linked to shrinking glaciers, rising sea levels, alterations in plant and animal habitats, and other 

global impacts. Th e IPCC concluded that it is very likely that most of the increase in the average 

global temperature since the mid-20th century has been caused by emissions of GHGs from human 

activities. Continued GHG emissions at or above current rates will lead to further warming and very 

likely to global impacts, some of which may be irreversible (IPCC, 2007).

Th e IPCC examined several scenarios to reduce and soon reverse increase in emissions of GHGs 

and thus limit future climate change; most studies fi nd that a range of strategies will need to be 

employed. In its Special Report on Carbon Dioxide Capture and Storage, the IPCC identifi ed CO2 

capture and storage (CCS) as one of several approaches with the potential to address climate change 

(IPCC, 2005). CCS is intended to mitigate climate change eff ects by decreasing emissions from 

stationary sources such as power plants (IPCC, 2005). Although several CCS technologies have 

been proposed, including ocean storage and mineral carbonation, GS has been identifi ed as the most 

technically viable approach (IPCC, 2005). GS involves injecting captured CO2 into deep, subsurface 

rock formations for long-term storage. It has been estimated that available capacity for GS in the 

United States ranges from 1,300 to 3,900 gigatons of CO2, with most of the capacity in deep saline 

formations (NETL, 2007). For reference, the total energy-related CO2 emissions in the United States 

in 2005 was 5.9 gigatons, with fossil fuel combustion accounting for 5.8 gigatons (U.S. EPA, 2007).

1 Note, sequestration is also sometimes referred to as storage, see for example, IPCC 2005.
1

introduction

2

1.2 Vulnerability Assessment Approach

Th e vulnerability assessment incorporated in the VEF was developed to systematically identify those 

conditions that could increase the potential for adverse impacts from GS, regardless of likelihood 

or broad applicability. It is not a quantitative, probabilistic risk assessment tool. Vulnerability 

assessment examines conditions that lead to increased or decreased susceptibility to consequences, 

whereas risk assessment measures the probability and severity of consequences. It is recommended 

that assessing vulnerability be an iterative process where new information is incorporated into the 

evaluation as it is generated. Many of the principles, approaches, and areas of focus in the VEF, 

discussed in this report, are similar to and refl ect conclusions reached in other GS assessments (see, 

for example, Oldenburg et al., 2002a, 2002b; Friedman and Nummedal, 2003; Maul et al., 2003; 

Celia and Radonjic, 2004; Celia et al., 2004; Le Gallo et al., 2004; Quintessa, 2004; Walton et al., 

2004; Zhang et al., 2006). 

Th e VEF described in this report is not designed to be a generalized site selection tool, to establish 

performance standards for GS sites, or to specify data requirements. Th e VEF represents a fi rst 

step toward a conceptual framework designed to aid regulators, and other technical experts, in 

framing key site-specifi c considerations and in identifying key areas that require in-depth evaluation 

for project design, site-specifi c risk assessment, monitoring, and management. It could serve as 

a reference document for regulators responsible for approving environmental impact statements, 

approving GS sites, or issuing permits for GS projects. Applying the VEF would necessitate detailed 

technical information on the proposed GS site extracted from existing data sources or specifi cally 

collected by a project operator. 

Current challenges to developing a quantitative risk assessment that is applicable across all GS sites 

are related to limited fi eld experience and the heterogeneity of GS sites. Attempting to quantify 

risks before suffi ciently understanding GS systems could result in understating or overstating the 

risks associated with GS. Development of a high-level quantitative risk analysis will become more 

feasible as information is generated from pilot- and commercial-scale projects. Due to the inherent 

heterogeneities of GS systems, site-specifi c quantitative risk assessments similar to approaches taken 

at the Weyburn (Canada), Gorgon (Australia), and Otway (Australia) sites provide invaluable insight 

into understanding and managing potential risks. (Zhou et al., 2004; Chevron, 2005; U.S. DOE, 

2007a; CO2CRC, 2008).

As with all such frameworks, the VEF is limited in that only vulnerabilities based on currently 

known or understood physicochemical or biological processes are considered. Uncertainties that go 

beyond current understanding of these fundamental processes or are related to issues of scale are 

not explicitly incorporated into this vulnerability framework. Uncertainties associated with models 

may also present challenges to both operators and regulators. Acknowledging these uncertainties and 

employing a strategy to manage them is a critical aspect of any assessment. Probabilistic approaches 

may be applied to handle uncertainties that arise from variability in the GS system (e.g. description 

of deep geologic storage systems, variation in types of cement materials used in wells, uncertainty 

regarding the location of existing wells, etc). Uncertainties from incomplete characterization of 

the GS site may be addressed through incorporating additional information as it is generated, thus 

reducing uncertainty and increasing the precision of the evaluation. It may be more diffi cult to 

address uncertainties arising from a lack of understanding of processes involved, and from potentially 

unreliable, inexplicable, or confl icting data. Approaches are being developed that can be applied to 

manage uncertainties associated with GS (see Benbow et al, 2006). 
1

introduction

3

Th e VEF concept was developed based on an extensive literature review as well as input from EPA’s 

Regional and Headquarters offi ces, researchers at the DOE national laboratories, experts from 

academia, and members of nongovernmental organizations working on issues related to GS. Th e IEA 

GHG R&D Programme also facilitated a peer review of this document by GS experts. 

It is important to emphasize that the use of the VEF in framing site-specifi c considerations for site 

selection does not replace or supersede other statutory or regulatory requirements for the protection 

of human health and the environment. Owners and operators must obtain all necessary permits from 

appropriate state and federal authorities under the Safe Drinking Water Act (SDWA) and any other 

applicable statutes and regulations.

1.3 Report Organization

Th e goals of this report are to:

1. Outline key characteristics of a GS system that need to be evaluated to understand potential 

impacts to human health and the environment.

2. Provide approaches for conducting “fi rst-order” assessments of GS systems to identify 

circumstances where additional study and/or monitoring may be warranted.

3. Identify potential mechanisms for unanticipated migration2, leakage3, and or pressure 

changes that could cause adverse impacts.

4. Describe potential impacts of unanticipated migration, leakage, and pressure changes based 

on available literature.

Th e report chapters address these goals as follows:

Chapter 2: For GS to be an eff ective climate change mitigation tool, large volumes of CO2 must 

remain underground for long periods of time (hundreds, if not thousands, of years). Chapter 2 

discusses the types of geologic formations that are currently being considered for GS and the types of 

mechanisms that could trap CO2 underground. Th e VEF was developed with a focus on deep saline 

formations (described in Chapter 2), but many of the concepts apply to other geologic settings under 

consideration for GS. Th e chapter also provides background on GS as a climate change mitigation 

strategy by identifying natural and industrial analogs for GS and reviewing U.S. experience in 

regulating subsurface injection. 

Chapter 3: Chapter 3 describes geologic attributes that could infl uence (i.e., increase or decrease) the 

vulnerability of a GS system to unanticipated migration, leakage, or pressure changes. Th is chapter 

also discusses the GS footprint component of the spatial area of evaluation.

2 Th e term “migration” refers to subsurface movement of CO2 (or other fl uids) within or out of the injection zone.

3 Th e term “leakage” refers to the movement of CO2 (or other fl uids) to the surface (for example, to the atmosphere or 

oceans). 
1

introduction

4

Chapter 4: Five impact categories and associated key receptors are described that could be aff ected 

by unanticipated migration and leakage or pressure changes: human health and welfare, atmosphere, 

ecosystems, groundwater and surface water, and the geosphere. Chapter 4 identifi es approaches to 

evaluate adverse impacts to receptors (human and environmental) that might occur in the event of 

unanticipated migration, leakage, or pressure change, and to evaluate spatial area of evaluation with 

respect to the receptors that may be impacted.

Chapter 5: Th e overall vulnerability of a GS site to adverse impacts is not dependent on the presence 

of a single attribute or receptor, but is a combined function of the identifi ed geologic attributes and 

receptors. Chapter 5 provides a qualitative discussion of this concept of an holistic approach, the 

linkage between attribute vulnerabilities and impacts, and how vulnerabilities may change over time. 

Th is chapter also elaborates on two key attributes considered likely conduits, wells and faults/fracture 

zones (both existing and pressure-induced). Th e discussion highlights that even for these individual 

attributes, it is the interplay of multiple characteristics that will determine the level of vulnerability, 

and not their simple presence.

Chapter 6: Th e VEF assists in identifying situations that could result in elevated vulnerability to 

adverse impacts from GS and often recommends monitoring in such instances. Th e potential for 

adverse impacts may be minimized in many cases by careful monitoring and mitigation. Chapter 6 

reviews monitoring technologies that can be used to measure how much CO2 is injected, to track 

the location of stored CO2, and to detect any CO2 releases4 to the atmosphere. Th is chapter also 

discusses potential mitigation actions in the event of leakage, unanticipated migration, or pressure 

changes. 

Chapter 7: Chapter 7 provides a summary of the VEF, its structure, development, purpose, and 

potential applications. Th e chapter also describes next steps that may be taken to further develop, 

refi ne, and validate the VEF.

4 Release is another term used for leakage.
2

5

CHAPTER 2

BACKGROUND ON GEOLOGIC 

SEQUESTRATION

Th e value of GS as a climate mitigation tool is, in part, contingent on CO2 remaining stored 

underground for a long period of time. Long-term storage could be accomplished by injecting 

CO2 into appropriate geologic formations with eff ective trapping mechanisms that will not be 

compromised over the storage period. It has been estimated that CO2 storage will need to function 

for at least several hundred years (Chalaturnyk and Gunter, 2004; IPCC, 2005; Holloway et al., 

2007). While a desirable timeframe for eff ective sequestration of CO2 may be as much as thousands 

of years, eff ective storage of CO2 for even several hundred years may provide valuable fl exibility in 

reducing CO2 emissions and addressing climate change impacts. 

Th is chapter describes likely formations where CO2 might be injected and the mechanisms that can 

trap CO2 underground. It reviews natural and industrial analogs and related fi eld experience that 

are often cited to support the view that GS can be an eff ective climate change mitigation technology 

(Benson et al., 2002; Heinrich et al., 2003; IEA, 2004; IPCC, 2005, 2007; Dooley et al., 2006; 

MIT, 2007). U.S. regulation of subsurface injection of fl uids also provides relevant experience that is 

summarized here. 

Th e chapter is organized into the following sections: 

• Section 2.1 outlines the kinds of geologic formations being considered for GS, including 

deep saline formations, depleted oil and gas reservoirs, coal seams, and other geologic 

formations. 

• Section 2.2 explains the various mechanisms and properties of CO2 and geologic formations 

that control the underground movement and trapping of CO2. 

• Section 2.3 examines practical experience relevant to sequestration, including existing GS 

projects, natural and industrial analogs for GS, and relevant EPA regulatory experience.

• Section 2.4 summarizes the chapter.

2.1 Geologic Settings under Consideration for Sequestration

Th e behavior of CO2 in the subsurface and potential vulnerabilities of a GS project are functions 

of both the type of geologic formation into which CO2 would be injected and the prior uses of the 

geologic setting, if any. Figure 2.1 is a general illustration of how GS could be implemented in a 

variety of geological settings. Th e VEF was developed mainly to evaluate deep saline formations, but 

many of the concepts also apply to other geologic settings. 

Geologic formations and operational processes typically considered for GS include the following:

• Deep saline formations: In these sedimentary formations, the pore space between the 

formation rock is fi lled with water containing elevated concentrations of dissolved salts 

(brines). Th ese formations are being considered for GS because they form very large basins, 

are located at signifi cant depth (generally below 800–1,000 meters), and typically are not 

considered viable sources of potable groundwater because of their salinity and depth. 
2

background on geologic sequestration

6

Figure 2.1. Examples of Geologic Formations Being Considered for GS. Source: CO2CRC, 2007

 Deep saline formations are believed to have the greatest potential storage capacity and 

are more widespread than other storage options (Dooley et al., 2006; NETL, 2007), and 

therefore are the primary focus of the VEF. Stacked reservoirs, in which multiple formations 

are overlain vertically, separated by lower permeability rock, may be considered particularly 

advantageous for GS. For a given surface area, such settings would provide greater vertical 

storage space, and the multiple layering may provide additional CO2 structural and 

stratigraphic traps (see section 2.2 for a discussion of CO2 trapping mechanisms). In 

addition, because of their depth, deep saline formations are penetrated by few wells or other 

artifi cial penetrations that could serve as pathways for CO2. Furthermore, the pressure 

and temperatures typically encountered at such depths are suffi cient to maintain CO2 as a 

supercritical fl uid; and supercritical CO2 requires much less storage space than it does in 

gaseous form. Finally, there is currently little competing demand for the resources contained 

in these formations, including saline pore waters (IPCC, 2005; Dooley et al., 2006). 

However, advances in desalination technologies and increasing water demand in certain 

regions could lead to increased competition in the future. A challenge in this GS option is 

the displacement of large volumes of water to create the pore space for CO2. Th e Sleipner 

Project in the North Sea is an example of a commercial operation in which captured CO2 is 

injected into a saline formation.

• Depleted oil and gas reservoirs: Th ese formations have eff ectively stored oil and natural gas 

for hundreds of thousands to millions of years before human extraction (Benson et al., 2002; 

IPCC, 2005; Haszeldine, 2006). As a result, there is reason to believe these same formations 
2

background on geologic sequestration

7

could eff ectively store injected CO2. Although a wealth of geologic data are available on 

the characteristics of these reservoirs, they are also known to have many abandoned oil 

and gas production and exploration wells that could be conduits for CO2 to escape from 

the subsurface. Chapter 5 contains more information on the potential for such wells to be 

unanticipated migration or leakage pathways.

• Enhanced oil/gas recovery: CO2 is currently injected in some U.S. oil fi elds to enhance 

oil production. Because these formations eff ectively stored oil for hundreds of thousands to 

millions of years, it is believed that they can be used to store injected CO2 for long periods 

of time. However, enhanced oil recovery (EOR) operations are not currently designed 

to maximize CO2 storage but rather to increase production of oil (Advance Resources 

International, 2006). Th e Weyburn site in Saskatchewan, Canada, is an example of an 

EOR operation using CO2 that is captured and shipped via pipeline from a commercial 

coal gasifi cation plant in Beulah, North Dakota. Geologic sequestration in EOR fi elds faces 

the same issues associated with depleted oil and gas reservoirs. Specifi cally, EOR fi elds have 

substantial geological information, but are known to have active and abandoned oil and 

gas production and exploration wells that could be unanticipated migration and leakage 

pathways (Celia et al., 2004; Heller, 2005).

• Coal seams: Coal seams are being considered for GS because of coal’s high affi nity for CO2 

(Haszeldine, 2006). Micropores in the coal matrix, made more accessible through coal seam 

fractures, can adsorb gases, including CO2 and methane (CH4). However, the fi ne cleats that 

create coal micropermeability can become plugged as CO2 replaces CH4, thereby restricting 

fl ow and causing localized matrix swelling. Matrix swelling around an injection wellbore may 

reduce permeability and injectivity and, if reduced injectivity is not overcome, may lead to 

reduced eff ective storage capacity. 

 Methane is a GHG that can contribute to climate change eff ects if discharged into the 

atmosphere (IPCC, 2007). While there are few examples in practice, the CH4 released by 

the adsorption of CO2 could be recovered for commercial use. Coal seams are sometimes 

underground sources of drinking water (USDWs), and may therefore be subject to additional 

requirements. Th ough coal seams are considered an option for CO2 storage, considerable 

research and development is still needed to understand coal seam CO2 sequestration. 

• Other geologic settings: Other rock types such as basalts and oil or gas rich shale, geologic 

repositories such as salt caverns, and abandoned mines may also be considered for GS, but 

are not the subject of current focus. Each of these settings has advantages and disadvantages 

with regard to its potential to eff ectively store CO2 based on its specifi c geologic 

characteristics. For example, basalt has the disadvantage of low porosity, permeability, and 

fractures that may result in the unanticipated movement of CO2 out of the injection zone; 

but has the advantage that CO2 could be permanently trapped in mineral form through 

chemical reactions of the CO2 with silicates in the basalt to form carbonate minerals (IPCC, 

2005). Additionally, these settings may not accommodate the anticipated scale of GS.
2

background on geologic sequestration

8

2.2 Subsurface CO2 Movement and Storage Mechanisms 

Knowledge of subsurface movement and trapping of CO2 assists in understanding the potential for 

CO2 to migrate out of the injection zone.

2.2.1 Factors that Control the Rate of CO2 Movement in the Subsurface

Th e rate of CO2 fl uid fl ow depends on the properties of CO2 and other fl uid phases present in the 

injection formation, properties of the formation itself, and physical and geochemical interactions 

that may occur in the subsurface. Th e primary fl uid transport mechanisms that control the rate of 

movement of CO2 in the subsurface include:

• Fluid fl ow caused by injection-induced pressure gradients (where higher gradients result in 

faster fl ow rates).

• Fluid fl ow caused by existing hydraulic gradients in the injection formation.

• Buoyancy-driven fl ow caused by the density diff erences between CO2 and the formation 

fl uids (which may result in upward migration of CO2).

• Dispersion and fi ngering caused by formation heterogeneities and viscosity contrasts between 

CO2 and the formation fl uid(s) (CO2 is less viscous than water and will preferentially “slide” 

over saline waters and channel into high permeability zones).

• Diff usion (this has a relatively minor eff ect).

Properties of the injection zone formation(s) that aff ect the rate of CO2 movement include its 

permeability, thickness, and heterogeneity. A higher permeability results in faster CO2 migration, 

and a greater thickness means that a greater total volume of CO2 can migrate at the given rate. 

Geologic heterogeneities also can control CO2 fl ow. For example, zones of high permeability such as 

a sand lens or an open fracture can act as conduits that allow CO2 to move much faster than would 

be expected based on the bulk properties of the rock. In contrast, low permeability zones such as 

shale can slow down or even stop fl ow. Some of these physical and geochemical processes also can 

aff ect the movement of CO2 in the subsurface by retarding CO2 fl ow and acting as CO2 trapping 

mechanisms, as discussed below.

2.2.2 Physical and Geochemical Trapping Mechanisms

Geologic sequestration of CO2 occurs through a combination of structural and stratigraphic 

trapping, residual CO2 trapping, solubility trapping, mineral trapping, and preferential adsorption 

trapping. Th ese mechanisms are functions of the physical and chemical properties of CO2 (see 

Appendix A for a summary of these properties) and the geologic formations into which the 

CO2 is injected. Figure 2.2 illustrates the relative eff ectiveness of the diff erent mechanisms (with 

the exception of preferential adsorption trapping) in trapping CO2 over time. Impermeable 

physical barriers are considered to be the most eff ective physical trap in the near term. Although 

mineralization is the most permanent trapping mechanism, it occurs relatively slowly compared to 

the others. Th e various trapping mechanisms, based on the discussion in the IPCC Special Report on 

Carbon Dioxide Capture and Storage (IPCC, 2005), are as follows. 
2

background on geologic sequestration

9

• Structural and stratigraphic 

trapping occurs when injected 

CO2 rises within the storage 

formation because of its relative 

buoyancy and/or the applied 

injection pressure and then 

reaches a physical barrier that 

inhibits further upward migration. 

Th e physical barrier could be a 

stratigraphic trap (a formation or 

a group of formations that act as a 

permeability and capillary barrier 

which impedes or prevents upward 

migration of supercritical CO2) 

or a structural trap such as one 

formed by folded or faulted rocks 

(IPCC, 2005). 

Figure 2.2. Eff ectiveness of Trapping Mechanisms Over 

Time. Source: IPCC, 2005, Figure 5.9. • Residual CO2 trapping occurs 

when CO2 moving through the 

formation(s) is retained in the pore 

space by capillary forces. Th is retention, also called capillary trapping, may range from 15% 

to 25% for typical storage formations (Holtz, 2002), but could exceed 25%, depending upon 

the porosity and permeability of the formation(s). When the degree of capillary trapping 

is high and CO2 is injected at the bottom of a thick formation or group of formations, 

capillary trapping can prevent CO2 from reaching overlying structural and stratigraphic traps 

(IPCC, 2005; Kumar et al., 2005).

