TECHNICAL SUPPORT DOCUMENT FOR

INJECTION AND GEOLOGIC SEQUESTRATION

OF CARBON DIOXIDE

PROPOSED RULE FOR MANDATORY

REPORTING OF GREENHOUSE GASES

MONITORING PLANS FOR

GEOLOGIC SEQUESTRATION

March 2010

U.S. Environmental Protection Agency
i

TECHNICAL SUPPORT DOCUMENT:

MONITORING PLANS FOR GEOLOGIC

SEQUESTRATION

Office of Air and Radiation

U.S. Environmental Protection Agency

Developed with support from Stratus Consulting, Inc
ii

CONTENTS

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

1.1 Geologic Sequestration: A Climate Change Mitigation Technology.....................................1

1.2 Importance of Monitoring .....................................................................................................1

1.3 Purpose of this Technical Support Document ......................................................................1

1.4 Overview of this Technical Support Document ....................................................................2

2. Need for Monitoring at Geologic Sequestration Sites........................................................4

2.1 Geologic Sequestration System ..........................................................................................4

2.2 Geologic Settings ................................................................................................................4

2.3 Subsurface CO2 Movement..................................................................................................5

2.4 Trapping Mechanisms..........................................................................................................6

2.5 Potential CO2 Migration and Leakage Pathways ..................................................................7

2.6 Monitoring for Accounting and Reporting on Geologic Sequestration...................................8

2.7 Other Monitoring Purposes ..................................................................................................9

2.8 Need for Monitoring Plans....................................................................................................9

2.9 Summary ...........................................................................................................................10

3. Key Components of a Robust Monitoring Plan ................................................................11

3.1 Tailoring Monitoring Plans to Site-Specific Conditions........................................................11

3.1.1 Identifying and Evaluating Geologic Attributes............................................................11

3.1.2 Identifying and Evaluating Potential Receptors and Impact Categories ......................19

3.1.3 Defining the GS Footprint for a Site-Specific Monitoring Plan.....................................21

3.2 Monitoring During the Different Phases of a Geologic Sequestration Project .....................22

3.2.1 Monitoring Needs for All Phases of a Project..............................................................22

3.2.2 Pre-Injection Phase....................................................................................................23

3.2.3 Injection Phase...........................................................................................................24

3.2.4 Post-Injection Phase ..................................................................................................26

3.2.5 Site Closure Phase ....................................................................................................26

3.3 Identifying Appropriate Monitoring Techniques...................................................................27

3.3.1 Monitoring Techniques to Ensure Accurate Accounting and Reporting of CO2

Emissions...................................................................................................................27

3.4 Ensuring a Flexible and Responsive Plan ..........................................................................29

3.5 Summary ...........................................................................................................................31

4. Summary .............................................................................................................................32

References ..............................................................................................................................33

LIST OF FIGURES

Figure 1. Vulnerability Evaluation Framework Conceptual Model..............................................12

Figure 2. VEF Confining System Evaluation Process................................................................15

Figure 3. VEF Wells Evaluation ................................................................................................16

Figure 4. VEF Faults Evaluation................................................................................................17

Figure 5. VEF Injection Zone Evaluation...................................................................................19

Figure 6. VEF GS Footprint Delineation....................................................................................21
1. INTRODUCTION

1

1.1 Geologic Sequestration: A Climate Change Mitigation Technology

The Intergovernmental Panel on Climate Change (IPCC) has identified carbon dioxide capture

and sequestration (CCS) as one of several approaches to reduce emissions of carbon dioxide

(CO2) to mitigate climate change (IPCC, 2007). While various CCS technologies have been

explored, including ocean storage and mineral carbonation, geologic sequestration (GS) has been

identified as the most technically viable approach (IPCC, 2005). GS is the long-term

containment of CO2 in subsurface geologic formations.. The CO2 is trapped for long periods of

time as a result of both physical and geochemical mechanisms. The climate change mitigation

benefits of GS depend on the long-term isolation of injected CO2 from the atmosphere.

1.2 Importance of Monitoring

While experience from existing projects and natural and industrial analogs to GS demonstrates

that CO2 can be safely sequestered in geologic formations, there is the potential for unanticipated

migration and leakage of injected CO2. Because of the potential for such migration and/or

leakage, monitoring is a necessary component of GS projects.

Monitoring of GS projects can be used to show that the injected CO2 is sequestered in the

subsurface, to verify the quantity of CO2 that has been injected, to detect leakage or unanticipated

subsurface movement of CO2, and to provide an early warning of any unanticipated migration or

leakage. Additionally, if leakage does occur, monitoring can be used to quantify how much CO2

was released.

Monitoring can also serve other purposes, including informing and evaluating the success of

mitigation actions, detecting induced microseismicity, and assessing potential environmental and

human health impacts of leaked CO2. Thus, monitoring is essential to ensuring the long-term

accountability and success of GS projects.

1.3 Purpose of this Technical Support Document

The purpose of this technical support document (TSD) is to identify and characterize key

components of monitoring plans for GS sites. A robust monitoring plan should describe a

monitoring process that is capable of demonstrating that the injected CO2 is sequestered in the

subsurface and of quantifying amounts of CO2 leaked, if leakage occurs. A robust monitoring

plan is tailored to site-specific conditions and to the different stages of a GS project, justifies the

selection of monitoring technologies, and is developed as a flexible, dynamic plan.

This TSD is based on the U.S. Environmental Protection Agency’s (EPA’s) “Vulnerability

Evaluation Framework for Geologic Sequestration of Carbon Dioxide” or VEF (U.S. EPA, 2008;

see docket EPA-HQ-OAR-2009-0926). The VEF covers a broad suite of potential surface and

subsurface impacts associated with GS. One example of potential subsurface impacts include

pressure-induced migration of saline waters into overlying aquifers which may serve as
introduction

2

underground sources of drinking water. All potential adverse impacts are important

considerations when assessing and designing monitoring plans for GS projects.

This TSD focuses on monitoring for the release of CO2 to the atmosphere, which is the focus of a

monitoring, reporting, and verification (MRV) plan under Mandatory Greenhouse Gas Reporting

Rule (see docket EPA-HQ-OAR-2009-0926). EPA considers the key components identified

above and described in further detail in this TSD as integral to MRV plans. It is important to

emphasize that the use of the VEF in framing site-specific considerations for site selection and

the use of this TSD for developing monitoring plans does not replace or supersede statutory or

regulatory requirements for the protection of human health and the environment. Owners or

operators must obtain all necessary permits from appropriate state and federal authorities under

the Safe Drinking Water Act and any other applicable statutes and regulations.

The terminology in this TSD is consistent with the VEF terminology. The reader is referred to

that document (U.S. EPA, 2008; see docket EPA-HQ-OAR-2009-0926) for further explanation

of the GS process and for further definitions of terms.