• Solubility trapping occurs when injected CO2 contacts a fl uid formation (e.g., saline water) 

and dissolves into the fl uid (also known as dissolution trapping). CO2 that is dissolved in a 

formation fl uid is not buoyant; it is instead trapped within the formation fl uid. Th e density 

of formation fl uids with dissolved CO2 increases, so formation fl uids containing dissolved 

CO2 may sink within the formations (GEO-SEQ Project Team, 2004; Streit and Watson, 

2004; IPCC, 2005), though this eff ect may be limited by geologic heterogeneities (Lindeberg 

and Wessel-berg, 1997; Ennis-King and Paterson, 2003). 

• Mineral trapping occurs when the injected CO2 reacts with the formation waters or 

formation rocks, or both, to form carbon-containing minerals such as carbonates. Although 

some of the injected CO2 may react relatively quickly to form solid mineral phases, it is 

generally believed that converting all the injected CO2 into solid minerals could take several 

thousand years. Nevertheless, the permanence of mineral trapping makes it a desirable 

feature for long-term storage (Wilson, 2004; IPCC, 2005). 

• Preferential adsorption trapping occurs when coal and certain organic-rich shales have a high 

affi nity for CO2, meaning that CO2 can be adsorbed to the coal and shale surfaces. Coal may 

contain up to 25 cubic meters of CH4 per metric ton of coal (IPCC, 2006). Because coal 

has a greater affi nity for CO2 than CH4, CO2 injected into coal seams can displace CH4, be 
2

background on geologic sequestration

10 5 http://www.netl.doe.gov/technologies/carbon_seq/core_rd/world_projects.html

adsorbed and trapped, and the released CH4 could be recovered for commercial use, though 

application of this concept is still in early developmental stages. Carbon dioxide will remain 

trapped in coal and certain organic-rich shales under stable pressure and temperature (IPCC, 

2005; Haszeldine, 2006).

2.3 Practical Experience Relevant to Geologic Sequestration

Existing project experience, natural and industrial analogs, research and current regulatory experience 

with underground injection contribute to the understanding of CO2 behavior in geologic systems.

2.3.1 Existing Projects

Ongoing GS projects show that CO2 can be successfully injected and sequestered in geologic 

formations (see, for example, IEA, 2004). Currently operating commercial CCS sites include the 

Sleipner Project in the North Sea (Norway), the Weyburn Enhanced Oil Recovery Project (Canada), 

and the In Salah Gas Formation (Algeria). Th ese projects provide valuable fi eld experience which will 

help improve understanding of GS systems, CO2 behavior, and storage mechanisms. For example, 

they provide information that diff erent geologic formations have the injectivity, containment, and 

storage eff ectiveness needed for long-term sequestration. Th ese projects have practically applied site 

selection tools and monitoring techniques and programs. Th e existing projects have also highlighted 

the importance of establishing a baseline as a part of an eff ective monitoring program (see chapter 

6), and have provided insight into the use of models to help predict the behavior of CO2 in the 

subsurface. However, it should be noted that these sites have been operating for only a relatively 

short period of time (up to a decade), and hence do not demonstrate the effi cacy of GS over the 

longer required storage time periods of hundreds to thousands of years.

A new set of commercial GS projects will be implemented in the very near future, including the 

Gorgon Joint Venture (Barrow Island, Australia). In addition, U.S. fi eld experiments, including 

the Frio Brine Experiment (Texas) and regional projects supported by DOE’s Regional Carbon 

Sequestration Partnerships Program, will contribute valuable information about GS in coming years 

(see U.S. DOE [2007b] for a summary of this program). For a more comprehensive list of current 

and planned GS projects, see NETL’s CO2 Storage Web site5. 

Existing projects indicate that GS is a viable technology. However, commercial-scale deployment 

of GS will involve substantially larger volumes of CO2, and individual GS projects will need to 

inject greater volumes of CO2 than current DOE pilot projects and other international sites. 

Commercial-scale GS projects will encompass areas that may be miles in diameter (as opposed to 

the small fraction of a mile encompassed by most pilot projects). Th erefore, current analogs may 

not demonstrate the full range of scenarios that are likely to be encountered in commercial-scale 

deployment. Commercial-scale projects may be more likely to: 

• Encounter geologic heterogeneities that may serve as unanticipated migration and leakage 

pathways, including faults and fractures and other geologic features such as high permeability 

sand lenses or “pinches” in the confi ning system. 

• Intersect potential anthropogenic pathways such as unplugged wells. 
2

background on geologic sequestration

11

• Experience adverse pressure eff ects that can cause fracturing or regional eff ects on 

groundwater fl ow.

• Encounter basin-wide eff ects, and infl uences of neighboring projects.

It is also noteworthy that pilot demonstration projects are generally designed to exhibit the viability 

of the technology (e.g., minimal unanticipated migration of CO2). Sites are chosen for these projects 

because they are anticipated to successfully contain the smaller volumes of CO2, and hence may 

not portray the full range of geologic and anthropogenic features that could be encountered in 

commercial-scale deployment (Friedmann, 2003). However, pilot projects can nevertheless provide 

useful information, and some of their inherent limitations may be overcome by evaluating multiple 

projects implemented across a variety of geologic settings.

2.3.2 Natural and Industrial Storage Analogs

Natural and industrial systems that have stored or are storing CO2 and other fl uids (e.g., liquids 

and gases) may also provide insights into the feasibility of GS. Although, as noted in the previous 

section, the quantity of CO2 that may need to be injected for GS may be much greater than these 

analogs, the evaluation of these analogs has improved the understanding of storage mechanisms and 

processes. 

Carbon dioxide accumulates underground naturally in a variety of geologic settings. For example, 

200 million metric tons of naturally occurring CO2 have remained trapped in the Pisgah Anticline in 

central Mississippi, northeast of the Jackson Dome, for more than 65 million years with no evidence 

of unanticipated migration or leakage (IPCC, 2005). Such natural analogs provide information 

about the ideal conditions for long term storage.

Industrial practices of injecting and storing fl uids underground may also serve as analogs for GS. Th e 

oil and gas industry, for example, has been storing natural gas in underground reservoirs for nearly 

100 years (IPCC, 2005). Experience from natural gas storage operations suggests that it is possible to 

store gases eff ectively in the subsurface. However, there are examples of gas escaping through wells, 

faults, and fracture zones (Perry, 2005). Furthermore, these sites are generally used for temporary 

storage and hence only provide insight, but not a demonstration of, the long-term feasibility of 

underground storage of fl uids and gases. Th ese sites also provide valuable evidence that confi ning 

systems can be exposed to repeated stress cycling (i.e. depressurizing and pressurizing) without 

adverse eff ects on seal integrity.

Th e oil and gas industry also has more than 35 years of experience in site characterization and 

injection of CO2 through enhanced product recovery projects (Benson et al., 2002; Heinrich et al., 

2003; IPIECA, 2007). EOR projects contribute substantial knowledge about the design of CO2 

injection wells and technologies for handling, injecting, and monitoring injected supercritical CO2. 

However, such projects are designed to maximize oil production rather than provide storage of CO2 

for long periods. 

2.3.3 Regulatory Experience

Federal and State regulations protecting underground sources of drinking water under SDWA 

address the injection of fl uids into the subsurface (including liquids, gases and semisolids). Th ese 

regulations are designed to ensure that injected fl uids do not endanger USDWs and address siting, 
2

background on geologic sequestration

12

well construction, monitoring and site closure. For example, the program regulates the injection of 

CO2 for enhanced oil and gas recovery. Th e EPA Underground Injection Control (UIC) regulations 

refl ect a great deal of technical expertise by operators and regulators on relevant geological issues and 

well construction and operations associated with injection. Th e UIC Program has been successfully 

regulating the injection of billions of gallons of fl uids annually into tens of thousands of injection 

wells for more than 30 years (Benson et al., 2002). Th is regulatory experience will provide useful 

insight for GS projects and is seen as a clear indication that GS projects will be addressed through an 

established and eff ective regulatory system. For more information, please visit 

http://www.epa.gov/safewater/uic/wells_sequestration.html.

2.4 Chapter Summary

Th is chapter described the geologic formations and trapping mechanisms necessary for the eff ective 

storage of CO2. It also examined the feasibility of GS as a climate mitigation technology by 

reviewing natural and industrial analogs as well as EPA’s current regulatory experience in subsurface 

injection. Th e next chapter provides an overview of the VEF and discusses the geological attributes 

that could result in vulnerabilities to adverse impacts.
3

13

CHAPTER 3

VULNERABILITY EVALUATION FRAMEWORK FOR 

GEOLOGIC SEQUESTRATION: GEOLOGIC 

SYSTEMS AND ATTRIBUTES

Th e VEF identifi es attributes of GS systems that may lead to increased vulnerability to adverse 

impacts, identifi es potential impact categories, and provides a series of decision-support fl owcharts 

that are organized, systematic approaches to assess the attributes and impacts. Th ese attributes and 

impact categories were carefully selected by EPA as the key factors of GS systems to be included in a 

vulnerability evaluation, through the process described in Chapter 1 of literature review, consultation 

with experts, and professional knowledge. Th e conceptual approach to the VEF shown in Figure 3.1 

has the following components:

• Th e GS system fi rst is characterized in terms of the injected CO2 stream, the confi ning 

system, the injection zone, and a series of geologic attributes that could infl uence (i.e., 

increase or decrease) the vulnerability of the GS system to unanticipated migration, leakage, 

and undesirable pressure changes (fi rst column). 

SPATIAL AREA 

OF EVALUATION

POTENTIAL IMPACT CATEGORIES 

AND RECEPTORS

GEOLOGIC SEQUESTRATION SYSTEM 

AND GEOLOGIC ATTRIBUTES

Atmosphere

Human 

health/welfare

Lateral extent

Capillary entry 

pressure

Permeability

Travel time

Wells

Geomechanical 

processes

Tectonic activity

Geochemical 

processes

Geosphere

Groundwater and 

surface water

Ecosystems

Unanticipated 

migration and 

leakage (of CO2

and other fluids) 

Faults/fracture 

zones

Injection 

zone

Physical capacity 

Injectivity

Geochemical and 

geomechanical 

processes

Human populations

Populations covered 

by Executive Orders

Cultural/recreational 

resources

Economic resources

Sensitive species

Water quality

Protected/sensitive 

drinking water 

supplies

Regional 

groundwater

flow

Pressure 

changes

Confining 

system

CO2 STREAM

Legislatively 

protected species

Figure 3.1. VEF Conceptual Model
3

vulnerability evaluation framework for geologic sequestration: geologic systems and attributes

14

• An approach is then provided for defi ning the spatial area that should be evaluated for 

adverse impacts associated with unanticipated migration, leakage, or undesirable pressure 

changes (middle column).

• Potential impact categories and associated key receptors are then identifi ed, including human 

health and welfare, the atmosphere, ecosystems, groundwater and surface water, and the 

geosphere (last column).

Th ough not explicitly shown in Figure 3.1, the VEF further recognizes that pressure changes in the 

injection zone and overlying geosphere may be linked to unanticipated migration or leakage pathways 

and the associated impacts depicted in Figure 3.1, through processes like induced fracturing, fault 

reactivation, and the exceedence of capillary entry pressure. Th ese concepts are also discussed in 

this chapter. Furthermore, the VEF also recognizes that impacts to the geosphere, groundwater and 

surface water, ecosystems, and the atmosphere may also impact human health and welfare. 

Th is chapter is an overview of the geologic sequestration system, the geologic attributes that have 

been identifi ed as aff ecting vulnerability, and the related spatial area that would be evaluated for 

unanticipated migration, leakage, and pressure changes. Th ese are represented in the fi rst two 

columns of the conceptual framework shown in Figure 3.1. Chapter 4 discusses the potential adverse 

impacts of GS and potentially aff ected receptors, the last column of the VEF conceptual framework. 

For the purposes of the VEF and this report, a binary classifi cation of low and elevated vulnerability 

are qualitatively defi ned as follows:

• Low vulnerability: Adverse impacts are not expected to be associated with the attribute or 

receptor under evaluation.

• Elevated vulnerability: Particular attention should be paid to the attribute or receptor under 

evaluation. In some cases, adverse impacts may occur if actions are not taken to further 

examine and/or manage the vulnerability associated with the attribute or receptor. Examples 

of actions that may be taken include corrective action at wells, targeted monitoring, and the 

development of mitigation plans.

Th e metrics and binary classifi cation schemes for each attribute in this chapter can be used to 

qualitatively evaluate the level of vulnerability associated with each attribute. Th e classifi cation 

schemes indicate whether vulnerability is expected to be low or elevated on a qualitative basis. 

Elevated vulnerability associated with a single attribute does not imply that overall vulnerability 

associated with the GS site is elevated. In some cases, there may be actions that could be taken to 

minimize vulnerability (e.g., targeting questionable wells for corrective action). Th ese instances 

are indicated where applicable in the accompanying decision-support fl owcharts that have been 

developed for the components of the GS system. 

Th e fl owcharts represent fi rst-order evaluation approaches and, as such, should be used only 

with additional information and analysis to support more comprehensive risk assessment and/or 

decision-making. In the future, binary classifi cation schemes could be further developed to refl ect 

a more refi ned, multiscaled classifi cation scheme, as warranted by available information, data, and 

expert opinion. Future steps could also include developing the decision-support fl owcharts into an 

integrated evaluation tool that has a more quantitative and numerical basis, presented in a userfriendly format.
3

vulnerability evaluation framework for geologic sequestration: geologic systems and attributes

15

Th is chapter contains the following sections:

• Section 3.1 describes the GS system and geologic attributes that may aff ect unanticipated 

migration, leakage, and pressure changes.

• Section 3.2 discusses the spatial area of evaluation.

• Section 3.3 summarizes the chapter. 

3.1 Geologic Sequestration System and Geologic Attributes

Descriptions of geological attributes that can infl uence the vulnerability of a GS system to 

unanticipated migration, leakage, or pressure changes include a variety of metrics and classifi cation 

schemes. Th e metrics and schemes can be used to determine whether there is low or elevated 

vulnerability to unanticipated fl uid migration, leakage, and/or pressure changes associated with 

each attribute. Th e attributes presented here are organized according to those that are relevant to 

the confi ning system and those that are relevant to the injection zone. Th e section also describes 

evaluation processes (decision-support fl owcharts) for the confi ning system, the injection zone, and 

the CO2 stream.

3.1.1 Confi ning System and Related Geologic Attributes

A confi ning system is defi ned as a geologic formation, group of formations (e.g., shale or siltstone), 

or part of a formation (sometimes referred to as an aquitard) that is composed of impermeable or 

distinctly less permeable material which acts as a barrier to the upward fl ow of fl uids stratigraphically 

overlying the injection zone. Geologic attributes of the confi ning system identifi ed as infl uencing the 

potential for unanticipated migration and leakage include the following.

• Lateral extent. Lateral extent is defi ned in the VEF as the surface area of the confi ning 

system that overlies the GS footprint. Th is attribute can be evaluated using a metric of total 

surface area measured in appropriate units such as square miles. Elevated vulnerability is 

associated with a confi ning system with a lateral extent that is less than the GS footprint. 

• Capillary entry pressure. Th e capillary entry pressure is defi ned in the VEF as the added 

pressure that is needed across the interface of two immiscible fl uid phases (e.g., supercritical 

CO2 and water or brine) for CO2 to enter the confi ning system. Appropriate evaluation 

metrics for determining suffi cient capillary entry pressure include CO2 column height 

and injection pressure (Harrington and Horseman, 1999). Elevated vulnerability may be 

associated with the exceedence of the confi ning system capillary entry pressure.

• Permeability. Permeability refers to the ability of a geologic material to allow transmission of 

fl uid through pore spaces. Appropriate metrics for evaluating permeability include the darcy 

unit. Elevated vulnerability may be associated with geologic materials with a permeability 

greater than clay, shale, or siltstone.

• Travel time. Travel time refers to the interval of time that is required for a fl uid (e.g., CO2 

or brine) to migrate across the thickness of the confi ning system. Factors that will infl uence 

travel time include the confi ning system thickness, permeability, diff usion, retardation (as a 
3

vulnerability evaluation framework for geologic sequestration: geologic systems and attributes

16

result of sorption or desorption), and geochemical reactions. Th us, all other parameters being 

equal, GS systems at greater depth will have longer associated travel times. Th e metric for 

travel time is a unit of time such as years. Travel times that compromise the integrity of the 

project are considered to result in elevated vulnerability.

• Wells. Wells (and other artifi cial penetrations such as boreholes) may serve as conduits for 

fl uid movement and hence could result in elevated vulnerability to adverse impacts. Numerous 

metrics have been identifi ed as relevant to the evaluation of vulnerability associated with 

wells, including well depth, well integrity (including construction materials, and seal and plug 

materials), and spatial density of well occurrence. Th e level of vulnerability associated with wells 

can be evaluated by considering all of these metrics. Wells and faults have been identifi ed as 

one of the most likely unanticipated migration and leakage pathways in GS systems; as such, 

they are discussed in greater detail in Chapter 5 of this report, which also includes an evaluation 

approach.

• Faults/fracture zones. Faults are breaks in the Earth’s crust that occur when the crustal 

rock is either compressed or pulled apart. A fracture is any local separation or discontinuity 

plane in a geologic formation that divides the rock into two or more pieces. Fractures are 

commonly caused by stress exceeding the rock strength. For the purposes of this report, 

fractures are defi ned as distinct from faults by their smaller scale. Faults may serve as 

either barriers or conduits to fl uid fl ow (Omre et al, 1994; Lewicki et al., 2006; Wilkens 

and Naruk, 2007). Numerous metrics may be appropriate for evaluating faults and/or 

fracture zones, including density, stratigraphic position, connectivity, sealing/conductive 

(transmissive), stress level, orientation, and fault reactivation pressure (multiplied by a safety 

factor). Th e level of vulnerability associated with faults and fractures can be evaluated by 

considering all of these metrics. Chapter 5 discusses faults and fracture zones in greater detail.

• Geochemical processes. Geochemical processes are chemical reactions that may cause 

alterations in mineral phases. A number of diff erent geochemical processes could infl uence 

the confi ning system. Acidity caused by the reaction of CO2 with water may partially dissolve 

confi ning zone geologic materials, which could have the unfavorable eff ect of opening fl uid 

migration pathways within the confi ning zone. Geochemical reactions could also have 

favorable eff ects, such as the formation of mineral phases as result of the reaction of CO2 

with the geologic material of the confi ning system and/or formation waters that could help 

to improve the seal of the confi ning system, by plugging pores and fractures (Johnson et al., 

2005). Appropriate metrics for evaluating geochemical processes include dissolution rates, 

buff ering capacity, molar volume, and pH level. Mineralogy and pH that favor the formation 

of conduits in the confi ning system through dissolution and/or decreases in molar volume 

increase vulnerability; those that do not favor the formation of conduits through dissolution 

and/or increases in molar volume decrease vulnerability. 

• Tectonic activity. Tectonically active settings may be more likely to have transmissive faults 

and/or fractures, and may be unsuitable for GS (IPCC, 2005). Seismic activity can be used as 

a measure for tectonic activity. Seismic activity is defi ned as the shifting of the Earth’s surface 

due to changes at depth, and it may cause seismicity or earthquakes. An appropriate metric 

for evaluating tectonic activity is the seismic hazard rating. Areas with seismic hazard ratings 

that indicate the potential for seismicity to cause adverse impacts are considered to have 

elevated vulnerability (see USGS, 2007). 
3

vulnerability evaluation framework for geologic sequestration: geologic systems and attributes

17

 Tectonic activity also covers volcanism, which is the process by which magma and gases are 

transferred from the Earth’s interior to the surface. Hotspots are shallow areas of molten rock 

below the surface that persist long enough to leave a record of uplift and volcanic activity. 

Distance to volcanic activity or hotspots serves as a metric for vulnerability, and the presence 

of active volcanoes or hotspots within the spatial area of evaluation may indicate whether 

there is low or elevated vulnerability.

• Geomechanical processes. Th ese are processes that may result in alterations of the structural 

integrity of geologic material. Appropriate evaluation metrics for this attribute include 

fracture pressure, fracture/fault reactivation pressure, and orientation of the fracture or fault 

relative to the orientation of the principal regional stress regime. If the fracture pressure 

and the fracture/fault reactivation pressure (multiplied by a safety factor) are exceeded, 

vulnerability is considered to be elevated. It should be noted that geomechanical processes 

occur at a continuum of scales, and potential impacts, such as deformation of geologic 

formations, can occur without necessarily adversely aff ecting the integrity of the confi ning 

system.

Confi ning System Evaluation Process

An evaluation process that can be used to examine the confi ning system based on the attributes 

described above is shown in Figure 3.2. In the fl owchart, elevated vulnerability refl ects a 

determination that the confi ning system is inadequate and may increase the potential for adverse 

impacts, and low vulnerability indicates that the confi ning system is anticipated to be adequate for 

the proposed project. Elevated or low vulnerability determinations in the VEF refer to a specifi c 

attribute, system, or impact being evaluated and not to the GS site as a whole. Chapter 5 discusses 

key considerations in assessing the overall vulnerability of a GS site. 

Th e confi ning system evaluation provides an approach for assessing and reducing vulnerability 

through: 

• Establishing that a confi ning system is present over the necessary lateral extent. To 

ensure the confi ning system acts as an eff ective barrier to fl uid fl ow, it is important that the 

geologic formations (rock layers) of the confi ning system are suffi ciently laterally extensive 

and continuous to cover the entire area aff ected by the CO2 injection. Th is includes the area 

occupied by the CO2 and a potentially larger area aff ected by pressure changes associated 

with injection6. Th e continuity of the barrier can be maintained, despite pinch-outs or 

other discontinuities, if such features in one rock layer are blocked by overlying layers 

of the confi ning system. In some cases, there may also be a confi ning system underlying 

the injection zone that serves as a lower barrier to the GS system. If the lateral extent is 

insuffi cient, it may be possible to alter operational conditions to improve site suitability, for 

example, by injecting into multiple formations of the injection zone, thereby reducing the 

surface area of the CO2 plume. 

• Evaluating the physical properties of the confi ning system to determine if it provides 

adequate confi nement of fl uids under the proposed operating conditions. Attributes 

of a confi ning system that will help to prevent the upward movement of fl uids include 

6 Deep saline formations that are laterally unconstrained may in particular have larger areas aff ected by pressure, in contrast for 

example, to depleted oil and gas fi elds, where the extent of pressure changes may be limited by the geologic structures of the system.
3

vulnerability evaluation framework for geologic sequestration: geologic systems and attributes

18

Elevated vulnerability

Low

vulnerability

Evaluations determine that confining 

system integrity is interrupted

Evaluate wells and faults using 

evaluation flowcharts

Yes

No

Site

suitability

may be 

improved by altering 

operational 

conditions?

No

Yes

Confining

system present 

over the projected

footprint?

Evaluate the physical properties of the confining 

system:

Overlying the CO2 plume:

• Permeability

• Capillary entry pressure

• Susceptibility to geochemical

 degradation

For 

the CO2

footprint: 

Projected CO2

column height and/or

injection pressure 

exceed the capillary

entry

pressure?

Evaluate fluid travel time 

through the confining system

Evaluate confining system integrity

Well and fault evaluations indicate that the integrity of the confining system is adequate, or 

adequate with application of corrective action and/or targeted development of monitoring 

and mitigation plans





Travel time 

compromises 

the project? 

Determine the necessary lateral extent of the confining system

No Yes

No Yes

Confining

system is

susceptible to 

induced fracturing 

under proposed 

operational 

conditions?

Elevated 

vulnerability

Confining system evaluation 

approach: 

Is the confining system present 

over the necessary lateral extent?

 Do the physical properties of the 

confining system provide adequate 

confinement of fluids under the 

given operating conditions?

 Is the confining system integrity 

interrupted by “natural” (faults) or 

anthropogenic (wells) features or 

compromised by operational 

conditions? 

Overlying the area 

affected by pressure

changes:

• Permeability

No Elevated 

vulnerability

Yes

Operating

conditions can be 

altered?

Low

vulnerability

Yes

Elevated vulnerability

No



Figure 3.2. Confi ning System Evaluation
3

vulnerability evaluation framework for geologic sequestration: geologic systems and attributes

19

its capillary entry pressure7. A high capillary entry pressure will make it diffi cult for CO2 

and other fl uids to enter the confi ning system. Travel time, a function of the permeability 

and thickness of the confi ning system, may also contribute to an eff ective barrier. A long 

travel time would ensure that if any CO2 or other fl uids entered the confi ning system, they 

would travel only minimal distances within the confi ning system over the timeframes that 

are relevant to GS systems. A low permeability will contribute to a long travel time. A low 

susceptibility to dissolution and other geochemical degradation processes will also contribute 

to an eff ective confi ning system, thus avoiding thinning of the barrier and the opening of 

potential fl uid pathways.

• Evaluating the integrity of the confi ning system. Another important factor that may limit 

the ability of the confi ning system to act as a barrier to fl uid fl ow includes the presence of 

fl uid-conducting perforations such as unsealed or unplugged wells, and transmissive faults 

and fractures (both natural and those induced/reactivated by injection). Vulnerabilities 

associated with wells and faults may be managed through altering operational conditions 

such as injecting at a lower rate at a greater number of wells to reduce injection pressure. Th e 

integrity of the confi ning system could also be interrupted by other high permeability zones 

such as sand lenses, and other discontinuities, such as pinchouts in the confi ning system 

formation(s). Targeted monitoring can also detect unanticipated migration and leakage and 

pressure changes, and inform the development of mitigation actions and plans. Mitigation 

actions, such as corrective action for wells, are discussed in more depth in Chapter 6. 

3.1.2 Injection Zone and Related Geologic Attributes 

Th e injection zone is a geologic formation, group of formations, or part of a formation of suffi cient 

areal extent, thickness, porosity, and permeability to accommodate CO2 injection volume and 

injection rate. Th e injection zone is characterized by several geologic attributes identifi ed to have the 

potential to infl uence pressure changes and are described below. 

• Physical capacity. Physical capacity is defi ned as the volume within a geologic formation 

that is available to accept CO2. Injected CO2 will occupy the pore space between the 

grains of the rock that makes up the injection zone by displacing the fl uids that are 

already occupying these pores. If storage capacity is found to be insuffi cient for the 

proposed operation due to pressure constraints capacity could be increased by dewatering. 

Appropriate metrics for evaluating this attribute include thickness, surface area, eff ective (or 

interconnected) porosity, CO2 density (CO2 density generally increases with depth, because 

density increases with pressure, and pressure generally increases with depth), and residual 

water saturation also referred to as irreducible water saturation. Induced pressure changes 

can also aff ect physical capacity. For example, if a geologic system is constrained laterally, 

the pressure buildup associated with the CO2 injection may restrict how much CO2 can be 

stored without exceeding fracture pressure, sometimes referred to as eff ective pore volume 

vs. absolute pore volume (Zhou, et al., 2008). Other non-technical considerations could 

also potentially aff ect storage capacity, including economic considerations such as competing 

demands for the injection zone. 

• Injectivity. Injectivity characterizes the ease with which fl uid can be injected into a 

geological formation. It will be infl uenced by both properties of the injection zone and 

7 Th is is true for CO2 and other nonaqueous displaced fl uids. However, in the case of brines, there is no capillary entry pressure to 

overcome because the fl uid in the confi ning system and the brine are both aqueous.
3

vulnerability evaluation framework for geologic sequestration: geologic systems and attributes

20

operational factors. Th e properties of the injection zone that control injectivity include 

injection zone thickness, permeability, and pressure within the injection zone8. Th ough the 

VEF is focused on the geologic system, operational factors that infl uence injectivity, and 

which may be altered to reduce vulnerability, are discussed below in the description of the 

injection zone evaluation process. 

• Geochemical and geomechanical processes. Th e geochemical and geomechanical processes 

may infl uence vulnerability to pressure changes by aff ecting injection zone capacity to store 

CO2 and injectivity. 

- Geochemical processes. Geochemical processes are the chemical reactions that may 

cause alterations in mineral phases. A number of diff erent geochemical processes could 

infl uence the injection zone. Dissolution of CO2 into the formation waters (e.g., brine) 

will reduce the volume of CO2 that is stored as a supercritical fl uid, thus reducing needed 

storage volume (Doughty et al., 2001). Acidity caused by the reaction of CO2 with water 

may partially dissolve injection zone geologic materials, which could improve porosity 

and injectivity. Geochemical processes may also have unfavorable impacts. For example, 

new mineral phases may form as a result of the reaction of CO2 with the geologic 

materials and/or formation waters. Th ese new minerals may partially plug pores and 

thereby have the unfavorable impact of reducing permeability and porosity (Knauss et al., 

2003). 

 Appropriate metrics for evaluating the geochemical processes that could lead to pressure 

changes include dissolution rates, buff ering capacity, molar volume, and pH level. 

Unfavorable geochemical processes are defi ned as involving pH and mineralogy that 

favor precipitation of minerals and/or increases in molar volume, resulting in elevated 

vulnerability. Favorable geochemical processes, those with low vulnerability, are defi ned 

as involving pH and mineralogy that favor increased injection zone porosity through 

dissolution and/or decreases in molar volume.

- Geomechanical processes. Th ese are processes that may result in alterations in the 

structural integrity of a geologic material. If pressure in the injection zone is increased 

because of CO2 injection, the injection zone geologic material may be deformed, and in 

the extreme, crack or fracture, or faults might be reactivated. Fracturing of the injection 

zone is intentionally used to increase production in oil and gas operations, and whether 

or not this method should be used for GS could be considered based on further study of 

the applicability of this technique to GS and site-specifi c factors. As a result, appropriate 

metrics and thresholds for injectivity may be site-specifi c.

Injection Zone Evaluation Process

An evaluation process that can be used to examine the injection zone based on the attributes just 

described is shown in Figure 3.3. As indicated in the fl owchart, the vulnerability to adverse impacts 

associated with the injection zone of a GS system may be considered low if the storage capacity is 

adequate, the injectivity is suffi cient, and geochemical and/or geomechanical processes produce 

favorable conditions for injection and storage. In particular, this evaluation provides an approach for 

assessing and reducing vulnerability through:

8 To introduce CO2 into the injection zone, the downhole injection pressure must be higher than the injection zone formation(s) fl uid 

pressure.
3

vulnerability evaluation framework for geologic sequestration: geologic systems and attributes

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No

Sufficient

injectivity under

the proposed 

operating

conditions?

No

Yes

Geochemical

and geomechanical

properties facilitate 

storage under the 

proposed operating conditions?

No

Operating 

conditions can 

be altered to 

improve 

injectivity?

Yes

Sufficient 

physical 

capacity?

Elevated vulnerability due to 

inadequate storage capacity

No Elevated 

vulnerability

Yes

Yes

Elevated 

vulnerability

Low 

vulnerability

Figure 3.3. Injection Zone (IZ) Evaluation

• Determining that the injection zone capacity is suffi cient. Th e physical capacity of the 

injection zone is proportional to the total pore volume of the injection zone. Total pore 

volume can be determined by multiplying the injection zone thickness by the injection 

zone surface area and by the fraction of the injection zone that is taken up by pores (i.e., its 

porosity). However, only a fraction of this total pore volume will actually be available for 

CO2 storage. 

 Numerous factors will infl uence the physical capacity of the injection zone. Carbon 

dioxide will enter only interconnected pores. Most of the pores in rocks such as sandstones 

are interconnected, but some fraction of the pore volume will be “dead space” and not 

interconnected. Carbon dioxide will displace only a fraction of the water in the pores, 

because of residual water saturation. Furthermore, CO2 is not likely to occupy the entire 

thickness of the injection zone because of its buoyancy and heterogeneities within the 

injection zone. Being relatively buoyant, it will tend to rise upward in the injection zone and 

spread out laterally under the confi ning system or under lower permeability lenses within the 

injection zone. Vertical features such as sealed faults or other discontinuities may also limit 

the pore volume that is accessible to CO2, and may put pressure constraints on the system 

that could also limit the physical capacity. 
3

vulnerability evaluation framework for geologic sequestration: geologic systems and attributes

22

 Properties of CO2 that will infl uence the amount of space it occupies include its density and 

ability to dissolve in water. Th e space needed to store a given amount of CO2 will decrease 

with increasing density. Density increases with increases in pressure and depth and decreases 

with increases in temperature. Dissolution will be greater at higher temperatures and 

pressures, and less dissolution occurs at higher salinities.

• Determining that the injectivity is suffi cient. As described above, injectivity is the rate at 

which CO2 injection can be sustained over the duration of injection, and is a function of 

properties of the injection zone and operational conditions. Th e former include injection 

zone thickness, permeability, and pressure within the injection zone. Operational factors that 

may infl uence injectivity include the length and orientation of the injection well. Injectivity 

may be increased by maximizing the slotted length of the injection wells (i.e., the part of the 

well that is open and allows passage of CO2 into the injection zone), installing horizontal 

wells that are slotted over much longer reaches than their vertical counterparts, or increasing 

the number of wells used for injecting CO2. 

• Evaluating the geochemical and geomechanical properties of the injection zone. By 

virtue of their impact on porosity, geochemical and geomechanical processes may also aff ect 

the physical capacity of the injection zone. Depending on whether there is net dissolution or 

precipitation of minerals associated with the injection of CO2, injectivity may be improved 

or decreased correspondingly. Impurities in the CO2 stream may also react with the injection 

zone rock and formation waters to form new minerals that may locally decrease porosity. 

Fracturing is intentionally used to increase production in oil and gas operations, and 

intentional fracturing of the injection zone could improve permeability and hence injectivity 

of the injection zone. However, the appropriateness of this technique for GS applications 

may need further study. 

 Initial estimates of physical capacity can be made using equations such as those in the NETL 

Carbon Sequestration Atlas (NETL, 2007), or the approaches developed by Bachu et al. 

(2007) and Brennan and Burruss (2003). More refi ned evaluations of the attributes of the 

confi ning system, the physical capacity, injectivity, and geochemical and geomechanical 

processes within the injection zone are likely to rely on use of numerical modeling and site 

characterization data (Xu et al., 2003; Pruess et al., 2004; Celia et al., 2005; IPCC, 2005). 

3.1.3 Carbon Dioxide Stream 

Th e CO2 stream and its characteristics may aff ect geologic attributes within the GS system and 

ultimately contribute to increased vulnerability to adverse impacts. A captured CO2 stream from 

a power plant or industrial source would probably not be pure CO2. Th e specifi c impurities and 

their concentrations in the CO2 stream will diff er depending on the fuel source, the capture process, 

constraints (i.e., concentration limits) associated with the mode of conveyance to the injection site 

(e.g., pipeline), and injection concentration limits. For example, trace amounts of sulfur dioxide 

(SO2), nitrogen oxide (NO), hydrogen sulfi de (H2S), hydrogen (H2), carbon monoxide (CO), 

methane (CH4), nitrogen (N2), argon (Ar), mercury (Hg), cyanide (Cn), and oxygen (O2) could 

be found in a captured CO2 stream (IPCC, 2005; U.S. DOE, 2007c). Some of these may be 

of potential concern because of their toxicity (e.g., Cn, Hg), others because of their potential to 

accelerate corrosion processes (e.g., H2S), and still others simply because they may increase the total 

amount of needed storage space (e.g., Ar). Under diff erent capture scenarios, the volumes of these 
3

vulnerability evaluation framework for geologic sequestration: geologic systems and attributes

23

impurities can vary signifi cantly. For example, the concentration of impurities as a percentage of total 

CO2 volume for post-combustion capture at coal-fi red power generation plants is approximately 

0.01%. In contrast, the concentration of impurities for precombustion capture at gas-fi red power 

generation plants is approximately 4.4% (IPCC, 2005). 

Given the potential presence of these impurities, characterization of the CO2 stream is very 

important. Impurities could create more acidic conditions, possibly accelerating the formation 

of fl uid-conducting pathways through the corrosion of well seal materials and the dissolution of 

geologic materials in the subsurface. Impurities could also impact storage capacity and well integrity. 

Th e CO2 Stream Evaluation, depicted in Figure 3.4, outlines key considerations for evaluating 

CO2 stream impurities. Th ough not a focus of the VEF, it is acknowledged that impurities may 

also impact surface infrastructure (see, for example, Rhudy 2004). Chapter 4 discusses potential 

receptors that might be aff ected by those impurities, and these receptors include human populations 

in general, populations covered by Executive Orders, economic and cultural/recreational resources, 

legislatively protected species and other sensitive species, and groundwater and surface water. Th e 

CO2 Stream Evaluation will help identify where impurity-specifi c targeted monitoring could indicate 

the need for mitigation actions and help reduce the potential for adverse impacts.

Evaluate the potential effects of impurities on confining system and injection zone 

attributes, including wells, faults/fracture zones, and geochemical processes 

Evaluate level of vulnerability, as appropriate, consider corrective actions 

and/or targeted monitoring and mitigation plans that are impurity-specific

If there is a high level of uncertainty or heterogeneity, consider identifying 

impact categories that may be affected by CO2 stream impurities

Characterize the CO2 stream: identify impurities

Figure 3.4. Carbon Dioxide Stream Evaluation

3.2 Spatial Area of Evaluation: Geologic Sequestration Footprint

Determining the full geographical extent of a GS system is essential for site characterization and for 

establishing the spatial area to be monitored for potential adverse impacts. Th e size and shape of 

the CO2 subsurface plume and associated pressure front will depend on the total volume of CO2 

injected over the duration of the injection, operational factors, and the geologic characteristics of the 

confi ning system and injection zone, including their geometry, heterogeneities, and other geologic 

features. Th e size of the CO2 plume will increase during the period of injection, which might last 

25 to 50 years. Because CO2 is relatively buoyant and has lower viscosity than water and brine, the 

plume may also continue to change position and shape for some time after injection ceases. However, 
3

vulnerability evaluation framework for geologic sequestration: geologic systems and attributes

24

secondary storage mechanisms, including dissolution, capillary, and mineral trapping, are anticipated 

to eventually minimize the size and immobilize the CO2 plume (see Chapter 2 for a discussion of 

secondary storage mechanisms). 

Th e pressure front9 associated with the injected CO2 may extend signifi cantly farther than the 

CO2 plume itself. Th is is particularly true in saline aquifers, where the geographical area aff ected by 

elevated pressure may be several orders of magnitude larger than the area occupied by CO2 (Nicot et 

al., 2006; Birkholzer et al., 2007). As discussed in Chapter 5, the pressure front may dissipate after 

injection stops, or may remain elevated, depending on the lateral boundaries of the system. 

For the purposes of this VEF, the spatial area encompassed by the CO2 plume and associated 

pressure front is termed the GS footprint. Figure 3.5 provides diff erent geologic scenarios that might 

be encountered, and approaches to delineate the footprint for each scenario. Estimates of the size of 

the footprint associated with the total amount of CO2 to be injected over the lifetime of a project are 

anticipated to be an element of the initial evaluation of the adequacy of a GS system. Th is footprint 

may also serve as the basis for defi ning the area where baseline conditions are established prior to 

injection at a GS site (see chapter 6 for more discussion on monitoring and baseline). Delineation 

of the footprint may also be performed at multiple stages of the project, as the footprint expands 

and changes shape during injection and due to buoyancy driven fl ow, and to predict the eff ect of 

secondary trapping mechanisms. In most instances, modeling in combination with site-specifi c data 

will most likely be used to delineate the GS footprint. Data collected during monitoring can be used 

to refi ne and calibrate models and confi rm the location and dimensions of the CO2 plume and the 

pressure front. 

Delineate the GS footprint. 

Possible footprint scenarios 

include:

Footprint delineation may occur at different stages of a project. Site characterization data may be used to refine footprint 

and calibrate models.

The GS footprint may be 

defined by delineating the 

surface area of the traps. 

Modeling may be used to 

determine if the storage 

capacity of the trap(s) will 

be sufficient.

The aerial extent of the 

footprint may be initially 

estimated using generic 

approaches and then 

refined using an 

appropriate model, and 

site-specific data.

Difficult to predict the GS 

footprint; modeling may be 

used; site may require 

additional data collection or 

may be inappropriate due to 

higher levels of uncertainty.

Difficult to predict the GS 

footprint; modeling may be 

used to determine CO2

migration up-dip.