1.4 Overview of this Technical Support Document

The goals of this technical support document are to: 1) provide general background information

on GS and monitoring; and 2) identify the key components of a robust monitoring plan and

provide information on how to develop a monitoring plan that includes these key components.

This TSD addresses these goals in the following sections:

 Section 2 – Need for Monitoring at Geologic Sequestration Sites. Successful GS depends on

CO2 trapping mechanisms. Potential leakage and migration pathways could potentially

compromise the ability of a geologic formation to sequester the injected CO2. Because of this

potential for leakage and unanticipated migration, accurate accounting and verification1 of

the injected CO2 are essential to assessing the performance of GS projects. This section

describes GS systems, the types of geological settings that may be used for GS, the physical

and geochemical processes that help ensure long-term sequestration of injected CO2, and the

potential pathways and mechanisms that may result in leakage and migration of CO2 from the

subsurface. The section also describes the purposes of monitoring, including the role of

monitoring in ensuring the accuracy of accounting and reporting efforts, and the importance

of developing monitoring plans to facilitate accurate accounting and reporting.

 Section 3 – Key Components of a Robust Monitoring Plan. This section details key

components of a robust monitoring plan, and discusses how to develop a plan that contains

these key components. The section provides background and information on: 1) tailoring the

1

 For the purposes of GS, CO2 accounting is the practice of quantifying and recording the volume of CO2 that

is injected into underground formations. Verification refers to the aggregate actions that are taken to ensure the

reliability of the information recorded when accounting for injected CO2. For more information on accounting

and verification of greenhouse gases (GHGs) in general, see e,g, ISO Standard 14064 (ISO, 2006; NETL,

2009).
introduction

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plan to site-specific conditions, based on an evaluation of the GS system, relevant geologic

attributes, and potential impact categories and receptors; 2) developing the plan such that it

addresses the different phases of the GS project; 3) selecting appropriate techniques for

different monitoring needs; and 4) ensuring a flexible plan that addresses current

uncertainties and can accommodate future changes to a GS project.

 Section 4 – Summary. This section provides a concise summary of the information presented

in this TSD.
2. NEED FOR MONITORING AT GEOLOGIC

SEQUESTRATION SITES

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Experience from existing projects and natural and industrial analogs to GS demonstrate that CO2

can be sequestered in geologic formations. In addition, a GS site that is selected and managed

properly can sequester CO2 safely and securely over time.. Monitoring is needed to confirm and

quantify the injected CO2, detect leakage and adverse impacts to subsurface and surface

receptors,

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 and enable accounting and reporting of CO2 sequestration. Monitoring plans are

needed because they provide the basis and justification for using and applying selected

monitoring approaches, and for determining the spatial and temporal scope of measurements.

Monitoring plans help to ensure that the collected information and data are reliable, of high

quality, and comprehensive.

This section provides a brief description of GS systems, including an overview of the types of

geologic settings being considered for GS, the factors that influence the subsurface movement of

CO2, CO2 trapping mechanisms, and the ways in which CO2 can migrate or leak from GS

systems. This section also summarizes the purposes of monitoring and the importance of

monitoring plans.

2.1 Geologic Sequestration System

For the purposes of this document, a GS system consists of an injection zone into which CO2 is

injected, an overlying confining system, and the injected CO2 stream. An injection zone is the

geologic formation (with sufficient areal extent, thickness, porosity, and permeability) into which

CO2 is injected and in which the CO2 is intended to remain sequestered. The confining system is

the geologic formation, group of formations, or section of a formation that serves as the lowpermeability barrier to upward migration of injected CO2. The CO2 stream consists of the CO2

captured from a power plant or industrial source.

2.2 Geologic Settings

The types of geologic settings that may be suitable for GS include the following:

 Deep saline formations. Deep saline formations are geologic formations in which the

pore space is filled with saline water or brine. These formations are considered strong

candidates for GS because of their formation characteristics, such as size and depth. The

estimated CO2 sequestration capacity of deep saline formations is greater than any other

type of geologic setting. In addition, these formations are generally deeper than other

geologic settings. This comparative depth may result in a lower vulnerability of leakage

2

 For the purposes of this document, and as described in U.S. EPA (2008), a receptor is a surface or

underground feature whose presence at a GS site could affect the vulnerability of adverse impacts in the event

of unanticipated migration, leakage, or pressure changes.
need for monitoring at geologic sequestration sites

5

and migration of CO2 from the subsurface compared to other geologic formations

because with greater depth, there are fewer existing wells that may act as pathways for

leakage (Celia et al., 2009).

 Oil and gas reservoirs. Oil and gas reservoirs are potential candidates for GS because

they have stored oil and gas below the subsurface for thousands if not millions of years,

the structural characteristics of the reservoirs are generally known, and existing

infrastructure for oil and gas withdrawal can be used to inject CO2. Currently, CO2 is

injected into some of these formations to enhance oil recovery. Enhanced oil recovery

(EOR) involves injecting CO2 into geologic formations to extract oil. These geological

formations may, however, have a number of deep wells that could serve as pathways for

the leakage of injected CO2.

 Coal seams. Unminable (deep or thin) coal seams are a potential formation for GS

because CO2 has a high affinity for coal (higher than that of methane) and will adsorb to

the micropores (cleats) within the coal seams. The technical feasibility of GS in coal

seams depends on the presence of a confining system, the permeability of the coal over

the life of the GS project, the rank of the coal, and other factors (IPCC, 2005).

 Other geologic settings. A variety of other potential rock types or geologic formations or

settings could sequester injected CO2, including basalts, oil or gas rich shale, salt caverns,

and deep abandoned mines. Each of these potential reservoirs has a variety of advantages

and disadvantages for sequestering CO2. Basalts, for example, can be an effective rock

type for sequestering CO2 because of the formation of carbonates from CO2 interacting

with minerals in the basalt. Basalts may not be ideal for sequestration, however, because

they typically have low porosity and permeability, and may have fractures that can pose a

risk for CO2 migration or leakage.

2.3 Subsurface CO2 Movement

Several factors control how CO2 moves in the subsurface. The rate of CO2 fluid flow in the

subsurface depends on both fluid transport mechanisms and the properties of the injection zone

formation. Key fluid transport mechanisms include injection-induced pressure gradients that

increase fluid flow, pre-existing hydraulic gradients in the injection formation, differences in

density between CO2 and formation fluids (known as buoyancy-driven flow), and formation

heterogeneities and viscosity contrasts that result in dispersion or fingering of CO2. In addition,

diffusion can affect CO2 fluid flow, but it is typically less important than the above mechanisms

that affect the flow of CO2 in the subsurface.