Scenario 4: Complex 

geology

Scenario 3: Sloped 

geologic formations

Scenario 2: Large regional 

trap or series of traps

Scenario 1: Horizontal 

geologic formations

Figure 3.5. Geologic Sequestration (GS) Footprint Delineation

9 Th e pressure front can be defi ned by the pressure diff erential that is signifi cant enough to cause adverse impacts to overlying receptors 

(e.g., fl uid displacement into an overlying aquifer).
3

vulnerability evaluation framework for geologic sequestration: geologic systems and attributes

25

Th e spatial area to be evaluated for potential adverse impacts is dependent on geologic characteristics 

and potential receptors. Depending on the particulars of a GS system, the area to be evaluated may 

be larger than the GS footprint itself. Chapter 4 describes receptors and how they may infl uence the 

spatial area of evaluation.

All of the attributes of the confi ning system and injection zone should be considered within the 

GS footprint. However, it is particularly important to understand the pathways that could serve as 

conduits for CO2 and other fl uids, and to understand the causes and changes in subsurface pressure. 

Th ese pathways are introduced below and described in further detail in Chapter 5. Appendix B 

summarizes and compares some VEF geologic attributes with critical CO2 unanticipated migration 

and leakage pathways or characteristics that would aff ect the suitability of a proposed storage site 

identifi ed by the IPCC (2006) and IPIECA (2007).

Migration and Leakage Pathways 

To understand the potential for unanticipated migration and leakage, it is necessary to identify 

pathways that could allow the movement of CO2 and fl uids displaced by the CO2 (e.g., brines) 

and other geologically stored fl uids (e.g., oil and CH4). Th e IPCC (2005) identifi es three principal 

pathways:

• Th e pore system of the confi ning system if it is of low permeability, or around the lateral 

extent of the confi ning system if it is insuffi cient. 

• Anthropogenic artifi cial penetrations such as abandoned wells.

• Openings in the confi ning system, such as fractures or faults, which may be natural or 

induced by pressure changes.

Pressure Changes

Injecting CO2 into geologic formations will, in most cases, cause subsurface changes in pressure. 

Increased pressure can lead to the unanticipated migration of fl uids either through existing pathways 

or induced fracturing. It can also have impacts on overlying receptors, even if the CO2 remains 

contained within the injection zone. Th e applied pressure could force CO2 and other fl uids through 

existing conduits, such as abandoned wells and faults. Induced fracturing and fault reactivation 

can also occur if injection pressure exceeds formation pressures. Even if the CO2 and other fl uids 

remain contained, pressure changes associated with the injection may still cause adverse impacts to 

groundwater and surface water, including causing changes in groundwater fl ow direction and water 

table levels, and pressure-induced displacement of brine and other fl uids through the pore structure 

of the rock into overlying aquifers. Th e nature of pressure changes in the subsurface associated with 

injection will be determined by both geologic attributes of the GS system and operational factors. 

Th ese eff ects can be minimized through an understanding of the relevant geologic attributes, careful 

site characterization, careful operation of GS systems, and monitoring.
3

vulnerability evaluation framework for geologic sequestration: geologic systems and attributes

26

3.3 Chapter Summary

Th is chapter presented an overview of the geologic sequestration system, which comprises the 

confi ning system, the injection zone, and the CO2 stream. Geologic attributes that may infl uence the 

vulnerability to unanticipated migration, leakage, and pressure changes and evaluation processes for 

the confi ning system, the injection zone, and the CO2 stream were described. Finally, the spatial area 

that may be evaluated for unanticipated migration, leakage, and pressure changes was discussed, and 

a fl owchart that depicts GS footprint delineation scenarios was presented. Chapter 4 discusses the 

potential adverse impacts of GS and potentially aff ected receptors.
4

27

CHAPTER 4

VULNERABILITY EVALUATION FRAMEWORK 

FOR GEOLOGIC SEQUESTRATION: IMPACTS 

AND RECEPTORS 

Unanticipated migration, leakage, and changes in subsurface pressure could potentially cause adverse 

impacts to human health and welfare, the atmosphere, ecosystems, groundwater and surface water, or 

the geosphere. Furthermore, impacts to the latter four categories could in turn impact human health 

and welfare. For example, groundwater contaminated via unanticipated migration could have adverse 

impacts to human health. Adverse impacts to forests or fi sheries or other harvested natural resources 

could also result in adverse economic (welfare) impacts to humans. 

Th e vulnerability of a GS system to these adverse impacts is a function of both the presence of the 

key receptors in the impact categories and the levels of exposure. A number of factors aff ect exposure, 

including but not limited to the concentration and volume of the release, the rate of release (i.e., 

slow vs. sudden), the proximity of the release to the receptor, and wind or wave dispersion. Impacts 

are also aff ected by whether the release is acute but limited (in time or spatial extent) or chronic. 

Unanticipated migration of CO2 from the injection zone will not necessarily result in leakage and/or 

subsequent adverse impacts. A qualitative discussion of the links between geologic attributes and 

potential impacts and receptors is provided in Chapter 5. 

Th e VEF includes decision-support fl owcharts for evaluating the impact categories, which can help 

identify key receptors and applicable qualitative exposure thresholds. Additionally, the VEF may be 

further developed to quantitatively account for exposure and threshold levels. 

Th is chapter focuses on the last component of the VEF conceptual model: potential impact 

categories and key receptors associated with each category (last column of Figure 3.1). Building 

on the GS footprint described in Chapter 3 the spatial area of evaluation is expanded to take the 

receptors that may be impacted into account. Th is chapter includes the following sections:

• Section 4.1 discusses potential human health and welfare receptors and impacts. 

• Section 4.2 presents potential impacts to the atmosphere.

• Section 4.3 covers potential ecosystems receptors and impacts.

• Section 4.4 presents potential groundwater and surface water receptors and impacts.

• Section 4.5 considers potential impacts to the geosphere.

• Section 4.6 examines how receptors may infl uence the area of evaluation.

• Section 4.7 summarizes the chapter.

4.1 Potential Human Health and Welfare Impacts

Adverse health eff ects caused by high levels of CO2 can range from minor, reversible eff ects to 

mortality, depending on the concentration of CO2 and the length of the exposure. Release of CO2 

may also adversely impact recreational and economic resources by restricting access or use or by 

changing the quantity and quality of the resource. 
4

vulnerability evaluation framework for geologic sequestration: impacts and receptors

28

Th e Human Health and Welfare Evaluation, depicted in Figure 4.1, outlines the components of 

human health and welfare impacts for the purposes of the VEF. Key receptors that could be adversely 

impacted in the event of unanticipated migration and leakage and the associated potential impacts 

are identifi ed. Suggestions are made for assessing and managing vulnerability of these receptors to 

adverse impacts. 

Adverse health eff ects from the release of high levels of CO2 (and other fl uids) could be experienced 

by the general human population and sensitive subpopulations, which, for the purposes of the 

VEF, are identifi ed as those subpopulations covered by Executive Orders. In addition, cultural, 

recreational, and economic resources (human welfare) could be negatively aff ected by CO2. Potential 

impacts to these key receptors are detailed below.

4.1.1 Human Populations

Th e vulnerability of a human population to the release of CO2 (and other fl uids) is aff ected by the 

population’s size and sensitivity to CO2 (and other fl uids that may leak from the GS system), and the 

proximity to and concentration of the release.

Carbon dioxide is a naturally occurring gas present in ambient air at a concentration of roughly 

0.04% [i.e., 380 parts per million (ppm)]. However, exposure at much higher concentrations can 

have a variety of impacts on human health10. Th ese impacts result when concentrations of CO2 or 

other constituents in the sequestered stream exceed toxicity threshold concentrations, and can range 

from mild discomfort to more permanent eff ects, including death (Benson et al., 2002). 

Figure 4.1. Human Health and Welfare Evaluation

10 Projected increases in CO2 concentrations from anthropogenic emissions range from 41 to 158 % above present levels or 535 

to 983 ppm by 2100 (Meehl et al., 2007). Such increases would result in atmospheric CO2 concentrations of 0.054 to 0.098 % by 

volume in 2100, which is well below published thresholds for adverse health effects.

Environmental 

Justice (EJ) 

populations

Tribal 

populations

Evaluate the potential to exceed relevant thresholds and consider developing monitoring and 

mitigation plans as appropriate

Children

Identify human health and welfare receptors

General human 

populations

Cultural/ 

recreational 

resources

Populations covered by executive orders Economic 

resources

Evaluate and 

address any 

disproportionate 

impacts

Consult and 

coordinate with 

tribal 

governments

Assess potential 

adverse impacts 

to children
4

vulnerability evaluation framework for geologic sequestration: impacts and receptors

29

Short-term (acute) exposure to CO2 levels at or below 3% causes only temporary and reversible 

health eff ects such as increased breathing rate, mild headache, and sweating. When CO2 levels 

exceed 3%, breathing rate increases substantially (e.g., from 11 liters per minute at 3% to 26 

liters per minute at 5%), hearing and vision may become impaired, and headaches can occur. 

Symptoms become more severe as CO2 levels exceed 5% (Benson et al., 2002; CEC, 2007). At high 

concentrations, CO2 can be fatal to humans following a relatively short exposure, because CO2 

displaces oxygen, causing suff ocation (Benson et al., 2002; Oldenburg et al., 2002a; Rice, 2003; 

Rice and Rhudy, 2004). When CO2 levels reach 10%, unconsciousness can occur after one to several 

minutes of exposure, and at levels exceeding 15%, unconsciousness occurs in less than one minute. 

When CO2 levels reach 30%, death occurs within a few minutes (Benson et al., 2002).

Measuring the eff ects of long-term (chronic) exposure to elevated CO2 levels is more complicated, 

but studies have found no evidence of any adverse health impacts from chronic exposure to levels 

below 1% (IPCC, 2005). Furthermore, there is evidence that exposure to air with up to a 3% 

CO2 concentration can result in physiological adaptation with no negative long-term health eff ects 

(Benson et al., 2002).

Small (low-fl ux), continuous leaks are less likely to result in adverse impacts unless there are 

topographic lows where CO2 could accumulate (Oldenburg and Unger, 2004; Shipton et al., 2005) 

and a lack of dispersion based on wind patterns (Heller, 2005). Anthropogenic features may also 

accumulate CO2, such as basements and other underground structures, including mines. Th ere is a 

greater potential for adverse aff ects to human health associated with higher fl ux leaks such as those 

observed at natural analogs like Mammoth Mountain in California, and Latera and Ciampino 

in Italy (Chiodini and Frondini, 2001; Eichhubl et al., 2005; NASCENT, 2005; Shipton et al., 

2005). However, Holloway et al. (2007) report that natural analogs underline the signifi cance of 

understanding the nature of a release and subsequent dispersion, rather than focusing exclusively on 

the volume of CO2 released; for example, the impacts of large sudden releases may be minimized 

by atmospheric dispersion. Furthermore, large sudden releases are anticipated to be unlikely if GS 

systems are appropriately characterized, designed, and monitored (Benson et al., 2002; IPCC, 2005; 

Lewicki, 2006; Holloway et al., 2007). Hence, events such as the 1986 fatal occurrence at Lake 

Nyos, Cameroon, in which 0.24 million metric tons of CO2 (Benson et al., 2002) were suddenly 

released, are highly unlikely to occur in association with GS. 

Impurities in the CO2 stream (introduced in Chapter 3) may independently pose a health risk to 

humans, but these health risks are not currently well characterized. For example, H2S is of particular 

signifi cance because of its toxicity. Th erefore, the release of stored CO2 with H2S to the atmosphere 

could have greater health and safety impacts than the release of pure CO2. 

To mitigate adverse impacts to the general population, it may be necessary to identify proximate 

populations and develop monitoring plans that reference appropriate exposure thresholds. 

Appropriate thresholds can be defi ned as the lowest concentration identifi ed by a regulatory agency 

as causing adverse health eff ects in humans. Regulatory agencies that prescribe short-term and 

chronic exposure thresholds for humans include the National Institute for Occupational Safety and 

Health and the Occupational Safety and Health Administration. 

 A Lake Nyos type event is even less likely to occur with GS, because it occurred as a result of a set of relatively unique conditions 

associated with the accumulation of CO2 at the bottom, and turnover of, a deep stratified lake, and there are very few deep, stratified 

lakes in the United States.
4

vulnerability evaluation framework for geologic sequestration: impacts and receptors

30

4.1.2 Populations Covered by Executive Orders

Additionally, it may be necessary to give special consideration to the potential for adverse impacts 

to populations covered by Executive Orders. Although these groups may experience impacts similar 

to general human populations, Executive Orders require proposed federal regulations and programs 

to specifi cally address potential impacts to environmental justice populations, children, and tribal 

governments; therefore they may need to be given particular attention.

• Environmental justice populations. Executive Order 12898, Federal Actions to Address 

Environmental Justice in Minority Populations and Low-Income Populations, establishes 

requirements to ensure enhanced protection from disproportionate impacts for minority 

and low-income populations. Th e order directs federal agencies to identify and address 

disproportionately high adverse human health or environmental eff ects of its activities on 

minority and low-income populations (Executive Order 12898, 1994). 

 If there is a potential for disproportionate impacts to environmental justice populations, it 

may be necessary to address these impacts by using targeted monitoring, altering the project 

design, or using other mitigation methods. 

• Tribal populations. Executive Order 13175, Consultation and Coordination with 

Indian Tribal Governments, establishes requirements to ensure enhanced protection from 

disproportionate impacts for American Indian tribal populations. Th is order requires 

that federal agencies consult and coordinate with tribal governments in formulating or 

implementing policies that have substantial direct eff ects on tribal governments (Executive 

Order 13175, 2000). Th us, it may be necessary to consult and coordinate with potentially 

aff ected tribal governments.

• Children. Children are more sensitive than adults to many substances. Th erefore, safety 

thresholds for areas occupied by children (e.g., schools, day-care centers) may need to be 

more stringent than those set for the general population. Executive Order 13045, Protection 

of Children from Environmental Health Risks and Safety Risks, establishes requirements to 

ensure enhanced protection from disproportionate impacts for children. Th e order requires 

federal agencies to prioritize identifi cation and assessment of environmental health and safety 

risks that may disproportionately aff ect children, and requires federal agencies to ensure that 

these risks are addressed (Executive Order 13045, 1997).

Other specifi c population subgroups that may need to be considered can be defi ned by conditions 

that lead to increased vulnerability to CO2 exposure. Th ese populations can include individuals with 

pulmonary disease, panic disorder patients, and patients with cerebral disease or trauma (Rice, 2003; 

Rice and Rhudy, 2004). For example, exposure to elevated levels of CO2 is an increased risk to those 

with head trauma injuries or who have certain cerebral diseases, because CO2 can inhibit blood 

clotting. As with the general human populations, monitoring and mitigation plans can be developed 

for these subpopulations that take into account relevant exposure thresholds.

4.1.3 Cultural and Recreational Resources

Unanticipated migration and leakage resulting from GS activities could adversely aff ect cultural and 

recreational resources by precluding human use (e.g., due to elevated CO2 levels) or by impacting 

the resource itself (e.g., chemical degradation of historical artifacts by reaction with CO2 or other 
4

vulnerability evaluation framework for geologic sequestration: impacts and receptors

31

stream constituents). Cultural sites include places of archaeological, historic, or natural signifi cance; 

American Indian resources; and other cultural resources, including cemeteries and paleontological 

resources (U.S. DOE, 2007c). In addition, certain recreational activities that are dependent on the 

health and welfare of ecological receptors (e.g., fi sh and game) could be aff ected if those ecological 

receptors are adversely impacted. If cultural or recreational resources are present, it may be necessary 

to develop a monitoring and mitigation plan, based on appropriate exposure thresholds.

4.1.4 Economic Resources (Surface and Subsurface)

Unanticipated migration and leakage resulting from GS activities has the potential to adversely 

aff ect certain surface or subsurface economic resources such as forestry, agriculture, mineral resources 

(mining), and oil and gas reservoirs. Impacts resulting from unanticipated migration, leakage, or 

pressure changes from GS might include restricted access to resources, restricted use of resources, or 

changes in the quantity and/or quality of resources. 

A GS project could also indirectly impact human welfare even in the absence of fl uid migration 

and/or release, if it precludes alternative land use or subsurface activities (e.g., resource extraction). 

Th is would include, for example, future restrictions on the use of saline formations as drinking water 

sources (if appropriate desalination technologies became available) or for the extraction of metal ions 

such as lithium. In addition, access to overlying or underlying oil and gas reservoirs and storage space 

for other substances (e.g., natural gas) could be limited because of GS operations. Further, changes 

in underground pressure caused by GS could increase seismic activity (discussed in Section 4.5) or 

change quantities and qualities of groundwater by altering some combination of fl ow rate and/or 

direction (discussed in Section 4.4).

It may be necessary to address these potential adverse impacts with, for example, targeted monitoring 

using appropriate thresholds and/or alteration of the GS project design. 

4.2 Potential Atmospheric Impacts

According to the National Oceanic and Atmospheric Administration, atmospheric concentrations 

of CO2 in 2006 totaled approximately 382 ppm (NOAA, 2007), and the current rate of increase 

in atmospheric CO2 concentrations is approximately 1.9 ppm per year (IPCC, 2007). Geologic 

sequestration is intended to mitigate local and global climate change impacts by providing longterm storage of CO2 emissions that would otherwise have contributed to global atmospheric CO2 

concentrations. 

In some cases, small releases of CO2 or other GHGs (e.g., CH4) as a result of releases from GS may 

not adversely impact local environmental receptors (e.g., ecological receptors, groundwater and 

surface water, the geosphere, and human health). However, releases can reduce the climate benefi ts 

of capturing CO2, thus decreasing the overall eff ectiveness of GS as a climate change mitigation 

strategy. 

An eff ective monitoring plan can help minimize adverse atmospheric impacts as a result of GS 

by ensuring that releases of CO2 or other GHGs are quickly identifi ed and remedied. (For more 

information on GS monitoring approaches, see Chapter 6.) In addition to identifying and remedying 

releases, it is important to record and account for them. Th e IPCC develops and publishes guidelines 

for estimating and inventorying national GHG emissions. Th e 2006 IPCC inventory guidelines 
4

vulnerability evaluation framework for geologic sequestration: impacts and receptors

32

See Faults Evaluation for 

vulnerability assessment 

approach and monitoring and 

mitigation considerations

See Confining System 

Evaluation for vulnerability 

assessment approach, and the 

CO2 Spatial Area of Evaluation, 

the Human Health and Welfare 

Evaluation and Ecosystems 

Evaluation for monitoring and 

mitigation considerations

See Wells Evaluation for 

vulnerability assessment 

approach and monitoring and 

mitigation considerations

Identify faults and fracture 

zones and evaluate their 

potential to conduct fluids

Evaluate the lateral extent and 

adequacy of the confining 

system

Identify and evaluate the 

integrity of wells

Identify and evaluate potential 

leakage pathways to the 

atmosphere

Figure 4.2. Atmosphere Evaluation

for the fi rst time included transport, injection, and GS of CO2 as sources of GHG emissions that 

should be considered in national inventories (IPCC, 2006). Th e EPA is currently evaluating how to 

implement the guidelines and accurately account for emissions associated with these activities.

Th e purpose of the Atmosphere Evaluation, depicted in Figure 4.2, is to identify fl uid-conducting 

pathways and identify approaches that can be used to evaluate their potential to conduct CO2 to the 

atmosphere. Monitoring strategies can be implemented to reduce the potential for CO2 to reach the 

atmosphere in the event of unanticipated migration. 

4.3 Potential Ecosystems Impacts 

Th e Ecological Receptors Evaluation, Figure 4.3, is used to frame the discussion of ecosystems 

impacts in the following sections. As shown in Figure 4.3, legislatively protected species and other 

sensitive species are key ecological receptors that could be adversely impacted by unanticipated 

migration, leakage, or pressure changes resulting from GS activities. Th ere are very few studies on 

the eff ects of CO2 at an ecosystem level (West et al., 2005), and adverse impacts to ecosystems from 

GS is currently an active area of research (West et al., 2006). Additional receptors not identifi ed in 

the VEF may also be impacted by GS. As the VEF is a iterative evaluation process, these may be 

included as new information comes to light.

4.3.1 Sensitive Species

Sensitive species are organisms that are especially vulnerable to high CO2 concentrations. Numerous 

ecological receptors may be sensitive to exposure to CO2, other stream constituents, brine, or other 

gases that may be released as a result of GS activities either because of their behavior (e.g., burrowing 

mammals, ground-nesting birds, reptiles that preferentially occupy low-lying areas, and soildwelling microorganisms) or because their physiology makes them unusually sensitive to increased 

concentrations. 
4

vulnerability evaluation framework for geologic sequestration: impacts and receptors

33

It is important to understand the condition (e.g., stressed by other factors or not) of these receptors 

as well as their potential ranges or stationary locations. Sensitive species can serve as sentinels for 

potential impacts to a wider array of organisms. If sensitive species are protected, then a reasonable 

assumption could be made that less sensitive species within that area are protected as well. Some 

species may also be sensitive to secondary eff ects of CO2, including increased acidity (in aquatic 

environments and in terrestrial soils), mobilization of metals caused by increased acidity, and loss of 

food resources and habitat in the case of vegetation loss caused by CO2 (e.g., fi sh and other species 

that are sensitive to increased acidity and aqueous metal concentrations, and mollusks and other 

species that are reliant on precipitation of carbonates).

It may be necessary to identify habitat range or location for each identifi ed sensitive ecological 

receptor, and to consider targeting for monitoring and mitigation plans. Sensitive species and 

potentially relevant thresholds are summarized below. 

Soil-dwelling animals and microbes. Soil CO2 concentration varies between 0.2% and 4% (Benson 

et al., 2002) and is a function of depth, water content, soil type, and time of year (Bouma et al., 

1997). Soil CO2 concentrations between 20% and 30% are thought to be suffi cient to signifi cantly 

alter the dynamics of ecosystems (Benson et al., 2002). Th ere is a relatively wide range, and some 

overlap, in the CO2 concentrations causing diff erent adverse eff ects in burrowing invertebrates such 

as worms and insects. For example, depending on the specifi c species, behavioral changes may be 

exhibited at CO2 concentrations between 2% and 39%, paralysis at concentrations between 10% 

and 59%, and death at concentrations between 11% and 50% (Sustr and Simek, 1996; Benson et 

al., 2002). Soil-dwelling animals may begin experiencing negative physiological eff ects at 2% CO2, 

and concentrations of approximately 15% CO2 can be lethal (Benson et al., 2002). In another 

study examining CO2 tolerance of dung insects, 80% CO2 was the threshold for paralysis (Holter, 

1994). Microbes have a wide range of sensitivity to CO2. For fungi, signifi cant inhibitory eff ects are 

observed at 30% CO2, with concentrations of 40% CO2 generally lethal (Benson et al., 2002).

Evaluate the potential to exceed relevant thresholds and consider developing monitoring and mitigation 

plans as appropriate

Identify species and their habitat range or 

location

May be necessary to demonstrate that the 

endangered species and their habitat will not 

be adversely impacted

State or federally listed threatened or 

endangered species or species of special 

concern

Identify habitat range or location

Other sensitive species

Identify ecological receptors

Figure 4.3. Ecological Receptors Evaluation
4

vulnerability evaluation framework for geologic sequestration: impacts and receptors

34

Plants. Elevated atmospheric CO2 can enhance plant photosynthesis if adequate amounts of 

other nutrients are available. However, because plant roots take in O2 and expel CO2, elevated 

CO2 in the soil can inhibit root respiration. Th e specifi c physiological limits for plant roots (the 

time and concentration required for CO2 toxicity) are unclear (USGS, 2001). However, plant 

growth at a natural CO2 spring site was accelerated at concentrations up to 10%, but inhibited at 

concentrations between 30% and 55% (Vodnik et al., 2006). At Mammoth Mountain, California, 

a soil concentration of 30% caused death of all trees, regardless of age or species (Saripalli et al., 

2002). Evaluations at landfi ll sites show that soil CO2 concentrations as low as 10% may inhibit root 

function, but that some species can tolerate concentrations up to 20% CO2 without detrimental 

eff ects on root function (Flower et al., 1981; Ehrlich, 2002). In addition to inhibition of root 

respiration, increased CO2 concentrations may increase soil acidity. Increased soil acidity can make 

potentially toxic metals more bioavailable, which can aff ect the dynamics of soil ecosystems (McGee 

and Gerlach, 1998; Saripalli et al., 2002).

Surface-dwelling animals. Few data are available indicating the toxic thresholds of CO2 in surfacedwelling animals, except for humans. Research suggests that birds may begin exhibiting behavioral 

changes or paralysis when CO2 concentrations reach 40% (U.S. Patent 5435776), and reptiles (and 

possibly amphibians) have a higher CO2 tolerance than other ground-dwelling vertebrates (Benson 

et al., 2002). Additionally, thresholds for human exposure to CO2 may be appropriate proxies for 

surface-dwelling animals (Benson et al., 2002; IPCC, 2005). As noted above, wildlife that rely on 

vegetation for food, cover, or nesting/breeding habitat can suff er secondary, habitat-related eff ects if 

plants are aff ected adversely.

Aquatic organisms. Adverse impacts to aquatic organisms can result if CO2 leaks into overlying 

water bodies, including lakes, streams, and oceans. High concentrations of CO2 reduce the amount 

of oxygen reaching the blood in aquatic organisms with gills, causing suff ocation (Maina, 1998). 

Fish, for example, are likely to experience negative eff ects with a three-fold increase of ambient 

aquatic CO2 concentrations. Aquatic organisms and ecosystems are also vulnerable to changes in 

pH resulting from increased CO2. In general, a pH level of less than 5 or greater than 10 is lethal for 

most fi sh; low pH can cause acidifi cation of body tissues (Benson et al., 2002; Turley et al., 2004, 

2006; Miles et al., 2007). 

Other invertebrates may be sensitive to increased CO2 as well. Miles et al. (2007) examined CO2 

tolerance of the sea urchin (Psammechinus miliaris), an ecologically important organism with a 

sensitive pH balance, and found that a pH of 6.16 was lethal to 100% of the test organisms after 7 

days, and a pH below 7.5 was severely detrimental to the test organisms. Negative eff ects included 

hypercapnia (excess CO2 in the blood) due to elevated CO2 concentrations and decreased pH in 

coleomic fl uid (Miles et al., 2007). Turley et al. (2004) reported that in saltwater, a pH ranging from 

5 to 6.7 was lethal for zooplankton in 72 hours, and a pH ranging from 4.8 to 6.2 was lethal in 24 

hours. However, they also found that a pH ranging from 5.6 to 7.8 was not lethal after 96 hours. 

An increase in aquatic CO2 concentration is unlikely to signifi cantly reduce aquatic photosynthetic 

productivity in phytoplankton that have CO2-concentrating mechanisms (Turley et al., 2004). 

However, some organisms’ photosynthetic rate is more sensitive to CO2 concentration, meaning that 

they may be more susceptible to changes in CO2 (Turley et al., 2006). 

Calcifying aquatic organisms. Calcifying organisms are dependent on dissolved carbonate in the 

water to produce their protective shells. As CO2 concentrations in water increase, pH decreases (a 
4

vulnerability evaluation framework for geologic sequestration: impacts and receptors

35

process sometimes referred to as ocean acidifi cation) and carbonate becomes less available in the 

aquatic environment. Organisms such as corals may be severely aff ected by acidic conditions if they 

are either unable to form shells or if the shells are dissolved by the acidic water. Turley et al. (2006) 

found reduced calcifi cation in cultures of coccolithophore species that were in water in contact with 

atmospheric concentrations of 750 ppm CO2. Spicer et al. (2007) examined the eff ects of elevated 

CO2 and low pH on the velvet swimming crab (Necora puber) and found that after 16 days at a pH 

level of 6.74, the animal experienced hypercapnia. To compensate, the test animals dissolved their 

exoskeletons to make bicarbonate available in their blood (Spicer et al., 2007).

Deep geologic ecosystems. Deep geologic ecosystems support signifi cant communities of microbial 

life but are poorly characterized. Microbial life is ubiquitous and extends several miles into the 

Earth’s crust (as deep as 4,000 meters), and in all of the types of locations being considered for GS 

(IPCC, 2005). At extreme depths, microbes are adapted to very high pressures and temperatures and 

can catalyze reactions that involve compounds such as hydrogen sulfi de (H2S), sulfate (SO), nitrate 

(NO), iron (Fe), and CO2 to derive energy (West and Chilton, 1997; Ehrlich, 2002). Th e eff ect 

of a change in CO2 concentration at extreme depths is unknown; deep-earth organisms may not 

respond to CO2 as organisms living nearer the atmosphere do, unless the organism uses CO2 in its 

metabolism. Relatively little is known about the importance or unimportance of such organisms in 

ecosystems and carbon cycles. Further research is needed to address this uncertainty. 

Microbial activity could also potentially impact the GS system (Quintessa, 2004). For example, 

microbes could catalyze the precipitation of minerals, thus decreasing storage capacity. Alternatively, 

they could produce organic acids, thereby enhancing the corrosion of well seals and the dissolution 

of geologic materials.

4.3.2 Legislatively Protected Species

State and federal government agencies use legislation to identify certain species populations as 

threatened or endangered (i.e., as defi ned in the Endangered Species Act of 1973). Impacts similar 

to those listed in the previous section are possible for these species as well. Because of their increased 

sensitivity, it is important to understand the legal status of these species (e.g., endangered or 

threatened) as well as their potential ranges or stationary locations. Further, it may be necessary to 

demonstrate that an endangered species and its habitat will not be adversely impacted by evaluating 

the potential for adverse impacts and targeting the species for appropriate monitoring based on 

current and known thresholds.

4.4 Potential Groundwater and Surface Water Impacts

Geologic sequestration could potentially impact groundwater and surface water. EPA is mandated to 

protect the nation’s waters (both USDWs and surface waters) under the SDWA and the Clean Water 

Act. USDWs could potentially be more susceptible to adverse impacts than surface waters because 

of their closer proximity to the injection zone. Although this discussion focuses on groundwater 

impacts, many of the concepts can be also be applied to evaluating surface waters. Th e evaluation 

depicted in Figure 4.4 identifi es key receptors, including protected and/or sensitive water bodies, 

water quality, and regional groundwater fl ow, and identifi es appropriate monitoring and mitigation 

strategies.
4

vulnerability evaluation framework for geologic sequestration: impacts and receptors

36

Groundwater and surface water may be vulnerable to adverse impacts associated with unanticipated 

migration of CO2 (and other fl uids) and pressure changes, including the water quality and 

groundwater fl ow eff ects described below. 

4.4.1 Water Quality

Geologic sequestration could impact groundwater if CO2 escapes the GS system or if brine or other 

fl uids are displaced as a result of pressure changes into overlying or underlying aquifers. Impacts 

include changes such as increased salinity, increased acidity (reduced pH), and mobilization of 

metals or other impurities. Th e eff ect of GS on a water body may be indicated by a change in specifi c 

relevant water quality parameters. Relevant water quality parameters that can be used to evaluate 

groundwater include: 

• Total dissolved solids (TDS). Migration of brine into overlying aquifers as a result of CO2 

injection could potentially endanger human health and the environment. Th ough there 

is not an EPA primary drinking water standard for TDS, water with TDS greater than 

500 milligrams per liter (mg/L) is not recommended for human consumption. EPA also 

considers aquifers with TDS below 10,000 mg/L to be underground sources of drinking 

water, therefore migration of brine into these sources may have potential implications if this 

threshold is exceeded. 

• Buff ering capacity. Systems made of geologic materials with lower buff ering capacities (e.g., 

sandstones as opposed to limestones) may be more susceptible to acidifi cation in the event of 

migration of CO2. 

Total 

dissolved 

solids (TDS)

Buffering 

capacity

Regional groundwater 

flow

Evaluate effects of pressure changes, and 

consider developing 

monitoring and mitigation 

plans based on sitespecific thresholds

Mineralogy Sorbed 

contaminants

Yes

Presence

of protected or 

sensitive 

drinking water 

supplies?

Identify groundwater and surface water

No

Consider targeted monitoring 

and mitigation plans

Microbial 

populations

Evaluate relevant water quality and quantity parameters

Evaluate the potential to exceed thresholds, such as drinking water 

standards, in the event of CO2 and/or brine migration and leakage, and 

consider developing monitoring and mitigation plans

Figure 4.4. Groundwater and Surface Water Evaluation
4

vulnerability evaluation framework for geologic sequestration: impacts and receptors

37

• Mineralogy. Migration of CO2 into overlying aquifers could cause acidifi cation, which 

in turn may cause dissolution of metal-containing minerals and desorption of metals and 

organic species from the aquifer matrix (Jaff e and Wang, 2003; Kolak and Burruss, 2003, 

2004; Wang and Jaff e, 2004). If metal and metalloid-bearing mineral phases (e.g., metal 

oxides, sulfi des, carbonates) are determined to be present, geochemical modeling may be 

necessary to evaluate whether metal aqueous concentrations will exceed drinking water 

standards in the event of migration into an overlying aquifer. Th e mobilization of metals and 

other contaminants within the injection zone could also potentially impact drinking water 

sources, if the metal-containing fl uids leaked into an overlying aquifer, or could preclude the 

use of the injection zone as a future drinking water source without treatment.

• Sorbed contaminants. Sorbed metals, organic contaminants, and gases could be released 

under acidic conditions created by CO2 and other geochemical processes (e.g. competitive 

desorption). If impurities (e.g., organics, metals, gases) are present, geochemical modeling 

may be necessary to evaluate if aqueous concentrations will exceed drinking water standards 

in the event of migration of CO2 and other fl uids into overlying aquifers. 

• Microbial populations. As noted in Section 4.3.1, microbial populations may catalyze 

geochemical reactions. As such, their infl uence may need to be considered when determining 

aqueous concentrations of metals and other impurities. 

In addition, impurities in the CO2 stream, such as H2S, SO2, and other sulfur- and nitrogencontaining species, can increase the acidifi cation of groundwater compared to pure CO2.Th is could 

lead to increased mobilization of trace amounts of hazardous metals if a CO2 stream leak comes into 

contact with groundwater (IPCC, 2005). However, the concentrations of sulfur and nitrogen bearing 

gaseous constituents in the CO2 stream are likely to be low, and hence their impacts on injection 

zone rocks and groundwater may be relatively minor (Apps, 2006). 

Th e migration of CO2 through overlying soils may also cause mobilization of contaminants in 

the vadose zone. For example, the FutureGen risk assessment identifi es radon mobilization as 

a potential secondary eff ect of migration of CO2 and other impurities through soils. Radon is 

a naturally occurring element in some soils and might be mobilized if CO2 and other potential 

impurities diff use through soils at high enough rates (U.S. DOE, 2007c). However, this impact may 

be minimal because such elements are likely to be present only at trace concentration levels (Apps, 

2006). Consideration may also need to be given to the possible entrainment of organic species by 

migrating CO2 in the case of injection into oil and gas fi elds. Th ough Apps (2006) reports that 

concentrations of such organic species is likely to be low.

For surface water (including oceans), biotic water quality criteria may also need to be considered. For 

example, fi sh and other species may be sensitive to increased concentrations of metals such as copper, 

cadmium, and zinc, and increased acidic conditions may inhibit precipitation of calcareous shells in 

surface water and oceans.

Geologic sequestration that involves injection into subseabed geologic formations raises the potential 

for water quality impacts in the ocean. If suffi cient volumes of leaked CO2 were to come into contact 

with sea water, it could locally increase the acidity of the water and potentially induce a secondary set 

of water quality impacts (e.g., changes in concentrations of metals through interaction with surface 

sediments). Impurities and substances added to the CO2 stream (e.g. to enable or improve capture) 

could also impact marine water quality. Finally, the eff ects of supercritical CO2 on marine water 
4

vulnerability evaluation framework for geologic sequestration: impacts and receptors

38

quality parameters may need to be considered under certain circumstances. At great enough depths, 

the CO2 may be more dense than the surrounding seawater, and hence may tend to sink, rather than 

being dispersed through buoyancy–driven upward migration and dispersion.

4.4.2 Regional Groundwater Flow

Groundwater fl ow directions, water table levels, and the distribution of groundwater (areas where 

groundwater is gained and lost) could all be aff ected in shallower aquifers sensitive to pressure 

changes caused by CO2 injection (Nicot et al., 2006; Tsang et al., 2007). Th is could infl uence the 

quantity of water available for groundwater-based municipal water supplies and cause interferences 

with injection wells. Further, a rise in the water table level under a stream or lake fed by groundwater 

could result in elevated water levels, changes in fl ow rates, and changes in the geometry of the water 

bodies. Finally, there may be eff ects on intertidal zones, where displaced brine may ultimately be 

discharged into oceans.

Groundwater fl ow direction, fl ow rate, and quantity are metrics for this receptor in the VEF, and 

the determination of elevated vulnerability thresholds for these metrics is likely to be site specifi c. 

Physical groundwater and surface water modeling is likely to be necessary to evaluate eff ects of 

pressure changes.

4.4.3 Protected and Sensitive Drinking Water Supplies 

Protected and sensitive drinking water supplies may be particularly vulnerable to adverse impacts 

associated with migration of CO2 and other fl uids and pressure changes, including the water quality 

and groundwater fl ow eff ects described above. Because of their increased vulnerability, it is important 

to identify the presence or absence of such water supplies, and it may be necessary to consider 

targeted monitoring and the development of mitigation plans if these water bodies are present. 

4.5 Potential Geosphere Impacts

Changes in subsurface pressure from GS could have direct impacts on the geosphere. Subsurface 

pressure changes that cause an exceedence of the subsurface geologic formation’s geomechanical 

strength could cause fracturing or reopening of faults and fracture zones (Quintessa, 2004; IPCC, 

2005). Th is in turn could cause unanticipated movement of CO2 and other fl uids and increase the 

potential for the impacts discussed above. Other potential impacts include induced seismic activity 

such as earthquakes in the extreme case (Healey et al., 1968) and land deformation through uplift 

(Quintessa, 2004; Birkholzer et al, 2007).

4.6 Spatial Area of Evaluation: Infl uence of Receptors

Chapter 3 introduced the GS footprint, the geographical area that may be impacted by unanticipated 

migration, leakage, and pressure changes, and is the focus of site characterization and monitoring 

(column two of Figure 3.1). Under certain circumstances, it may be necessary to extend site 

characterization and monitoring beyond the GS footprint. Receptors outside the footprint might 

be adversely impacted if leakage occurs near or at the footprint boundary. For example, it might be 

appropriate to extend monitoring into the habitat of an endangered species that straddles the boundary 

of the CO2 plume. In addition, concentrating features such as topographic lows, wind patterns, or 
4

vulnerability evaluation framework for geologic sequestration: impacts and receptors

39

ocean current patterns could transport CO2 beyond the boundary of the GS footprint (Bogen et al., 

2006; U.S. DOE, 2007c). Furthermore, site characterization and monitoring for pressure eff ects, 

including displacement of brine into overlying aquifers, are likely to extend over an even larger area. 

It is also important to note that areas of evaluation are not expected to be static entities. As discussed 

in Chapter 3, the CO2 plume and pressure front will grow and change shape over the lifetime of a 

project, as will the corresponding spatial area of evaluation.

Th is section introduces two evaluation processes for spatial area evaluation processes. Th e fi rst focuses 

on characterization of the area that may be aff ected by the injected CO2, and the second focuses 

on the area that could be aff ected by pressure changes. Separate CO2 and pressure spatial areas of 

evaluation are delineated because of the anticipated much larger areal extent of pressure changes 

(particularly for deep saline formations). Additionally, a smaller set of potential impacts will be 

associated with pressure changes. Th e evaluation processes suggest approaches on how to delineate 

the geographical extent of the area aff ected by CO2 and pressure changes, and also recommend 

monitoring and mitigation plans. It is important to note that the geographical area aff ected by 

pressure may be evaluated for pressure-driven displacement of brine into overlying aquifers, as 

described below, but it will not need to be evaluated for direct adverse impacts of CO2 (and other 

impurities) on human health and welfare and ecological receptors.

4.6.1 Carbon Dioxide Spatial Area of Evaluation

Th e fl owchart depicted in Figure 4.5 represents an approach for delineating and examining the CO2 

spatial area of evaluation.

Spatial Area of Evaluation Delineation

Th e lateral extent of the CO2 spatial area of evaluation can be delineated by fi rst using the areal 

extent of the CO2 plume as the base and then expanding on that base to include the spatial extent 

over which adverse impacts to physical features and receptors within a determined distance of its 

perimeter may occur. Physical features can include CO2 -concentrating features such as topographic 

lows and dispersing features such as atmospheric conditions and ocean current patterns. As 

discussed in Sections 4.1 through 4.5, potentially aff ected receptors can include human populations, 

populations covered by Executive Orders, cultural and recreational resources, economic resources 

(surface and subsurface), legislatively protected and other sensitive species, and groundwater and 

surface water sources. 