Key properties of the injection zone that affect the rate of CO2 fluid flow include permeability,

thickness, and heterogeneity of the injection zone formation. If an injection zone has high

permeability, CO2 will migrate faster through the formation. Thicker injection zones also allow a

larger volume of injected CO2 to migrate. Heterogeneities such as faults, fractures, and bedding

planes may result in preferential flow pathways allowing CO2 to migrate.
need for monitoring at geologic sequestration sites

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2.4 Trapping Mechanisms

Sequestration of injected CO2 is the result of both physical and geochemical trapping

mechanisms. Physical trapping mechanisms include structural and stratigraphic trapping and

residual trapping. Geochemical trapping processes include solubility trapping, mineral trapping,

and preferential adsorption trapping. Based on IPCC (2005) and U.S. EPA (2008), these

mechanisms are described here.

 Structural and stratigraphic trapping. Structural and stratigraphic trapping are physical

trapping processes in which injected CO2 is trapped below a physical barrier that prevents

upward migration. Structural trapping results from folded or fractured geologic formations

that form a physical barrier to migration. Stratigraphic trapping specifically refers to

situations where injected CO2, being buoyant, is trapped below a low-permeability rock

formation that forms a capillary barrier, thus preventing upward migration (IPCC, 2005).

 Residual trapping. Residual trapping, also known as hydrodynamic or capillary trapping, is a

physical process that occurs when CO2 migrates to the top of a saline formation (because of

its lower density) and is retained in the pore spaces of the formation by capillary forces.

Residual trapping retention rates vary by geologic formation, but generally fall between 15%

and 25% of injected CO2 (Holtz, 2002). Residual trapping can be an effective means of

preventing injected CO2 from reaching structural and stratigraphic trapping formations

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

 Solubility trapping. Solubility trapping, also know as dissolution trapping, occurs when

injected CO2 dissolves into formation waters. The dissolved CO2 will travel with the water

rather than buoyantly rising to the top of the formation as a separate phase. In addition, when

CO2 dissolves in water, the water’s density increases, meaning waters that contain dissolved

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

IPCC, 2005).

 Mineral trapping. Mineral trapping is a geochemical process that occurs when injected CO2

reacts with formation waters, formation rocks, or both to create solid minerals such as

carbonates. These minerals can become immobilized, making this generally one of the more

desirable trapping mechanisms. Although mineral trapping can occur immediately for small

amounts of injected CO2, mineral trapping for large amounts of injected CO2 can take

millennia (Wilson, 2004; IPCC, 2005).

 Preferential adsorption. Injected CO2 can preferentially adsorb to coal seams or organic-rich

shales when injected into these types of geologic formations. The adsorption of CO2 will

displace methane bonded to the coal surface; this methane could potentially be recovered for

commercial processes (enhanced coalbed methane). Adsorbed CO2 will remain immobilized

in these formations under stable temperatures and pressures (IPCC, 2005; Haszeldine, 2006).
need for monitoring at geologic sequestration sites

7

2.5 Potential CO2 Migration and Leakage Pathways

As described in Section 2.4, several different physical and geochemical processes can trap

injected CO2 in underground geologic formations. Although natural and industrial analogs and

current GS experience support the assertion that GS projects can successfully prevent the

emission of CO2 to the atmosphere, it is important to consider processes and pathways which

could ultimately compromise the ability of a project to sequester CO2, and result in the need for

additional monitoring to quantify the injected CO2 and to detect potential leakage. Processes and

pathways that could lead to leakage are described in IPCC (2005) and U.S. EPA (2008). Three

potential leakage pathways are described here.

 Faults, fractures, and other geologic heterogeneities. Faults and fractures have been

identified, along with wells, as one of the most likely fluid conduits in GS systems. Other

geologic heterogeneities that could facilitate CO2 leakage include sedimentary rock bedding

planes, contacts between different rock types, and lenses of more permeable units (e.g., sand

lenses, or “pinches”). The role of fractures and faults in CO2 migration or leakage depends on

several factors, including their stratigraphic position within the geologic formation, whether

they are sealed or conductive, and the orientation of faults to regional stresses (Rutqvist et al.,

2007a, 2007b). These factors govern whether faults and fractures form barriers or conduits to

CO2 leakage and migration. Stable faults can be reactivated by increased pressure in the

injection zone. The potential for adverse impacts resulting from induced fractures and reactivated faults depends on the presence of receptors, the presence of additional fluidconducting pathways, and the reopening or resealing of faults by geochemical processes

(dissolution or precipitation) (IPCC, 2005).

 Wells. Wells (and other artificial penetrations such as boreholes) have been identified as one

of the most probable conduits for the escape of CO2 from the injection zone (Gasda et al.,

2004; Benson, 2005; IPCC, 2005; Carey et al., 2007). Improperly sealed and plugged wells

from previous operations in the area, such as oil and gas operations, can be open conduits to

the surface. Only wells that penetrate the injection zone can be direct leakage pathways;

however, shallower wells could be connected to faults or other features that penetrate the

injection zone. Thus, a combination of wells, faults, or other geological discontinuities can

potentially serve as a pathway for migration and leakage.

A project’s injection and monitoring wells also can be potential leakage pathways.

Experience from other analogous subsurface injection projects (such as those used in oil and

gas operations) has shown that leakage from the injection well itself, as a result of improper

completion or deterioration of the casings, packing, or cement well materials, can serve as a

well failure mode (Benson et al., 2002; IPCC, 2005).

 Excessive injection pressure. Injecting CO2 into a formation will usually result in changes in

subsurface pressure. If injection pressure exceeds fracture pressure when injecting CO2, it

can induce fracturing and fault reactivation and create potential fluid migration pathways

(Healey et al., 1968). The risk of injection pressure exceeding fracture pressure can be

reduced through understanding the relevant geologic attributes, careful site characterization,

careful operation of GS systems, and monitoring (IPCC, 2005).
need for monitoring at geologic sequestration sites

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2.6 Monitoring for Accounting and Reporting on

Geologic Sequestration

As described above, CO2 can be sequestered safely in subsurface geologic formations, but the

potential exists for CO2 to migrate from the injection zone and be released to the atmosphere or

to other receptors, such as underground sources of drinking water. Thus, monitoring the fate of

the injected CO2 is necessary to determine whether injected CO2 is being released to the

atmosphere. Monitoring can be used to confirm that injected CO2 remains in the intended

formation, to detect leakage if it occurs, and thus to accurately account for the injected CO2.

Monitoring site characteristics, project operations, and CO2 behavior in the subsurface can

ultimately contribute to the dual goal of demonstrating sequestration and detecting leakage if it

occurs. This section describes the different purposes for monitoring. More information on the

techniques used to monitor for these different purposes is provided in Section 3.

Specific purposes for monitoring for accounting and reporting could include:

 Establishing baseline conditions. Because CO2 is ubiquitous in the environment and

concentrations vary over space and time (e.g., diurnally, seasonally, annually), determining

background levels of CO2 and understanding natural fluctuations is necessary to discern

whether detected CO2 is attributable to leakage or to preexisting sources. It is also important

to establish baseline conditions before injection because changes in the parameters before

and after injection and over time assist in tracking the injected CO2.