Identifi cation of Receptors and Development of Monitoring and Mitigation Plans 

It can be useful to specifi cally locate potentially aff ected receptors within the expanded CO2 spatial 

area of evaluation and identify their vulnerability to adverse impacts, taking into account CO2-

concentrating physical features and geologic attributes. Four potential scenarios highlighted in 

Figure 4.5 suggest diff erent types of monitoring and mitigation plans. For example, if the location 

of receptors does not coincide with CO2-concentrating physical features or with elevated geologic 

attribute vulnerabilities, then generic monitoring may be implemented. However, if the location of 

receptors coincides with CO2-concentrating physical features, a receptor-specifi c monitoring and 

mitigation plan may be developed that takes those features into account. Th e scenarios highlight 

that the overall vulnerability associated with a GS system is determined based on a combination of 

multiple factors. Th is concept is discussed further in Chapter 5. 
4

vulnerability evaluation framework for geologic sequestration: impacts and receptors

40

Develop generic 

monitoring plan

Develop 

receptor-specific 

monitoring and 

mitigation plan that 

takes into account 

CO2- concentrating 

features

Develop 

receptor-specific 

monitoring and 

mitigation plan that 

takes into account 

geologic attribute 

vulnerability

Vulnerability to adverse 

impacts may be 

elevated

Scenario 2: Location 

of receptor(s) 

coincides with 

CO2-concentrating 

physical features that 

may concentrate CO2

in the event of leakage

Identify

and locate

receptors

None 

present

Evaluate the level of vulnerability to receptors

Evaluation complete

Identify physical features and receptors along or near 

the perimeter of the plume

Scenario 1: Location of 

receptor(s) does not 

coincide with 

CO2-concentrating 

physical features, or 

with elevated geologic 

attribute vulnerabilities

Scenario 3: Receptor 

location coincides with 

elevated vulnerability 

based on geologic 

attribute evaluation

Scenario 4: Receptor 

location coincides with 

CO2-concentrating 

physical features and 

elevated geologic 

vulnerability

If any of these physical features and receptors are present, the 

CO2 spatial area of evaluation may need to be expanded to 

include the area over which adverse impacts could occur

Delineate the lateral extent of the CO2 spatial area of 

evaluation, using the CO2 plume as the base

Figure 4.5. Carbon Dioxide Spatial Area of Evaluation

4.6.2 Pressure Spatial Area of Evaluation

Th e fl owchart depicted in Figure 4.6 provides an approach for delineating and examining the 

pressure spatial area of evaluation.

Spatial Area of Evaluation Delineation

Th e lateral extent of the pressure spatial area of evaluation can be delineated using the GS footprint 

as the base. In this case, the boundary of the GS footprint is defi ned by the pressure change that 

is signifi cant enough to cause adverse impacts to overlying receptors (e.g., fl uid displacement into 
4

vulnerability evaluation framework for geologic sequestration: impacts and receptors

41

an overlying aquifer). Th e perimeter can then be expanded to include the spatial extent over which 

receptors within a determined distance of the perimeter of the GS footprint could be aff ected. As 

noted in the previous section, receptors can include human populations, populations covered by 

Executive Orders, cultural/recreational resources, economic resources (surface and subsurface), 

legislatively protected and other sensitive ecological species, and groundwater and surface water 

sources.

Identifi cation of Receptors and Development of Monitoring and Mitigation Plans 

Potentially aff ected receptors within the expanded pressure spatial area of evaluation can then be 

considered. It is important to evaluate the potential impacts to these receptors, taking into account 

geologic attributes, and then develop monitoring and mitigation plans for receptors susceptible to 

adverse impacts associated with pressure changes.

Figure 4.6. Pressure Spatial Area of Evaluation

Develop monitoring and mitigation plan for receptors susceptible to 

adverse impacts associated with pressure changes

Identify receptors along or near the perimeter of the footprint

Delineate the lateral extent of the pressure SAE, using the GS 

footprint as the base

Identify 

and locate 

receptors

Evaluation complete

None present

Evaluate the level of vulnerability to receptors within the pressure 

SAE taking into consideration geologic attributes

If present, the pressure SAE may need to be expanded to include the 

spatial extent over which adverse impacts could occur
4

vulnerability evaluation framework for geologic sequestration: impacts and receptors

42

4.7 Chapter Summary

Th is chapter examined categories of potential adverse impacts resulting from unanticipated 

migration, leakage, and pressure changes, and associated receptors. Impact categories include human 

health and welfare, atmosphere, ecosystems, groundwater and surface water, and the geosphere. 

Th e chapter then discussed the delineation of spatial areas of evaluation for site characterization 

and monitoring, taking into account the receptors discussed in the previous sections. Chapter 5 

incorporates elements from Chapters 3 and 4 into a holistic approach for vulnerability evaluation 

and highlights key geologic attributes that should be considered carefully when characterizing a 

potential GS site. 
5

43

CHAPTER 5

VULNERABILITY EVALUATION FRAMEWORK FOR 

GEOLOGIC SEQUESTRATION: KEY CONSIDERATIONS

Adopting a holistic approach to vulnerability assessment is important because the overall 

vulnerability of a GS site is a combined function of all of the identifi ed geologic attributes and 

receptors. In most instances, evaluating a single attribute will not provide suffi cient information 

to characterize overall vulnerability. Rather, the entire site needs to be evaluated as a whole. Th e 

individual and combined infl uence of attributes on vulnerability as well as the presence, proximity, 

and sensitivity of receptors is likely to change over time. Generally, vulnerability and uncertainty will 

decrease after injection ceases and as monitoring data are incorporated into the assessment. 

Key attributes that have been identifi ed as particularly important when evaluating the potential 

vulnerability to unanticipated migration and leakage from a GS system include wells and faults/

fracture zones, as well as pressure changes that may induce fracturing or reactivate faults. However, 

even for these key attributes, it is the interplay of multiple characteristics that will determine the 

level of vulnerability that may be associated with them; the simple presence of a well or a fault does 

not automatically indicate elevated vulnerability. Th e prioritization of vulnerabilities associated with 

receptors, geologic attributes and their characteristics is an important, but challenging endeavor 

that is beyond the current scope of the VEF. However, vulnerabilities can nevertheless be managed 

by carefully evaluating and managing these key attributes, thus reducing the likelihood of adverse 

impacts. 

Chapters 3 and 4 identifi ed and described the geologic attributes that could contribute to 

vulnerability and the receptors that may be adversely impacted. Th ey also presented evaluation 

processes and suggestions as to how particular vulnerabilities may be addressed. Th is chapter 

examines key considerations when examining the vulnerabilities of a GS site as a whole and contains 

the following sections:

• Section 5.1 discusses a holistic approach to vulnerability evaluation in terms of GS system 

characteristics and temporal scales.

• Section 5.2 provides additional detail on attributes that are key to determining GS site 

vulnerability, specifi cally wells, faults, and pressure changes. Evaluation fl owcharts for these 

attributes are introduced as well as suggestions for reducing vulnerabilities. 

• Section 5.3 provides a summary of the chapter.

5.1 Holistic Approach to Evaluating Vulnerability

Th e overall vulnerability to adverse impacts is determined by the combination of geologic attributes 

and receptors that are associated with a GS site. Further, since storage of CO2 is intended for a long 

time period, it is important to examine how the infl uence of diff erent attributes may change over 

time.

5.1.1 Interplay of Geologic Attributes and Receptors

Th e overall vulnerability of a GS site to adverse impacts is a function of the vulnerability of a system 

to unanticipated migration, leakage, or pressure changes in combination with the vulnerability of 
5

vulnerability evaluation framework for geologic sequestration: key considerations

44 Figure 5.1. Risk Profi le for CO2 Storage. Source: Benson, 2007.

specifi c receptors of concern. Th e highest level of vulnerability emerges when there is a high potential 

that there are unanticipated migration or leakage pathways in the presence of important vulnerable 

receptors, such as a sensitive drinking water supply. If the vulnerability to unanticipated migration 

or leakage is low (for example, the confi ning system is laterally extensive, has a very low permeability, 

is not known to be perforated by wells or faults, and has a low vulnerability to induced fracturing or 

other pressure-related eff ects) then the overall vulnerability to adverse impacts may be relatively low, 

despite the presence of sensitive receptors such as drinking water supply. Th erefore, an evaluation of 

the combined presence and condition of GS attributes together with the presence and sensitivity of 

receptors is required for a clear understanding of overall vulnerabilities. 

5.1.2 Evaluation of Vulnerability at Diff erent Temporal Scales

Th e importance of diff erent attributes for aff ecting the probability and severity of adverse impacts is 

likely to change over the lifetime of a GS project. Although a quantitative discussion of the relative 

contribution of diff erent attributes and how this may change over the duration of a project is not 

possible at this time, some qualitative comments can be made. For example, during the injection 

phase, injectivity and geomechanical processes are anticipated to be key attributes in the evaluation 

of vulnerability to adverse impacts. Th e mechanical integrity of the injection wells is also very 

important, particularly the ability to withstand applied pressures. Th e vulnerability to induced 

fracturing is expected to be much higher during injection than post-injection. Post-injection, wells 

might continue to represent potential unanticipated migration or leakage pathways. However, the 

concern may be more for slow/small leaks that develop through slow geochemical degradation 

pathways (thus primarily aff ecting atmospheric vulnerability) as opposed to high-intensity pressure 

blow-outs that could also aff ect human health and welfare and environmental receptors. 

In general, the overall vulnerability to adverse impacts is expected to decline over time, as illustrated 

in Figure 5.1. Th is is based on a combination of factors, including the greater permanence of 

secondary trapping mechanisms such as dissolution, which also decreases buoyancy (see Chapter 2), 
5

vulnerability evaluation framework for geologic sequestration: key considerations

45

pressure recovery once injection stops, and improved characterization and modeling of GS system 

attributes over time. 

In some cases, however, vulnerabilities associated with certain aspects of a GS system may not 

decrease appreciably over time. For example, in open systems where the injected CO2 and associated 

induced pressure changes do not reach the lateral boundaries of the injection zone formation(s), 

pressure is expected to drop signifi cantly when injection ceases (Pawar et al, 2006; Birkholzer et al., 

2007), thus decreasing the vulnerabilities to induced seismic activity and faulting and fracturing. 

However, in closed systems where pressure remains elevated at the lateral boundaries of the system 

(for example in EOR geologic settings, where boundaries may be defi ned by geologic features such 

as an anticline or sealed fault), it may take much longer for pressure to dissipate. Th is can result 

in elevated vulnerability for a prolonged period of time after injection stops. It should be noted 

that this example re-emphasizes the necessity of evaluating the vulnerabilities associated with a GS 

system as a whole. Although pressure may remain elevated for a longer period of time in the closed 

system, this does not necessarily mean that the system has an overall elevated level of vulnerability. 

For example, EOR settings have withstood elevated pressure for geologic timescales in the past, thus 

demonstrating an eff ective confi ning system with low vulnerability to unanticipated migration or 

leakage. 

5.2 Key Attributes in Evaluating GS System Vulnerability

Although the overall vulnerability of a GS site is determined through a holistic evaluation of the 

interactions among all elements of a GS site, certain key attributes have been identifi ed as central to 

evaluating the vulnerability associated with a GS system. It is unlikely that any one of these single 

attributes alone will determine the overall vulnerability of a GS site. However, there are some basic 

screening considerations:

• Is the physical capacity of the injection zone suffi cient to store the total CO2 volume?

• Is the confi ning system present and of suffi cient lateral extent?

• Is the injectivity of the injection zone suffi cient?

• Is tectonic activity (earthquakes, volcanoes, hotspots, etc.) not a concern?

• Are wells, faults, fracture zones not a concern?

If one or more of these conditions is not met, there may be elevated vulnerability to unanticipated 

migration, leakage, or adverse pressure changes that render a GS system less suitable. As described in 

the evaluation processes presented in Chapter 3, the suitability of the GS system can be improved, 

for example, by altering operational plans and injecting less, through targeted monitoring and 

mitigation, and/or corrective actions at vulnerable wells. Th e discussion below focuses on wells 

and faults because they have been identifi ed as being the most likely attributes to contribute to 

unanticipated fl uid migration out of the injection zone. However, the presence of these attributes 

alone may not indicate elevated vulnerability; and it is important to examine them thoroughly to 

ascertain the level of vulnerability associated with a GS system. 
5

vulnerability evaluation framework for geologic sequestration: key considerations

46

5.2.1 Wells as Fluid-Conducting Pathways

Wells and other artifi cial penetrations such as boreholes have been identifi ed as one of the most 

probable conduits for the escape of CO2 and other fl uids from the injection zone (Gasda et al., 

2004; Benson, 2005; IPCC, 2005; Carey et al., 2007). Industrial analogs indicate that while gases 

such as CH4 can be stored eff ectively in the subsurface, there are examples of unanticipated gas 

leakage through poorly completed or improperly plugged and abandoned wells (Gurevich et al., 

1993; Lippmann and Benson, 2003; Perry, 2005). Hence, if not properly sealed and plugged, wells 

can provide an open conduit from depth to the surface. Even wells that are properly sealed may have 

fl uid-conducting pathways along the outside of the casing, where a complex environment involving 

well cements, drilling muds, and possibly damaged rock zones can provide opportunities for fl uid 

fl ow (Gasda et al., 2004). Further, the acid generated when CO2 contacts water could degrade well 

construction materials, possibly creating pathways for fl uid fl ow (Scherer et al., 2005). Th e relative 

depth of wells is also an important consideration when evaluating their potential to serve as fl uid 

pathways. Only wells that penetrate the injection zone can serve as direct leakage pathways to the 

surface. However, well characterization should include all wells within the area anticipated to be 

aff ected by CO2 injection, because shallower wells could be connected to faults or other features that 

do penetrate the injection zone. Th us, the combination of wells and faults can serve as a pathway for 

unanticipated migration and leakage. 

Experience from industrial processes underlines the importance of evaluating the potential for active 

and abandoned wells to serve as fl uid pathways and conducting a detailed GS site characterization 

before injection (Cawley et al., 2005). As illustrated in Figure 5.2, evaluating the potential for 

wells to act as conduits for CO2 and other fl uids involves considering multiple factors, including 

determining whether wells are present, measuring their depth relative to injection zone, and 

evaluating the integrity of the well construction materials and seal.

While operational factors are not the focus of the VEF, injection and monitoring wells may also have 

implications for the creation of fl uid-conducting pathways. Principal design considerations for CO2 

injection wells include pressure, production and injection rates, and corrosion. Th ere is substantial 

knowledge about the design of CO2 injection wells developed for EOR operations (IPCC, 2005). 

Equipping injection wells with packers can help isolate pressure eff ects to the injection interval. 

Th e slotted length of wells, use of horizontal wells (slotted over much longer reaches than vertical 

wells), and the number of injection wells used for injecting CO2 are all operational factors that 

may minimize pressure. Using corrosion-resistant well construction materials can help maintain 

the integrity of the well. Numerous monitoring and mitigation techniques also exist and could be 

applied to GS systems (Jarell et al., 2002; Skinner, 2003).

Identifying and evaluating active and abandoned wells may be particularly challenging in certain 

geologic settings, such as depleted oil and gas fi elds, where there may be numerous active and 

abandoned production and exploration wells of diff erent ages, depths, and general condition. 

Industry and state records exist that can help locate such wells, and determine their depth and 

mechanisms of sealing and plugging. However, the completeness of such records, particularly for 

historical (pre-1950s) wells, may be limited, and may need to be supplemented by fi eld verifi cation. 

Nevertheless, site characterization that includes the considerations outlined in Figure 5.2 may reduce 

uncertainties and manage vulnerability by helping identify those conditions involving wells that 

may be of concern and by providing approaches to address them. For example, a well that penetrates 

the injection zone and is not sealed properly, or for which records do not exist on abandonment 
5

vulnerability evaluation framework for geologic sequestration: key considerations

47

No Assessment complete. 

Low vulnerability.

Yes

Assessment complete. 

Low vulnerability.

Yes Assessment complete. 

Low vulnerability.

Well

penetrates

the IZ?

No Assessment complete. 

Low vulnerability.

Yes Yes

No

No

Well 

integrity is 

adequate?

Adequate 

records of 

well integrity 

exist?

Well

connected

to IZ through 

secondary 

conduits?

Possible

to conduct 

field 

investigation 

to evaluate 

integrity?

Yes

Elevated vulnerability. Consider 

corrective action and/or targeted 

monitoring and mitigation plans.

No

Yes

Elevated vulnerability. 

Consider corrective 

action and/or targeted 

monitoring and 

mitigation plans.

Evaluate individual wells

No

Yes

Conduct field investigation 

to determine depth

Unknown

Integrity is 

adequate 

along full 

penetration 

depth?

Secondary 

conduits to 

receptors 

exist?

Evaluate 

secondary 

conduits

Secondary 

conduits 

provide path 

to receptors?

Elevated vulnerability. Consider 

corrective action and/or targeted 

monitoring and mitigation plans.

No

Yes

High

density of 

wells?

Higher cumulative uncertainty and vulnerability. Corrective action and/or targeted 

monitoring and mitigation plans may need 

to be considered.

Yes

No

Locate pre-existing wells

Injection and monitoring wells should be carefully designed, operated, and monitored, 

taking into consideration injection rate, pressure, and corrosion conditions

Identify wells

No

Wells evaluation approach: 

Initial well screen

 Evaluate penetration depth relative to 

the Injection Zone (IZ) and confining 

system, and connection to other conduits

 Evaluate well integrity







Figure 5.2. Wells Evaluation 
5

vulnerability evaluation framework for geologic sequestration: key considerations

48

methods, could be managed by targeting the well for monitoring and taking mitigative steps 

such as those described in Chapter 6. Decisions may need to be made on a site-specifi c basis 

regarding whether corrective action or targeted monitoring, or a combination of both, may be most 

appropriate to address conditions at a given well, in addition to the development of mitigation plans.

5.2.2 Faults and Fractures as Fluid-Conducting Pathways

Although faults and fractures have been identifi ed, along with wells, as one of the most likely fl uid 

conduits in GS systems, the simple presence/absence of fault in many cases will not be suffi cient 

to evaluate the vulnerability to unanticipated migration and leakage. Th e potential for faults 

and fractures to act as fl uid pathways in GS systems is a function of numerous factors, including 

stratigraphic position. For example, a fault that does not cut across the full thickness of the confi ning 

zone may not pose the same level of vulnerability as a fault that is continuous from the injection 

zone to the surface. 

Whether a fault is sealed or conductive will also determine its ability to allow passage of CO2 and 

other fl uids; faults may serve as either barriers or conduits to fl uid fl ow (Omre et al., 1994; Lewicki 

et al., 2006; Wilkens and Naruk, 2007). Existing faults and fractures that are sealed may remain 

stable or may be reactivated by induced pressure changes and geochemical conditions caused by CO2 

injection. Th e orientation and geometry of the fault relative to regional stresses will also infl uence 

whether it may be re-activated (Rutqvist et al., 2007a, 2007b). New fractures could form if the 

fracture pressure of the formation(s) is exceeded (Healy et al., 1968; Gibbs et al., 1973; Raleigh et al., 

1976; Sminchak et al., 2002; IPCC, 2005; Streit et al., 2005; Wo et al., 2005). However, currently 

available geomechanical methods can assess the stability of faults and estimate the maximum 

sustainable pore fl uid pressures for CO2 storage (e.g., Streit and Hillis, 2003). 

Th e potential for induced fractures and re-opened faults to result in adverse impacts will depend on 

numerous additional factors, including whether they are connected to an overlying receptor, whether 

they are connected to other fl uid-conducting pathways (e.g., wells), and whether or not they may 

be resealed by geochemical processes. In addition, an adverse impact will occur only if the amount 

of CO2 that is transported along the fault is suffi cient to impact a receptor such as a drinking water 

supply. For example, if a small fault reopened that connects the injection zone to an overlying 

aquifer, the amount of CO2 transported along the fault may not adversely aff ect drinking water or 

other resources.