 Ensuring injection controls. Monitoring the condition of the injection well, the injection rate,

and wellhead and formation pressure is important to verify the amount of CO2 injected and to

avoid leakage.

 Confirming the quantity of injected CO2. Determining the amount (mass) of CO2injected is

important for accounting and verification purposes. Detecting unanticipated migration and

leakage. Monitoring in the subsurface can be used to detect unanticipated migration of CO2

in the subsurface. Monitoring at the surface can be used to help confirm that leakage has not

occurred. If CO2 has leaked, surface monitoring can help quantify the amount released to the

atmosphere. This monitoring information can also be used to calibrate models, to evaluate

model predictions, and to confirm trapping mechanisms.

 Providing oversight of targeted locations and site features. Specific locations and site

features should be identified for targeted monitoring if they are found to be contributing to

site vulnerability. For example, existing wells and faults should be targeted for focused

monitoring because of their elevated potential to act as CO2 conduits, which can result in

leakage.

2.7 Other Monitoring Purposes

In addition to monitoring for accounting and reporting of injected CO2, monitoring can also be

used for the following purposes:
need for monitoring at geologic sequestration sites

9

 Informing mitigation actions. Mitigation refers to actions taken to prevent CO2 from

migrating or leaking, or to prevent unanticipated pressure changes from occurring, as well as

actions taken to reduce the severity of impacts in the event of leakage, migration, or pressure

changes. For example, corrective actions such as well plugging and cementing can prevent

CO2 from migrating through, or leaking from, injection wells and other artificial

penetrations. In existing or induced fractures and faults, mitigation actions can include

modifying injection operations to prevent CO2 migration in the direction of the conduit.

Monitoring can enable early detection of vulnerabilities or impacts as well as effective

mitigation actions. It can also be used to inform and evaluate the success of mitigation

actions.

 Detecting induced microseismicity. Microseismic activity (minor earthquakes) may be

induced by CO2 injection operations under certain conditions (e.g., if certain pressure

thresholds are exceeded). Monitoring can help recognize such activity and mitigate impacts

before they become severe.

 Assessing environmental and human health impacts of leakage if they occur. If CO2 leaks to

the surface or to subsurface resources, adverse impacts to the environment and human health

can vary depending on the concentration of the CO2 and the exposure. Monitoring techniques

can help assess the severity of adverse impacts by providing information on the amount of

leaked CO2 and potential exposure scenarios. Site-specific receptors may also be targeted for

monitoring, such as sensitive ecosystems or underground sources of drinking water, to ensure

that they are not adversely impacted by unanticipated CO2 migration and leakage.

2.8 Need for Monitoring Plans

A monitoring plan provides the basis and justification for using and applying the selected

monitoring approaches, it describes the spatial and temporal scope of measurements, and it

provides information about the monitoring technologies. The plan can also describe analytical

and data interpretation procedures, quality control and quality assurance plans, actions to be

taken based on monitoring results, and data management practices. It can include reporting

schedules and regular equipment inspection schedules, and specify the need to use qualified,

appropriately trained personnel to conduct the monitoring. Without a robust plan, monitoring

data can be unintentionally biased, misleading, or of poor quality, and result in unreliable

information, which can hinder the ability to detect and address site vulnerabilities, including the

potential for leakage.

In sum, a robust monitoring plan can help:

 Demonstrate that selected monitoring methods are suitable for the GS project and conform to

the specific monitoring needs at the site and any regulatory requirements

 Ensure that qualified personnel are responsible for monitoring and assist them with

effectively generating accurate and consistent data

 Provide regulatory agencies with a starting point for analyzing and verifying the use of

appropriate monitoring technologies and interpreting monitoring results
need for monitoring at geologic sequestration sites

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 Ensure the usefulness of collected data through a thorough and properly managed data

system that allows for efficient and informative analysis.

2.9 Summary

CO2 can be safely sequestered in subsurface geologic formations through a combination of

physical and geochemical processes. There are pathways (such as wells, faults, and other

geologic discontinuities), however, that can potentially result in CO2 migration and/or leakage.

Measures can be taken to reduce the potential for migration and leakage of injected CO2.

Monitoring can be used to confirm the location, quantity, and physical status of injected CO2,

and to detect any leakage if it occurs. Monitoring plans are important because they can ensure

that the collected monitoring information and data are reliable, of high quality, and

comprehensive. Section 3 introduces the key components of monitoring plans and explains how

monitoring plans can be developed based on these key components.
3. KEY COMPONENTS OF A ROBUST

MONITORING PLAN

11

To address the purposes for monitoring summarized in Section 2, four key components of a

robust monitoring plan are described here. A robust plan: 1) is tailored to site-specific

conditions; 2) addresses the different stages of a GS project; 3) identifies appropriate techniques

for different monitoring purposes; and 4) is developed as a flexible, dynamic plan that allows for

modification or adaptation based on monitoring results. This section provides a description of

each of these key components, and the development of a monitoring plan that includes them.

3.1 Tailoring Monitoring Plans to Site-Specific Conditions

Tailoring to site-specific conditions is a key component of robust monitoring plans because

geologic systems are complex, and geologic attributes can be highly variable from location to

location. Receptors and potential adverse impacts also can vary from one site to the next. This

complexity means that a “one size fits all” monitoring approach may not be able to focus on the

specific features that increase the vulnerability of a particular GS project to CO2 migration and

leakage and potential impacts to human health and the environment. Site-specific monitoring

plans can identify and allow the monitoring program to be focused on the largest risks or

vulnerabilities associated with that particular site. A tailored approach may enable the most costeffective use of monitoring resources. Thus, a robust monitoring plan identifies the geologic

attributes and the potential impacts and receptors to be monitored, and lays out systematic

processes for evaluating and monitoring them.

Drawing upon EPA’s VEF for GS (U.S. EPA, 2008), this section identifies those geologic

attributes, impact categories, and receptors to be considered when developing a robust

monitoring plan. It also identifies the systematic evaluation processes that can be incorporated

into monitoring plans to assess them. The VEF was developed by EPA as a first step toward a

conceptual framework designed to aid regulators and other technical experts in framing key sitespecific considerations that might require in-depth evaluation for GS project design and sitespecific risk assessment, monitoring, and management.

The overall VEF conceptual model is shown in Figure 1. The VEF also includes a series of

evaluation processes depicted in flowcharts, some of which are shown in this section. These

processes and flowcharts reflect an approach developed by EPA to evaluate the vulnerabilities

associated with GS. They are not guidance, nor are they meant to supersede any regulations

regulations.