Identifying and evaluating how faults and fractures will behave presents a particular challenge in 

the characterization of GS systems. Figure 5.3 presents some of the factors that can be considered 

when evaluating the potential for faults and fractures to act as fl uid-conducting pathways. Th e 

fi gure highlights the need to identify whether faults are continuous from the injection zone to 

secondary conduits, overlying aquifers, or other receptors. Th is evaluation can be strengthened if 

multiple techniques for site characterization are used, such as surface mapping, interpretation of well 

bore data, and seismic techniques. Use of seismic techniques is generally the more comprehensive 

approach, because not all faults at injection zone depths have surface expressions allowing surface 

mapping to locate them, and the spatial discreteness of well bore data is generally insuffi cient 

to properly characterize fault and fracture zones. Th ree-dimensional seismic data may provide 

advantages for complete fault characterization, because two-dimensional seismic data have associated 

spacing and resolution constraints. As work progresses, additional methods may be identifi ed for 

fault characterization techniques within GS systems.
5

vulnerability evaluation framework for geologic sequestration: key considerations

49

Evaluating the stability of faults and fracture zones in GS systems also presents particular challenges. 

Faults and fractures are often associated with complex zones with altered properties, rather than 

being simple discontinuities. Th ere may be signifi cant uncertainty in being able to predict how they 

will behave under the conditions induced by GS. However, as described in Figure 5.1, careful design 

and monitoring can help manage vulnerabilities associated with faults and fractures. Approaches 

include using multiple site characterizing technologies and setting monitoring at a level appropriate 

to the level of uncertainty. Furthermore, the existing knowledge base is likely to improve as more 

data are acquired from fi eld projects.

5.2.3 Pressure-Induced Physical Eff ects

Injecting CO2 into geologic formations will in most cases cause subsurface changes in pressure. As 

discussed above, induced fracturing and fault reactivation can occur if injection pressure exceeds 

Low 

vulnerability 

Secondary 

conduit(s) 

identified?

No

Evaluate potential of Yes

combined pathway 

to conduct fluids

Locate faults based on 

site characterization

Evaluate if faults are 

expected to be present 

based on the regional 

tectonic environment

Presence

of fault(s) 

continuous from the 

IZ to overlying 

aquifer(s) or

other

 receptors?

Evaluate potential conduits for all other identified 

faults, considering secondary conduits, such as 

wells

None

 identified

Yes

Evaluate potential to 

conduct fluids

If conductive: 

elevated 

vulnerability

If sealing: evaluate 

fault stability under 

proposed 

operational 

conditions and set 

monitoring and 

mitigation plans

If uncertain/ 

unknown: consider 

further site 

characterization 

and targeted 

monitoring and 

mitigation plans

Figure 5.3. Faults Evaluation 
5

vulnerability evaluation framework for geologic sequestration: key considerations

50

fracture pressures, which may in turn result in the opening of fl uid migration pathways. If pressures 

are great enough, they could in extreme cases cause earthquakes (Healey et al., 1968). However, these 

eff ects could be minimized through an understanding of the relevant geologic attributes, careful site 

characterization, careful operation of GS systems, and monitoring. As summarized in Chapter 3, 

the geologic attributes that will infl uence pressure changes include physical capacity of the injection 

zone, its injectivity, and geomechanical and geochemical processes. Operational factors that infl uence 

pressure changes, and that can be managed, include the rate of injection, the slotted length of the 

injection well, the number of injection wells, and their orientation.

Th ere may be greater uncertainty about evaluating pressure eff ects in GS systems when examining 

the potential for pressure-induced regional scale impacts that do not involve fracturing or faulting. 

As discussed in Chapter 4, pressure changes in the injection zone could cause regional impacts on 

overlying aquifer systems, including changes in groundwater fl ow directions and water table levels. 

Th ese may result in alterations in the distribution and fl uxes of groundwater. Th is could in turn have 

other impacts, for example, changing the quantity of groundwater that is available for municipal 

drinking water supplies. Th ere also could be pressure-induced migration of brines and other fl uids 

through the pore structure of overlying formations into groundwater receptors, which may impact 

water quality. Furthermore, pressure-induced fl uid displacement could result in the release of brine 

at locations where injection zone formations outcrop at the land surface. Regional pressure eff ects 

have been the focus of relatively few studies, and uncertainties and vulnerabilities associated with this 

subject should be addressed through additional research.

5.3 Chapter Summary

Th is chapter discussed the importance of a holistic evaluation of vulnerability associated with GS 

systems and highlighted key attributes (i.e., wells, faults, and pressure changes) that should be 

examined extensively in a vulnerability evaluation. Chapter 6 summarizes monitoring and mitigation 

strategies to help decrease or avoid the potential adverse impacts of GS summarized in Chapter 4.
51

CHAPTER 6

MONITORING AND MITIGATION

Careful site selection and site characterization of GS systems and accompanying spatial areas 

of evaluation can help minimize the potential for adverse impacts. However, in the event of 

unanticipated migration and leakage, early detection through monitoring and application 

of mitigation techniques can help prevent or minimize adverse impacts. Data collected from 

monitoring can also help refi ne our understanding of GS systems and overlying receptors, thereby 

enhancing operations while minimizing vulnerabilities.

Th e VEF identifi es the key attributes that are important to site selection and site characterization 

(geologic attributes and receptors described in Chapters 3 and 4), and approaches to delineate the 

spatial extent of the GS system (also described in Chapters 3 and 4). Although monitoring and 

mitigation are not the focus of the VEF, application of the VEF can inform and prioritize monitoring 

and mitigation approaches, focused on both the subsurface geologic attributes, and overlying 

receptors. 

Th is chapter contains the following sections:

• Section 6.1 presents a brief overview of the purposes of monitoring, diff erent applicable 

monitoring technologies, and the timeframe for monitoring activities.

• Section 6.2 provides a brief summary of mitigation strategies in the event of unanticipated 

migration, leakage, and pressure changes.

• Section 6.3 provides a summary of the chapter.

6.1 Monitoring

6.1.1 Purposes of Monitoring

Most of the geologic attribute evaluation processes provided in the VEF qualitatively identify those 

conditions that may lead to low or elevated vulnerability, and recommend the development of 

monitoring and mitigation plans in instances of elevated vulnerability. Monitoring can provide an 

early warning mechanism in the event of unanticipated movement of CO2 and other fl uids from the 

injection zone (Oldenburg et al., 2003; IPCC, 2005). It can also be used to detect pressure changes 

in the subsurface. Early detection of unanticipated fl uid migration and increases in pressure provides 

the opportunity to put into place mitigation strategies to avoid or minimize adverse impacts. Th e 

goals of monitoring include regularly confi rming the location and containment of the injected CO2. 

Hence, the data collected during monitoring can also be used to develop a more comprehensive 

understanding of the GS system, and to calibrate and refi ne models of the behavior and location of 

CO2 in the subsurface. Operations may be modifi ed and enhanced based on the interpretation of 

monitoring data.

An important element of monitoring is establishing baseline conditions by collecting monitoring 

data prior to the injection of CO2. Measurements taken before injection will help evaluate how 

the subsurface might change in response to injection. Interpreting the data gained through many 

6
monitoring and mitigation 6

52

monitoring techniques (such as the seismic measurements discussed in the next section) can be 

greatly facilitated if post- and pre-injection data can be compared.

Monitoring can also be used to ensure proper functioning of injection wells and to optimize 

injection. Specifi c techniques can also be targeted at attributes and receptors of a GS site identifi ed 

during site characterization that may be of particular interest, including abandoned wells, faults and 

fracture zones, and sensitive overlying receptors (IPCC, 2005). 

6.1.2 Monitoring Technologies

Several types of monitoring technologies can be used to meet the purposes identifi ed above. 

Th e IPCC, in its Special Report on Carbon Dioxide Capture and Storage, describes monitoring 

technologies for the diff erent monitoring purposes:

• Location and movement of stored CO2. Th e location and movement of stored CO2 and 

other underground fl uids and gases can be monitored using direct or indirect approaches. 

Direct approaches, which are employed at many EOR projects, involve measuring the 

volume of injected quantities of CO2 at multiple production wells. A second direct approach 

to monitoring movement of stored CO2 and other underground fl uids and gases is to drill 

monitoring wells. Monitoring wells can also be used in other assessments of subsurface 

fl uid migration, including tracer tests. Injecting unique tracers (e.g., gases not found within 

the injection zone) and detecting for them at monitoring wells can help characterize CO2 

migration in the subsurface (IPCC, 2005). 

 Indirect approaches for monitoring movement of stored CO2 and other underground 

fl uids and gases include using seismic and non-seismic technologies (IPCC, 2005). Seismic 

technologies measure the velocity and absorption of energy waves through rock and provide 

a picture of underground layers of rocks and reservoirs. Non-seismic techniques such as 

electrical and electromagnetic techniques measure the relative conductivity of subsurface 

layers of various solids and fl uids (IPCC, 2005). A comprehensive list of technologies for 

monitoring movement of stored CO2 and other underground fl uids and gases is given in 

Table 5.4 of the IPCC Special Report on Carbon Dioxide Capture and Storage (IPCC, 

2005).

• Injection rates and pressures. Th e oil and gas industry has a long history of monitoring 

injection rates and pressures. Technologies used for this purpose are mature and commercially 

available. Gauges at wellheads are used to measure injection rates. Injection pressure is 

typically measured at most injection wells, and downhole formation pressure measurements 

are also routine, though the latter measurements may only be made on a periodic basis. 

Pressure gauges at injection wells are often linked to shut-off mechanisms that will slow or 

cease injection if injection pressure deviates from a determined range (IPCC, 2005). 

• Well integrity. Wells are often subjected to extensive induced-pressure testing for mechanical 

integrity before use. Continued integrity of the well during injection can be evaluated by 

monitoring pressure. Th e injection pressure can be continuously monitored at the wellhead 

by meters (IPCC, 2006). Th e pressure in the injection well annulus can also be monitored 

during injection to ensure the integrity of the injection well packer and casing. A decrease 

in pressure in the annulus may indicate unanticipated subsurface migration. In addition, 
monitoring and mitigation 6

53

temperature and “noise” logs can be used to detect well failures. Such logs have historically 

been used in natural gas storage projects. Th e integrity of the casing material can also be 

monitored using corrosion monitoring techniques such as caliper logs (IPCC, 2005).

• Local environmental impacts. In addition to technologies for monitoring CO2 movement 

underground, several technologies are available for monitoring the local environmental 

impacts of unanticipated migration and leakage. Several reviews have been published 

on near-surface monitoring technologies that can evaluate local environmental impacts, 

including both conventional and unconventional technologies (IPCC, 2005; Benson et al., 

2002; Oldenburg et al., 2003).

 Monitoring CO2 levels at the surface is a common practice in many occupational 

applications. Many heating, ventilation, and air conditioning systems, for example, have 

CO2 sensors. In addition, the literature on CO2 concentrations in soil and air is extensive (as 

described in Oldenburg et al., 2003; Miles et al., 2005) and monitoring occurs frequently 

in many areas (e.g., regular CO2 fl ux measurements by the U.S. Geological Survey at 

Mammoth Mountain, California) (USGS, 2001). 

 Satellite-based remote sensing and airborne imaging measurements of CO2 levels are also 

possible technologies for monitoring CO2 levels at the surface. However, these technologies 

may not be sensitive enough to detect the small CO2 fl uctuations that may be associated 

with GS. Monitoring ecosystems for exposure to CO2 is another technique for assessing 

local environmental impacts at the surface. Such monitoring often involves measuring 

the productivity and biodiversity of fl ora and fauna and measuring pH levels in aquatic 

ecosystems. Impacts on USDWs can be monitored by water sampling that tests levels of 

major ions and gases that may be carried by the leaked plume or mobilized by the plume’s 

constituents (IPCC, 2005). Hyperspectral imaging is also being considered for the detection 

of stressed vegetation.

6.1.3 Timeframe Implications for Monitoring

Th e long timeframe over which GS needs to be eff ective underlines the importance of regular 

monitoring. It is suggested that monitoring periods for GS can be divided into three periods: 

injection and operation; storage during pressure equilibration; and long-term storage (Chalaturnyk 

and Gunter, 2004).

Th e length of the injection and operation period will depend on the capacity of the injection zone 

and the rate of injection, but is generally expected to be 25–50 years. Th e period of time required 

for pressure equilibration after injection stops will be site specifi c. For open systems, where the 

injection zone formation is very large compared to the volume of the CO2 plume, the time period 

for pressure to decay back to the regional formation pressure may be 100 years or less (Birkholzer et 

al., 2007). For closed systems, pressure may remain elevated for a much longer period of time. After 

pressure stabilization (either with a return to the regional pressure regime in the case of open systems 

or a long-term stabilization at elevated pressures in closed systems), if unanticipated migration and 

leakage has not occurred, the GS system may be considered stable, and the formation of pressurerelated fl uid conduits no longer anticipated (Chalaturnyk and Gunter, 2004). Geochemical changes, 

however, can continue over a longer timeframe. Th e integrity of the caprock or wellbore cements, for 

example, may be aff ected by reactions with injected CO2 (IPCC, 2005). 
monitoring and mitigation 6

54

Monitoring may be more intensive in the earlier stages, with more frequent measurements and 

specifi c targeting of operational infrastructure. Th e frequency of monitoring may decrease as the 

system stabilizes post-injection, and eventually cease (Chalaturnyk and Gunter, 2004; IPCC, 2006).

Additionally, the area requiring monitoring is also likely to change over time, expanding during the 

injection period as the CO2 plume grows. Th e continued mobility of CO2 post-injection, because 

of its relatively high buoyancy and lower viscosity compared to water and brine, may also result 

in a dynamic post-injection area of evaluation, necessitating adaptive monitoring. Th e area to be 

monitored for pressure impacts will also expand during injection and may be signifi cantly larger than 

the area covered by the CO2 plume. Post-injection, if the pressure dissipates (e.g., in the case of open 

systems), the area requiring monitoring for pressure changes may decrease correspondingly. 

6.2 Mitigation

Unanticipated migration, leakage, and adverse pressure changes identifi ed through monitoring 

may be addressed in many cases with mitigation actions. For the purposes of this report, mitigation 

refers to actions taken to prevent unanticipated migration, leakage, or pressure changes that have the 

potential to cause adverse impacts. It also includes actions taken to prevent or reduce impacts once 

unanticipated migration, leakage, or pressure changes have been detected by monitoring. Th is report 

does not address actions taken to address impacts that have already occurred. Several important 

mitigation concepts are discussed below. Some of these approaches are routine within the oil and gas 

and other industries, while others are more experimental, and will likely require further study prior 

to implementation.

Wells. According to the IPCC, the development of mitigation plans for active or abandoned wells 

is particularly important, because they are known vulnerabilities (Gasda et al., 2004; IPCC, 2005; 

Perry 2005). Standard mitigation techniques, also referred to as corrective actions, exist to stop 

unanticipated migration and leakage from injection and abandoned wells. Th ese include injecting 

a heavy mud to plug the opening. If the well is not accessible at the surface, a new well can be 

drilled to intercept the casing of the compromised well below the ground surface and cement can be 

pumped in from the interception well. 

Further mitigation techniques also exist for injection wells. Th e integrity of CO2 injection wells 

can be repaired by replacing injection tubing and packers. If the space between the casing and the 

formation borehole leaks, the casing can be perforated to allow injection of cement behind the casing 

to seal the leak. If the well cannot be repaired, it can be plugged and abandoned using the standard 

procedures cited above, and replaced with a new injection well.

Faults and fractures. Th e unanticipated migration of CO2 and other fl uids along faults (and 

fractures) could be mitigated by lowering the pressure driving fl ow along the fault by injecting at a 

lower rate, or through more wells. Alternatively, the pressure in the injection zone could be lowered 

by removing water or other fl uids, or by possibly creating a pathway to access additional formations 

within the injection zone. Further, extraction wells could be used to intersect the pathway, or a 

hydraulic barrier could be created by increasing the reservoir pressure upstream of the fault. It may 

also be appropriate to consider ceasing injection to stabilize the project and, in the extreme case, the 

CO2 could be recovered from the formation and re-injected in a more suitable formation (IPCC, 

2005). 
monitoring and mitigation 6

55

Accumulation in indoor environments. Slow releases of CO2 can accumulate in confi ned spaces 

(e.g., basements) and cause harm to human health. Th ese slow releases into structures can be 

eliminated using techniques designed to dilute the CO2 before it enters indoor environments, such 

as basement/substructure venting or pressurization (IPCC, 2005).

Large releases to the atmosphere. For very large releases to the atmosphere spread over large areas, 

natural attenuation through atmospheric dispersion may be the main mitigation option. Smaller 

localized releases at wellheads or in buildings could be addressed using fans or venting (Benson and 

Hepple, 2005; IPCC, 2005).

Accumulation in soil gas. As discussed in chapter 4, accumulation of CO2 in soil gas can be 

detrimental to ecological receptors. Removing the CO2 from the vadose zone would also prevent its 

escape to the atmosphere. CO2 can be extracted from the vadose zone and soil gas using standard 

vapor extraction techniques that use horizontal or vertical wells to collect the CO2. Additionally, 

it may be possible to decrease or stop releases to the atmosphere from the vadose zone by the use 

of caps or gas vapor barriers. Th e CO2 accumulated under the cap could then be removed using 

extraction wells. Since CO2 is denser than air, it may be possible to collect it in subsurface trenches. 

Th e CO2 captured via these approaches could then be reinjected into the subsurface. Finally, in 

some cases, acidifi cation of the soils from contact with CO2 could be remediated by irrigation and 

drainage, acid-neutralizing substances such as lime could be applied to the soil (IPCC, 2005).

Groundwater and surface water. Fluids that reach groundwater could be captured using extraction 

wells and reinjected into the subsurface. Carbon dioxide that reaches a groundwater aquifer and 

then becomes immobile through residual trapping could be removed by fl ushing the aquifer with 

water, thereby enhancing CO2 dissolution, and then extracting the dissolved CO2 using groundwater 

extraction wells. Additionally, CO2 that has dissolved in shallow groundwater can similarly be 

removed with groundwater extraction wells. Th e mobilization of metals or other contaminants as a 

result of CO2 migration into groundwater could be addressed through pump and treat technologies, 

the creation of hydraulic barriers, or by in situ passive methods such as enhanced bioremediation 

(IPCC, 2005).

Shallow surface water bodies that have signifi cant turnover (shallow lakes) or turbulence (streams) 

may be naturally attenuated, due to their relatively quick release of dissolved CO2 back into the 

atmosphere. For deep, stably stratifi ed lakes, active systems for venting gas accumulations have been 

developed and applied (IPCC, 2005).

6.3 Chapter Summary

Chapter 6 summarized the importance of delineating the geographical extent of GS systems for site 

characterization and monitoring purposes, how monitoring can be used to achieve multiple goals, 

the types of monitoring technologies, and the timeframe for monitoring activities. It also described 

mitigation strategies that can be employed in the event of unanticipated migration and leakage of 

fl uids in order to avoid or minimize adverse impacts.
56

CHAPTER 7

SUMMARY AND NEXT STEPS

Th e VEF approach was developed by EPA to systematically identify those conditions that may 

increase or decrease susceptibility to adverse impacts from GS. Th e VEF is a reference document that 

can assist regulators and other technical experts in identifying key areas for in-depth site-specifi c risk 

assessment, monitoring, and management. Th e vulnerability assessment introduced in the VEF is 

intended to be an iterative process where new information is incorporated into the evaluation as it 

is generated. It is not intended to be broadly applicable or to measure the severity of an outcome, to 

establish performance standards for GS sites, or to specify data requirements.

Th e VEF identifi es attributes of GS systems that may lead to increased vulnerability to adverse 

impacts, identifi es potential impact categories, identifi es thresholds that may indicate low versus 

elevated vulnerability, and provides a series of decision-support fl owcharts that are organized, 

systematic approaches to assess the attributes and potential impacts. Th ough certain attributes or 

receptors may increase the overall susceptibility of a GS site to adverse impacts, it is important 

to recognize that assessing overall vulnerability is dependent on the interplay between individual 

attributes and receptors. 

Th e VEF, as described in this report, represents the fi rst step toward the development of a conceptual 

framework to evaluate potential vulnerabilities of GS projects and as such is a work in progress. 