3.1.1 Identifying and Evaluating Geologic Attributes

Several geologic attributes may affect a particular GS site’s vulnerability to CO2 migration and

leakage and unanticipated pressure changes which may induce leakage. These attributes, which

are described in further detail in EPA’s VEF (U.S. EPA, 2008), are listed and briefly described

here. Examples of systematic approaches from the VEF for evaluating these attributes are also

presented.
key components of a robust monitoring plan

12

The specific geologic setting may influence the relative importance of the geologic attributes

when tailoring the monitoring plan. For example, if the geologic setting is a depleted oil

reservoir with a well-characterized confining system and structural trapping mechanism, the

monitoring plan might focus on assessing the proper performance of wells in the area. However,

if a project is a deep saline formation where there are few wells, but the confining system is less

well defined, the monitoring plan might focus on characterizing the confining system and

tracking the migration of the plume. If solubility trapping is an important component of the GS

system, the monitoring plan might focus on demonstrating that the CO2 is dissolving in the

formation fluid and geochemical reactions are progressing as expected.

Attributes Related to the GS Confining System

The confining system is the geologic formation, group of formations, or section of a formation

that serve as the low-permeability barrier to upward migration of injected CO2. Based on

Figure 1. Vulnerability Evaluation Framework Conceptual Model

Source: U.S. EPA, 2008.
key components of a robust monitoring plan

13

U.S. EPA (2008), the following are geologic attributes of the confining system that can influence

the vulnerability of a GS site to migration or leakage, and hence may be considered when

developing a monitoring plan:

 Lateral extent. Lateral extent refers to the surface area of the confining system that overlies

the spatial extent of the CO2 plume and its associated pressure front.The potential for CO2

migration and leakage can increase if the lateral extent of the confining system does not

extend beyond the footprint of the CO2 plume and its pressure front.

 Capillary entry pressure. Capillary entry pressure refers to the additional pressure needed

across the interface of two immiscible fluid phases (e.g., CO2 and formation water) for CO2

to enter the confining system. The risk of CO2 migration and leakage increases if the

injection pressure exceeds the capillary entry pressure. The capillary entry pressure of a

geologic formation can be evaluated using CO2 column height and injection pressure

(Harrington and Horseman, 1999).

 Permeability. Permeability refers to the ease with which fluids flow through the pore spaces

of a geologic material. A GS site can have increased vulnerability to CO2 migration and

leakage if the rocks of the confining system have high permeability. Clay, shale, or siltstone

have low permeability and are thus desirable confining system rock materials.

 Travel time. Travel time is the period of time necessary for a fluid to move across the

thickness of the confining system. Travel time, which is typically measured in years, depends

on the confining system’s thickness, permeability, diffusion, ability to retard CO2 migration,

and other geochemical reactions that could slow or accelerate CO2 movement. Short travel

times increase vulnerability.

 Wells. Wells, boreholes, and other anthropogenic penetrations of the geologic formation can

provide direct pathways for the migration and leakage of injected CO2. Artificial penetrations

are considered one of the most likely pathways of CO2 leakage and migration. The

vulnerability posed by wells is a function of well depth, well integrity, and the density of

penetrations in the confining system.

 Faults/fracture zones. Faults result from the compression or separation of crustal rock.

Fractures, which are local breaks in the rock formation, can occur when stress on the rock

exceeds the shear strength of the rock. The vulnerability of a site to faults and fractures can

be evaluated based on the density, stratigraphic position, connectivity, stress level,

orientation, and fault reactivation pressure of a rock and whether the fault promotes or

inhibits fluid flow (Omre et al, 1994; Lewicki et al., 2006; Wilkens and Naruk, 2007).

 Geochemical processes. Geochemical reactions operating in the confining system can affect

the ability of the confining system to confine CO2. For example, increased acidity in geologic

formations from CO2 dissolved in formation fluids can dissolve minerals, thereby decreasing

CO2 travel times and increasing vulnerability to migration and leakage. In contrast, other

geochemical reactions between CO2, pore waters, and confining system formations can form

new minerals that may partially plug pores, and lower the porosity of the confining system,

and thus potentially lower vulnerabilities of migration and leakage. Geochemical processes
key components of a robust monitoring plan

14

can be evaluated using metrics such as changes in concentrations of chemical constituents

over time, for example, pH and dissolved carbon concentrations.

 Tectonic activity. Areas with elevated tectonic activity are more likely to experience faulting

and fracturing, which can result in increased vulnerability to CO2 migration and leakage, and

are typically not suitable for GS (IPCC, 2005). Volcanic activity, a particular form of

tectonic activity, can also increase the vulnerability of GS sites to CO2 migration and

leakage, with vulnerability increasing with proximity to volcanic activity (USGS, 2008).

 Geomechanical processes. Geomechanical processes affect the structural integrity of

geological formations. These processes can be measured using the following metrics: fracture

pressure, fracture/fault reactivation pressure, and the orientation of fracture/fault relative to

the orientation of the principal regional stress regime. The vulnerability of CO2 migration and

leakage is elevated if the fracture pressure or fracture/fault reactivation pressure is exceeded.

EPA has developed an evaluation process for the confining system that considers all of these

attributes. The process, illustrated in Figure 2, can inform site characterization and monitoring

plan development. The confining system evaluation process, described in greater detail in

U.S. EPA (2008), involves: 1) establishing that a confining system is present over the necessary

lateral extent; 2) evaluating the physical properties of the confining system to determine if it

ensures adequate confinement of fluids under the proposed operating conditions; and

3) evaluating the integrity of the confining system. Wells and faults are evaluated in this last

step. Given their potential to act as migration and leakage pathways, and the complexities

involved in their evaluation, EPA has also developed evaluation processes that can help

characterize the vulnerability caused by wells (Figure 3) and faults (Figure 4) at a GS site. These

evaluation processes can be taken into consideration when developing a robust monitoring plan

that is tailored to site-specific conditions.
key components of a robust monitoring plan

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Figure 2. VEF Confining System Evaluation Process

Source: U.S. EPA, 2008.

U.S. EPA, 2008.
key components of a robust monitoring plan

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Figure 3. VEF Wells Evaluation

Source: U.S. EPA, 2008.
key components of a robust monitoring plan

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Figure 4. VEF Faults Evaluation

Source: U.S. EPA, 2008.
key components of a robust monitoring plan

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Attributes Related to the Injection Zone

Key attributes that affect the capability of the injection zone to sequester CO2 were identified by

U.S. EPA (2008) and are described here. These attributes have implications for a site’s

vulnerability to CO2 migration, leakage, and unanticipated pressure changes, and may therefore

be evaluated during site characterization and considered when developing monitoring plans.

 Physical capacity. Physical capacity, also known as storage capacity, refers to the volume of

a geologic formation that can be used for sequestering injected CO2. Once CO2 is injected

into a formation, it will reside in the pore space of the formation, displacing fluids previously

held within the pore space. Factors that impact physical capacity include thickness, surface

area, effective (or interconnected) porosity, CO2 density, and residual water saturation of the

formation (Zhou et al., 2008).

 Injectivity. Injectivity is the relative ability of a fluid to be injected into a geologic

formation. Injectivity is a function of the physical characteristics of the injection zone and the

operational factors of the injection process. Key metrics for evaluating the injectivity of the

injection zone include injection zone thickness, permeability, and pressure within the

injection zone (IPCC, 2005).

 Geochemical and geomechanical processes. Geochemical and geomechanical processes

can influence the vulnerability of a potential geological sequestration site by altering

injection zone capacity. Geochemical processes can make the injection zone more or less

favorable for GS. For example, CO2-rich waters can dissolve minerals, which can increase

the porosity and storage capacity of a geologic formation (Doughty et al., 2001). In the

context of the injection zone, unfavorable geochemical processes are those that reduce a

formation’s physical capacity by precipitating minerals that can plug pore space and reduce

porosity (Knauss et al., 2003). Geochemical processes can be evaluated using metrics such as

changes in concentrations of chemical constituents over time, for example, pH and dissolved

carbon concentrations.

Geomechanical processes are physical processes that affect the structural integrity of host

rock formations. Generally, geomechanical processes include the formation of fractures or

reactivation of faults as injection pressures exceed the physical limits of the geologic

formation. Because of high variability in the structural integrity of different rock formations,

metrics for analyzing vulnerability resulting from geomechanical processes are highly sitespecific and will need to be determined prior to injection (Streit and Hillis, 2003; Streit et al.,

2005).

Figure 5 shows an evaluation process for the injection zone that incorporates all of these

attributes. The process is described in further detail in U.S. EPA (2008).
key components of a robust monitoring plan

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3.1.2 Identifying and Evaluating Potential Receptors and Impact Categories

Although the primary objective of this technical support document is to identify monitoring

needs for CO2 accounting and reporting purposes, a robust monitoring plan for a GS site may

also consider potential impacts to all potential receptors and impact categories. In addition,

unanticipated CO2 migration or leakage can sometimes first be detected through monitoring of

impacts to human health/welfare, ecosystems, groundwater and surface water, or the geosphere.

Key potential impact categories and specific impact receptors that are important in monitoring

plans are identified here. More information on these receptors and impact categories can be

found in U.S. EPA (2008).

 Potential human health and welfare impacts. Exposure to elevated levels of CO2 can result

in a variety of adverse health effects, because CO2 is an asphyxiant. The vulnerability of

human populations to migration and leakage of CO2 depends on proximity to the leakage

area, population density, the length of exposure, and the concentration of the release.

Exposure to CO2 can result in minor, reversible health effects such as increases in breathing

rate and headaches; higher concentrations can result in mortality. Some populations are at

Figure 5. VEF Injection Zone Evaluation

Source: U.S. EPA, 2008.
key components of a robust monitoring plan

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greater risk and are covered by executive orders, such as children, who are more sensitive

than adults to many substances (Benson et al., 2002).

Migration or leakage of injected CO2 can also negatively impact cultural and recreational

resources and places of archaeological importance by making human use unsafe or degrading

the actual resource (e.g., increased CO2 in the atmosphere can dissolve historical artifacts

more rapidly) (U.S. DOE, 2007). Economic resources can also potentially be affected by GS.

For example, leakage of CO2 from the subsurface can limit access to forestry, agricultural,

and mineral resources. Economic resources may be impacted even in the absence of leaking

and migration, because GS operations can limit other economic activities (e.g., by reducing

access to hydrocarbon or mineral reserves within the area of sequestration).

 Potential ecosystem impacts. Both sensitive species and legislatively protected species are

key ecological receptors that could be impacted in the event of leakage of CO2. Sensitive

species are organisms that, because of either their physiology or their behavior, are highly

vulnerable to adverse impacts associated with CO2 migration or leakage. Sensitive species

include soil-dwelling animals and microbes, plants, surface-dwelling animals, aquatic

organisms, calcifying aquatic organisms, and deep geologic ecosystems (i.e., microbes deep

in the earth’s crust). These species can serve as indicators of the potential impacts of CO2

leakage on larger ecosystems, because they will generally experience adverse impacts before

other less vulnerable species. If steps are taken to ensure the protection of sensitive species,

project owners and operators can be reasonably assured that less sensitive species in the area

are protected. Because of the legal status of protected species, GS projects may need to show

that these species and their habitat will not be adversely impacted (U.S. EPA, 2008).

 Potential groundwater and surface water impacts. GS has the potential to adversely

impact both groundwater and surface water, with groundwater at greater risk because of its

proximity to underground injection sites. Injected CO2 that migrates could increase acidity in

groundwater and mobilize metals and other impurities that can degrade the quality of these

resources. If native brines associated with GS migrate to overlying groundwater, total

dissolved solids concentrations can increase and adversely affect drinking water quality.

Total dissolved solids, buffering capacity, mineralogy, sorbed contaminants, and microbial

populations in surface water and groundwater can all be influenced by GS systems (Jaffe and

Wang, 2003;Wang and Jaffe, 2004; Apps, 2006; Nicot et al., 2006; Tsang et al., 2007).

Changes in pressure resulting from CO2 injection also can impact groundwater by affecting

groundwater flow direction, water table levels, and gaining and losing reaches of streams.

Variations in these parameters can affect the usefulness of groundwater as a source of

drinking water and can damage streams and intertidal zones (Nicot et al., 2006; Tsang et al.,

2007).

 Potential geosphere impacts. Injection of CO2 into the subsurface can result in increases in

pressure that directly alter the geosphere (e.g., by fracturing and faulting rock formations).

These changes can result in new pathways for CO2 to migrate or leak from the injection zone.

Other potential changes include increases in seismic activity and landscape deformation

through local uplifting of rock formations (Healey et al., 1968; Quintessa, 2004; Birkholzer

et al., 2007).
key components of a robust monitoring plan

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3.1.3 Defining the GS Footprint for a Site-Specific Monitoring Plan

Determining the geographical extent of the GS footprint is essential for establishing the spatial

area to be monitored for leakage and for other potential adverse impacts. The size and shape of

the GS footprint will depend on the geologic attributes of the confining system and injection

zone, as described earlier. It will also depend on the total amount of CO2 injected over the

lifetime of the project, and operational factors such as the injection rate and number and location

of injection wells. The size of the plume will increase during the injection period, and because

supercritical, separate-phase CO2 is buoyant and has lower viscosity than water and brine, the

plume may also continue to change shape and position for some time after injection stops. At

some point, trapping mechanisms, including dissolution, physical, and mineral trapping, are

anticipated to eventually minimize and immobilize the CO2 plume. (For more information on

trapping mechanisms, see Section 2.3 and U.S. EPA, 2008.)