Numerous reviewers have encouraged further development and refi nement of the VEF as well as 

demonstration of its practical applicability. Future work could also involve developing the decisionsupport fl owcharts into an integrated evaluation tool that has a more quantitative and numerical 

basis. Th is could include refi ning the binary vulnerability classifi cation (low versus elevated) into a 

multiscaled classifi cation scheme as warranted by available information, data, and expert opinion. In 

addition, the VEF may be validated and refi ned by applying it to case studies that represent a range 

of likely scenarios in which various aspects of the VEF framework can be tested. Th ese scenarios may 

be based on real, successful demonstration projects as well as more challenging situations where risk 

may be higher. Applying the VEF in this manner would improve the framework and could be an 

instructive exercise for regulators, operators and other stakeholders and can provide a means to test 

approaches developed to manage uncertainties. 

Th e VEF is designed to focus on vulnerabilities of GS systems that could increase adverse impacts 

associated with GS. It could be expanded to include vulnerabilities to adverse impacts associated 

with the capture or transport of CO2, operational aspects of GS such as surface infrastructure (e.g., 

buildings, pipelines, well construction), and specifi c monitoring techniques to ensure the integrity 

of the GS system recommended. Th ough not currently included in the VEF, these elements need 

to be considered because they may play an important role in defi ning a proposed project’s overall 

vulnerability.

7
57

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69

GLOSSARY

Term Defi nition

Abandoned Well A well that is no longer in use and has been closed according to 

standardized procedures, which may often include placing cement or 

mechanical plugs in all or part of the well. 

Adsorption The adhesion of molecules on the surface of a solid or a liquid. 

Artifi cial Penetration Man-made perforations that penetrate the subsurface (e.g., wells, boreholes, 

and mines).

Aquifer An underground geological formation or group of formations, containing 

water. Aquifers are sources of groundwater for wells and springs.

Attribute A characteristic of the geologic system that may influence the vulnerability 

to adverse impacts. Geologic attributes are characteristics that can aff ect the 

vulnerability to unanticipated migration, leakage, or pressure changes.

Borehole A hole drilled into the subsurface, typically to collect soil samples, water 

samples or rock cores. A borehole may be converted to a well by installing a 

vertical pipe (casing) and well screen to keep the borehole from caving.

Brine Water containing a high concentration of dissolved salts.

Buff ering Capacity A measure of the ability of a solution to resist change in pH when acid or 

base are added.

Capillary Entry 

Pressure

The additional pressure needed for a liquid or gas to enter a pore occupied 

by a different phase. For example, for CO2 to displace water and enter the 

pores of the confi ning system.

Capillary Trapping Retention of CO2 in pore spaces by capillary forces.

Caprock See confi ning system.

Carbon Dioxide 

Capture And Storage

A climate change mitigation strategy that involves capturing CO2 emissions 

from large stationary sources, transporting the CO2 to a storage location, 

and sequestering the CO2 for long periods of time.

Closed System A system where elevated pressure levels associated with the injection of CO2 

do not dissipate to background levels, but remain elevated at its physical 

boundaries, and may remain elevated for a long period of time after 

injection stops.

CO2 Plume The extent, in three dimensions, of an injected carbon dioxide stream.

CO2 Spatial Area Of

Evaluation

The CO2 spatial area of evaluation is delineated based on potential adverse 

impacts resulting from unanticipated migration or leakage. Th e CO2 

spatial area of evaluation is typically larger than the CO2 plume, because 

unanticipated migration or leakage of CO2 right along the boundary of the 

plume may affect areas beyond the plume.

CO2 Stream The content of the CO2 stream captured from large point sources for 

injection. The CO2 stream may include trace amount of impurities in 

addition to CO2.
glossary

70

Confi ning System Th e confining system is a geologic formation, group of formations (e.g., 

shale or siltstone), or part of a formation that is composed of impermeable 

or distinctly less permeable material stratigraphically overlying the injection 

zone that acts as a barrier to the upward flow of fl uids.

Corrective Action Use of methods to assure that wells do not serve as conduits for 

unanticipated migration or leakage.

Cultural/Recreational Cultural resources include gathering places for human populations, 

Resources including places of archaeological, historic, or natural signifi cance; 

American Indian resources, cemeteries and paleontological resources. 

Recreational resources include ecological features that provide fi shing, 

hunting, and hiking to the public.

Dip The angle between a planar feature, such as a sedimentary bed or a fault, 

and the horizontal plane. 

Dissolution Rate The rate at which a substance is dissolved in a fl uid. 

Dissolution Trapping The trapping of CO2 when it contacts the fluid formation and dissolves into 

the fluid. Also referred to as solubility trapping

Downhole Injection Injection pressure at the point where CO2 exits the well and is introduced 

Pressure to the injection zone formation(s). 

Economic Resources Surface and subsurface resources with economic value, including mineral 

and hydrocarbon reservoirs, forests, and croplands.

Endangerment The construction, operation, maintenance, conversion, plugging, or 

abandonment of an injection well, or the performance of other injection 

activities, by an owner or operator in a manner that allows the movement 

of fluid containing any contaminant into a USDW, if the presence of 

that contaminant may cause a violation of any primary drinking water 

regulations or may adversely affect the health of persons.

Enhanced Coal Bed The process of injecting a gas (e.g., CO2) into coal, where it is adsorbed to 

Methane Recovery the coal surface and methane is released. The methane can be captured and 

produced for economic purposes; when CO2 is injected, it adsorbs to the 

surface of the coal, where it remains sequestered.

Enhanced Gas Typically, the process of injecting a fluid (e.g., water, brine, or CO2) into 

Recovery a gas bearing formation to recover residual natural gas. The injected fl uid 

thins (decreases the viscosity) or displaces small amounts of extractable gas, 

which is then available for recovery. 

Enhanced Oil Typically, the process of injecting a fluid (e.g., water, brine, or CO2) into 

Recovery an oil bearing formation to recover residual oil. The injected fl uid thins 

(decreases the viscosity) or displaces small amounts of extractable oil, which 

is then available for recovery. 
glossary

71

Fault Faults are breaks in the Earth’s crust that occur as a result of when the 

crustal rock is either compressed or pulled apart. Faults may either serve

as barriers or conduits to fl uid flow, depending upon whether they are 

transmissive or sealed.

Flux Rate The rate of transfer of fluid, particles, or energy across a given area.

Formation A body of rock of considerable extent with distinctive characteristics that 

allow geologists to map, describe, and name it. 

Fracture A separation or discontinuity plane in a geologic formation, such as a joint 

that divides a rock segment into two or more pieces. Fractures can be caused 

by stress exceeding the rock strength.

Fracture/Fault The geologic formation pressure threshold that when achieved will cause reReactivation Pressure opening of a sealed fault or fracture.

Geochemical Process Geochemical processes refer to chemical reactions that may cause alterations 

in mineral phases.

Geologic The process of injecting captured CO2 into deep, subsurface rock 

Sequestration formations for long-term storage. This term does not apply to its capture or 

transport.

Geomechanical Processes that may result in alterations in the structural integrity of geologic 

Processes material.

GS System A system that is comprised of the confining system, the CO2 stream, and 

the injection zone. 

GS Footprint The areal extent of the CO2 plume and associated pressure front.

Hypercapnia A physical condition involving an excessive amount of CO2 in the blood.

Impact Category Term for classifying groups that might experience adverse impacts. Impact 

categories include human health and welfare, atmosphere, ecological 

receptors, surface and ground water, and the geosphere.

Injection The subsurface discharge of fluids through a well.

Injection Zone A geologic formation, group of formations, or part of a formation of 

sufficient areal extent, thickness, porosity, and permeability to accommodate 

CO2 injection volume and injection rate.

Injectivity Injectivity characterizes the ease with which fluid can be injected into a 

geological formation.

Lateral Extent The surface area of the confining system that overlies the GS footprint.

Leakage The movement of CO2 (or other fluids and gases) to the surface (for 

example, to the atmosphere or oceans). 

Legislatively Protected Species designated as threatened or endangered by State and federal 

Species government authorities. 
glossary

72

Measurement, Collectively, a comprehensive protocol for providing an accurate accounting 

Monitoring, And of stored CO2. 

Verifi cation

Mechanical Integrity The absence of fluid-conducting openings within the injection tubing, 

casing, or packer (known as internal mechanical integrity), or outside of the 

casing (known as external mechanical integrity). 

Mechanical Integrity Evaluations used to confirm that a well maintains internal and external 

Test mechanical integrity. These tests are a means of measuring the adequacy of 

the construction of an injection well.

Migration Subsurface movement of CO2 (or other fluids) within or out of the 

injection zone.

Mineral Trapping When injected CO2 reacts with the formation waters and/or formation 

rocks to form carbon-containing minerals such as carbonates, thereby 

effectively retaining the CO2 in the formation.

Mineralization The chemical process involving the reaction of CO2 with the formation 

waters and/or formation rocks to form carbon-containing minerals such as 

carbonates. 

Mineralogy The mineral content of a rock or geologic formation. Evaluating mineralogy 

can help determine whether metal and metalloid-bearing mineral phases 

(e.g., metal oxides, sulfides, carbonates) are present.

Molar Volume The volume occupied by one mole of a substance (chemical element or 

chemical compound) at a given temperature and pressure.

Monitoring Employing various technologies for the purposes of measuring quantities 

of injected CO2, tracking the location and movement of injected CO2 

and other fluids, ensuring the effectiveness of injection wells, assessing the 

integrity of abandoned wells. 

Open System A system in which pressure levels associated with injection of CO2 dissipate 

to background levels before reaching the physical boundaries of the system. 

Other Sensitive Ecological receptors that may be sensitive to exposure to CO2, other stream 

Receptors constituents, brine, or other fluids that may result from GS activities but are 

not legislatively protected. These receptors may be vulnerable either because 

of their behavior (e.g., soil-dwelling organisms) or because their physiology 

makes them unusually sensitive to increased concentrations of released 

substances. 

Packer A mechanical device set immediately above the injection zone that seals the 

outside of the tubing to the inside of the long string casing. A packer may 

be a simple mechanically set rubber device or a complex concentric seal 

assembly.
glossary

73

Permeability The ability of a geologic material to allow transmission of fl uid through 

pore spaces.

Permit An authorization, license, or equivalent control document issued by EPA 

or a State to operate an injection well. Permits may be individual permits 

(covering a single well) or area permits (covering multiple wells in one area).

Physical Capacity The volume within a geologic formation that is available to accept CO2.

Physical Trapping When injected CO2 rises owing to its relative buoyancy or the applied 

injection pressure, and reaches a physical barrier that inhibits further 

upward migration.

Plugging The act or process of stopping the flow of water, oil or gas into or out of a 

formation through a borehole or well penetrating that formation.

Populations Covered A series of executive orders require proposed federal regulations and 

By Executive Orders programs to specifically address potential impacts to environmental justice 

populations, children, and tribal governments.

Pore Space Open spaces in rock or soil. In the subsurface, these are typically fi lled with 

water, brine (i.e., salty fluid), or other fluids such as oil and methane.

Porosity A measure of the percentage of a rock that is occupied by pore space.

Precipitate A solid separated from a solution, especially as the result of a chemical 

reaction (i.e., the reaction of minerals within the confining system with 

CO2 and salt ions). 

Preferential When micropores in certain geologic formations tend to adsorb CO2 and 

Adsorption Trapping displace other present gases to which they have a lower affi nity.

Pressure Change A change in force per unit area. Pressure changes are likely to be associated 

with the injection of CO2 into the subsurface.

Pressure Equilibration A state of balance achieved when formation pressure levels reached during 

injection return to the original formation pressure levels. 

Pressure Spatial Area The pressure spatial area of evaluation is delineated based on the potential 

Of Evaluation for subsurface pressure changes that are suffi ciently significant to cause 

adverse impacts to overlying receptors (e.g., fluid displacement into an 

overlying aquifer).

Protected Or Sensitive Drinking water supplies that are vulnerable to endangerment or 

Drinking Water contamination. 

Supplies

Receptor A surface or underground feature whose presence within the CO2 and 

pressure spatial areas of review could affect the vulnerability of adverse 

impacts in the event of unanticipated migration, leakage, or pressure 

changes.

Regional Groundwater The direction and rate of groundwater movement in the subsurface. 

Flow
glossary

74

Release Another term for leakage, the movement of CO2 (or other fluids) to the 

surface (for example, to the atmosphere or oceans).

Remediation The process of correcting any source of failure to stop or control undesired 

CO2 movement if it occurs.

Residual Water Retainment of water in pore space due to capillary forces. 

Saturation

Risk Assessment An approach to measuring the probability and severity of consequences.

Safe Drinking Water The main federal law that ensures the quality of Americans’ drinking water. 

Act The Safe Drinking Water Act sets the framework for the Underground 

Injection Control Program to control the injection of fluids. EPA and states 

implement the UNDERGROUND INJECTION CONTROL Program, 

which sets standards for safe injection practices and bans certain types of 

injection.

Seismic Activity Seismic activity is defined as the shifting of the Earth’s surface due to 

changes at depth. Increased seismic activity can lead to earthquakes.

Seismic Technology A monitoring approach that involves measuring the velocity and absorption 

of energy waves through rock and provide a picture of underground layers 

of rocks and reservoirs.

Seismicity The episodic occurrence of natural or human-induced earthquakes.

Stratigraphic Position The order and relative arrangement of a specific layer of rock that is 

recognized as a cohesive unit based on lithology, fossil content, age, or other 

properties. 

Strike The line representing the intersection of a planar feature with the 

horizontal.

Subsidence Lowering, or “sinking,” of geologic formations due to dissolution of 

formation minerals. Dissolution of formation materials can result from 

CO2 acidification of formation waters.

Supercritical Fluid A fluid above its critical temperature (31.1 degrees Celsius for CO2) and 

critical pressure (73.8 bar for CO2). Supercritical fluids have physical 

properties intermediate to those of gases and liquids. 

Tectonic Activity Natural activity involving structural changes to the Earth’s geology.

Total Dissolved Solids The measurement, usually in mg/L for the amount of all inorganic and 

organic substances suspended in liquid as molecules, ions, or granules. For 

injection operations, TDS typically refers to the saline (i.e., salt) content of 

water-saturated underground formations.
glossary

75

Tracer A chemical compound or isotope added in small quantities to trace fl ow

patterns. 

Trapping Mechanism A physical or geochemical feature in the geologic system that retains 

injected CO2, immobilizing it under thick, low-permeability seals or by

converting it to solid minerals.

Travel Time The interval of time that is required for a fluid (e.g., CO2 or brine) to 

migrate across the thickness of the confi ning system.

Tubing A small-diameter pipe installed inside the casing of a well. Tubing conducts 

injected fluids from the wellhead at the surface to the injection zone and 

protects the long-string casing of a well from corrosion or damage by the 

injected fl uids.

Underground Source 

Of Drinking Water

An aquifer or portion of an aquifer that supplies any public water system or 

that contains a sufficient quantity of ground water to supply a public water 

system, and currently supplies drinking water for human consumption, or 

that contains fewer than 10,000 mg/l total dissolved solids and is not an 

exempted aquifer.

Uplift Rising of geologic formations due to increased pore pressure from injection. 

Vulnerability 

Assessment

An approach to examining conditions that lead to increased or decreased 

susceptibility to consequences.

Water Quality The characteristics of a source of water that determine its usefulness for a 

specifi c purpose.

Well A bored, drilled, or driven shaft or a dug hole whose depth is greater than 

the largest surface dimension. Wells can be used for production, injection, 

or monitoring purposes. 

Wellbore See borehole.
A

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APPENDIX A

Properties of Carbon Dioxide (CO2)

Th is appendix provides a brief overview of the physical and chemical properties of CO2 and a 

description of the supercritical phase in which CO2 will be injected for GS. Th e properties discussed 

in this section are important considerations addressed in the VEF.

Physical Properties of CO2

Carbon dioxide typically exists as a gas at normal pressure and temperature (i.e., at the earth’s 

surface). It can be processed into a solid (i.e., dry ice) at temperatures below -55°C (-67°F). At 

very high pressure, CO2 may exist as a solid, liquid, or a supercritical fl uid. A substance may be a 

supercritical fl uid at high pressure and temperature when the gaseous and liquid phases have the 

same density and cannot be further compressed with additional pressure. Carbon dioxide exists 

as a supercritical fl uid at temperatures greater than 31.3°C and pressures greater than 73.9 bar 

(IPCC, 2005).

Chemical Properties of CO2

Carbon dioxide is soluble in water, forming carbonate ions and acidity 

[CO2 (g) +H2O => H2CO3 (aqueous) => HCO3- + H+ => CO32- + 2H+] (IPCC, 2005, Chapter 6). 

Th e solubility decreases with increasing temperature and salinity and increases with increasing 

pressure. Solid hydrates may form in aqueous solutions of CO2 at high pressure and temperatures 

below 11°C (IPCC, 2005).

Characteristics of 

Supercritical CO2

Supercritical CO2 is approximately 

one order of magnitude less viscous 

than water and oil and is highly 

mobile (Oldenburg et al., 2002a; 

GEO-SEQ Project Team, 2004; 

Wilson, 2004). Supercritical CO2 is 

less dense and more buoyant than 

oil and 30% to 50% more buoyant 

than saltwater (Benson et al., 2002; 

GEO-SEQ Project Team, 2004; 

Wilson, 2004). As a result, injected 

supercritical CO2 will rise to the 

top of depleted oil or deep saline 

Figure A.1. Phase Diagram for CO2
A

appendix a

77

formations. In contrast, supercritical CO2 is denser than natural gas, and will sink when injected 

in depleted gas reservoirs. As temperature increases, the density of supercritical CO2 decreases at a 

given pressure, but increases with increasing pressure at a given temperature. Similarly, the viscosity 

of supercritical CO2 decreases with increasing temperature at a given pressure, but increases with 

increasing pressure at a given temperature. As temperature and pressure increase with subsurface 

depth, the density and viscosity of supercritical CO2 injected will be determined by the specifi c 

pressure and temperature conditions (Haszeldine, 2006). Finally, when CO2 dissolves, it shares the 

physical properties of the substrate (e.g., saltwater) and no longer behaves independently (IPCC, 

2005).
B

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APPENDIX B

Comparison of Attributes: VEF, IPIECA, and IPCC

Th e VEF geologic attributes are similar to characteristics identifi ed in GS summaries by IPCC 

(2006) and IPIECA (2007) as shown in the table below. However, there are two discrepancies 

between the IPCC and IPIECA summaries and the VEF. First, in the VEF, the seismic and volcanic 

risks in a proposed project area must be explicitly identifi ed and considered. Th is potential pathway 

is not explicitly addressed in the IPCC and IPIECA guidelines, although their recommended 

consideration of faults and fractures would presumably include consideration of seismically active 

areas. Second, the IPCC recommendations explicitly request consideration of potential methane 

displacement from GS projects. In the VEF this is not explicitly addressed by a specifi c attribute. 

Rather, the VEF addresses this potential release as part of the CO2 and Pressure Areas of Evaluation 

processes consideration of substances that could be released.

Table B.1. Comparison of Attributes/Pathways Identified as Being Likely to Aff ect 

Unanticipated Migration or Leakage from GS Projects

VEF geologic attributes IPIECA storage site attributesa IPCC potential emissions pathways and sourcesb

Lateral extent (of the 

confi ning system)

Effective seal provided by

overlying caprock. Local absence of cap rock.

Through the pore system in low permeability cap 

Capillary entry pressure rocks if the capillary entry pressure is exceeded or if 

the CO2 is in a solution.

Permeable distribution is suitable Through the pore system in low permeability cap 

Permeability for both injection and post- rocks if the capillary entry pressure is exceeded or if 

injection CO2 migration. the CO2 is in a solution.

Artificial penetrations (i.e., 

wells, mines)

New and existing wells will not 

compromise the integrity of the 

(caprock) seal. 

Operational or abandoned wells or future mining of 

CO2 reservoir.

Geomechanical processes 

or properties

Trapping mechanisms (capillary,

solubility, and mineralization) are 

eff ective.

CO2/water/rock reactions degrade cap rock.

Likelihood of contacted faults 

Faults/fracture zones reactivating and potential for Natural or induced faults and/or fractures.

existing fractures to re-open. 

Earthquakes, volcanoes, 

hot spots 

Physical capacity (of the 

injection zone) Suffi cient storage capacity.

Reservoir overfilling results in release from a spill 

point.

a. Source: Bulleted list in on page 23 of IPIECA (2007).

b. Source: Table 5.3 in IPCC (2006).