In the VEF, EPA has identified different example scenarios that might be encountered in GS

systems, and approaches to delineating the footprint for these example scenarios. Delineation of

the footprint may be refined at multiple stages of the project as the footprint expands and

changes shape. As indicated in Figure 6, it is anticipated that modeling in combination with sitespecific data may be needed to delineate the footprint. The footprint can serve as the basis for

defining the area where baseline conditions are established before injection and the area to be

monitored during the different project phases.

Figure 6. VEF GS Footprint Delineation

Source: U.S. EPA, 2008.
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3.2 Monitoring During the Different Phases of a Geologic

Sequestration Project

GS projects can be described in terms of four phases: pre-injection (or site

selection/characterization), injection (or operational), post-injection (or confirmation), and site

closure (or post-closure). Certain monitoring needs are common across the different phases of a

project, others are distinct to a particular phase with distinct monitoring activities and needs. This

section summarizes information pertinent to all phases of a project, it also briefly describes each

phase, and the type of monitoring activities associated with each phase.

3.2.1 Monitoring Needs for All Phases of a Project

A robust monitoring plan delineates the phases of a GS project, indicates the purposes for

monitoring during the different phases, and lists the specific monitoring techniques to be

employed during each project phase. Robust monitoring plans explain the selected techniques

and the rationale for their selection.

For each phase of the project, the plan may list the spatial area and depths to be monitored by the

selected techniques, and the locations where different measurements will be taken, including

locations or features targeted for monitoring because of elevated vulnerabilities of leakage or the

presence of particular receptors. The plan also can indicate the locations of fixed monitoring

instruments and the frequency of measurements for the different techniques. It is anticipated that

most of these details will be informed by the initial site characterization work undertaken during

the pre-injection phase, and will be modified and refined based on initial site data and modeling

results. Thus, a robust plan is flexible and adaptable as new information and data are collected.

The plan may describe what models will be used, the purpose of the modeling, a rationale for the

selected models, the required model inputs, expected model outputs, calibration and validation

procedures, and how the model results will be used to inform different aspects of the project,

including future monitoring efforts.

For all monitoring and modeling activities, reporting schedules and equipment maintenance

schedules may need to be developed for the different project phases. The monitoring plan can

specify, when appropriate, the use of qualified personnel for the installation and operation of

monitoring technologies, use of models, and collection and interpretation of monitoring data. A

robust monitoring plan would also detail quality assurance and quality control procedures, and

data and information management plans for each project phase. To the extent possible, the plan

would specify how monitoring results will be incorporated into the monitoring program, and how

results from one phase will inform activities during the next phase of operations.

The rest of this section presents additional phase-specific factors that may need to be taken into

consideration when developing the monitoring plan.
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3.2.2 Pre-Injection Phase

The data and information collected during the pre-injection phase inform both project design

parameters (e.g., capacity and injectivity) and GS system monitoring. Other activities during this

phase may include project design, obtaining permits, and infrastructure construction. Monitoring

activities during the pre-injection phase largely take place as a part of site characterization

(Benson, 2007a; Johnson, 2009; NETL, 2009) and include: establishing the spatial area to be

monitored during the injection phase; identifying locations of potential vulnerabilities or

sensitive receptors that may need to be specifically targeted for monitoring3; establishing

environmental baseline (pre-injection) conditions; and serving as the basis for tailoring the

monitoring plan to site-specific conditions. All of these aspects of site characterization are

important for establishing an effective monitoring program, and are discussed in further detail

below.

Establishing the spatial area to be monitored during the injection phase will be based on site

characterization results and on the total amount of CO2 planned to be injected over the lifetime of

the project, and can also be informed by predictive model simulations. Site characterization data,

along with vulnerability or risk assessment modeling, can identify areas of elevated

vulnerabilities that may need targeted monitoring beyond the base level set for the area of

evaluation. For example, targeted monitoring may be needed for wells or faults to detect

pathways of migration and leakage of CO2 and near populations to help in assessment of

environmental and human health impacts.

Baseline conditions may be established for the attributes and receptors identified in Section 3.1.

Establishing environmental baseline conditions before injection is important for several reasons.

Changes over time in the parameters used to monitor CO2 can be used to track the injected CO2.

Thus, successful monitoring requires establishing pre-injection baseline conditions over time and

space. This is also important for surface measurements of CO2 that could detect leakage. Because

CO2 is ubiquitous in the environment and concentrations vary over space and time

(e.g., diurnally, seasonally, annually), baseline measurements may be needed over a prolonged

period in multiple locations to account for such variability. Such baseline monitoring is

necessary to distinguish between background levels of CO2 and potential unanticipated migration

and leakage. Identifying impurities in the proposed CO2 stream can also aid in distinguishing

between injected and ambient CO2 because such impurities may be unique to the stream, and not

otherwise found in the atmosphere.

Finally, because monitoring is used both to confirm the quantity of injected CO2 and to detect

and quantify any unanticipated migration and/or leakage if it does occur, baseline measurements

will likely need to cover as wide an area as reasonably anticipated to be affected by the project

(possibly based on modeling multiple scenarios), the full project depth profile (from the injection

zone depth, through the near-surface and surface environments, to the atmosphere), and include

measurements using all of the techniques expected to be used during the subsequent project

phases.

3

 With regard to wells, boreholes, and other anthropogenic penetrations, establishing an accurate GPS location

database can be critical to facilitate rapid inspection.
key components of a robust monitoring plan

24

A tailored, site-specific monitoring plan can then be established based on the results of these site

characterization activities. A site-specific plan can focus on the GS footprint and depth profile

delineated during site characterization. If relevant, it can also specify locations for targeted

monitoring due to elevated vulnerability of adverse impacts or due to the presence of sensitive

receptors. The monitoring plan also can include mitigation options for areas of higher

vulnerability. Note that multiple techniques may be tested during pre-injection and early

injection to identify the most appropriate techniques for particular site-specific conditions.

Working through the systematic evaluation processes described in Section 3.1 would be

appropriate for the pre-injection phase.

3.2.3 Injection Phase

The injection phase of a GS project is when CO2 is injected into the subsurface. During the

injection phase, the main purposes of monitoring are to confirm and quantify the amount of CO2

injected, and to detect any unanticipated migration and leakage. During this phase, monitoring

can also inform any necessary mitigation actions and help to assess their success. Accordingly,

monitoring activities during this phase can be grouped into the following categories:

 Ensuring injection controls

 Monitoring the location and quantity of injected CO2

 Addressing targeted monitoring needs

 Detecting migration or leakage of CO2

 Informing mitigation actions (if such actions take place).

Ensuring injection controls includes monitoring the condition of the injection well materials,

wellhead and formation pressures, and injection rates. Monitoring the condition of well materials

is important because experience from other injection projects (such as those used in oil and gas

operations) has shown that leakage from the injection well itself, as a result of improper

completion or deterioration of the casings, packing, or cement well materials, is a potential t

failure mode (Benson et al., 2002). Existing well monitoring technologies can be adapted to GS

applications, and these, as well as other injection control monitoring technologies, are discussed

in Section 3.3. Data obtained from monitoring injection rates can be used in real time to avoid

overpressuring and, over the long term, can be used to help quantify and confirm the amount of

CO2 injected during the life of the project.

Monitoring the location and quantity (volume and mass) of injected CO2 is a key part of

confirming that the CO2 remains in the subsurface where intended and that leakage has not

occurred. The spatial area of evaluation can be delineated based on the results of the initial site

characterization and modeling simulations. Interpreting monitoring data can help refine initial

model predictions and alter the delineated spatial area of evaluation. If significant unanticipated

migration of CO2 in the subsurface occurs, the monitoring plan may need to be adapted and

monitoring locations, frequencies, and applied technologies may need to be changed to follow

the new CO2 trajectory. If there is concern that the unanticipated migration can lead to leakage,
key components of a robust monitoring plan

25

then the monitoring results can be used to inform mitigation actions. The monitoring information

also can be used to confirm the effectiveness of physical and geochemical trapping mechanisms

that can control where the CO2 comes to be located. Specific subsurface monitoring technologies

are discussed in Section 3.3.

Monitoring targeted locations can help track areas with elevated vulnerability of leakage or

elevated vulnerability for injection phase impacts. Higher vulnerability areas and pathways

would be identified during site characterization and vulnerability/risk assessment modeling. For

example, existing wells and faults may need to be targeted in monitoring plans if elevated

potential for leakage of CO2 along these features exists. The particular vulnerabilities of sensitive

species or human populations near the site also can be considered with targeted monitoring.

Detecting any unanticipated migration and leakage of CO2 is an important part of monitoring

during the injection phase. Monitoring at the surface can be employed to determine if any

injected CO2 has leaked into the atmosphere. If there is unanticipated migration of CO2 at depth,

or in the near-surface (e.g., if CO2 escapes from the confining system and migrates into

groundwater aquifers), monitoring can determine if the CO2 is reaching near-surface or surface

environments. If CO2 has leaked, surface monitoring can help to quantify the amount released to

the atmosphere.

Some key factors to consider when developing the monitoring plan for the injection phase of a

GS project include:

 Tailoring the frequency of measurements of different parameters to their specific monitoring

purposes. For example, measurements at the wellhead for ensuring injection controls will

most likely be made at a very high, if not continuous, rate. The frequency of measurements

taken for targeted monitoring of specific site features and receptors will be less frequent.

Measurements to confirm the location and quantity of injected CO2, such as conducting

seismic surveys, may be even less frequent.

 Tailoring the frequency of measurements based on the degree of site knowledge. Overall, the

frequency of measurements may be greatest during the early part of the injection phase, when

the least is known about the system and data are needed for model and instrument calibration.

Longer time intervals between measurements may be sufficient later in the injection phase. In

addition, the early part of the injection phase is likely to be the most active and intensive

period for modeling activities, including model calibration and refinement, and for

verification as initial field results are obtained. The intensity of modeling activities may slow

once models are refined and are able to adequately predict subsurface CO2 movement,

location, and quantity.

 Keeping the monitoring plan flexible and responsive to initial field results. Significant

changes to the monitoring plan may be needed during the injection phase, if monitoring

results reveal that actual CO2 movement and location in the subsurface are different than

predicted.
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26

3.2.4 Post-Injection Phase

The post-injection phase occurs when injection has stopped, and wells are plugged (Benson et

al., 2004; NETL, 2009). Injection control monitoring is no longer needed, but monitoring to

ensure effective closure and plugging of injection and monitoring wells is likely to be needed.

Subsurface and surface measurements can be taken to confirm the final amount of CO2 injected

into the subsurface. Continued monitoring of targeted locations or features of elevated

vulnerability may be needed, depending on long-term risk assessment results.

 Decisions are likely to be made about the types and frequencies of long-term monitoring for

the post-injection phase. These decisions would be based in part on site performance, and

might include selecting a smaller set of the monitoring techniques used during the injection

phase.

 The monitoring plan will by necessity need to be open and flexible until this site closure

phase is reached and decisions about site closure monitoring can be made based on actual site

performance during operations.

3.2.5 Site Closure Phase

The site closure phase occurs after a project has been allowed to close. During the site closure

phase, a base level of periodic monitoring can be used to demonstrate that the GS project is

continuing to perform as expected, that long-term sequestration mechanisms are working, and

that the CO2 is sequestered in the subsurface.

Specific considerations for this phase include the following:

 Monitoring needs, including techniques and frequencies of measurements, will be informed

by the monitoring results obtained during the injection and post-injection phases of the

project.

 Targeted monitoring of specific site features may need to continue if it is determined that

they pose continued, long-term vulnerabilities of leakage or other adverse impacts.

 Although less frequent monitoring may be needed during the site closure phase, the

monitoring plan would include contingency plans for leakage or other matters that could

arise that require additional monitoring.

3.3 Identifying Appropriate Monitoring Techniques

Monitoring techniques to be incorporated into a monitoring plan may be selected based on the

specific monitoring purposes identified. Section 2 described different monitoring purposes,

including accounting and verification of CO2. A robust monitoring plan identifies the parameters

that require monitoring, and then selects the techniques best suited to measure these parameters.

Once these steps are taken, baseline measurements can be taken before injection, followed by

measurements during and after injection, at an interval appropriate to the measurement technique

and purpose (NETL, 2009).
key components of a robust monitoring plan

27

Some technologies may be used across all phases of a project, while others may not. Selecting

monitoring techniques according to monitoring purpose in some instances will also implicitly

assign certain technologies to particular phases of a GS project. For example, monitoring to

ensure injection controls, such as measuring CO2 injection rates, will take place only during the

injection phase. In contrast, measurements for confirming the location and amount of injected

CO2 could be made during all phases. Similarly, instruments installed for targeted monitoring of

old wells, faults, or other site features may be needed through multiple stages of a project. The

design of the monitoring plan will need to reflect the use of specific monitoring equipment,

including specifications for equipment maintenance schedules and for selecting durable

instrument materials.

Although some technologies serve only one main monitoring purpose, such as eddy flux

detectors at the surface to detect leakage, others can serve multiple monitoring purposes. For

example, seismic surveys can help locate and quantify the sequestered CO2 and also can reveal

potential migration and leakage pathways of CO2 to the surface. Natural or introduced tracers

can help locate the injected CO2, and monitor targeted site features such as potential migration

through discontinuities in the confining system.

The remainder of this section provides brief descriptions of monitoring techniques. The

information provided here can guide the selection of techniques when developing a monitoring

plan, but it is not meant to be an exhaustive summary of all available or developing technologies.

More detailed information on monitoring technologies is available from other resources,

including the comprehensive index of monitoring techniques compiled by NETL (2009). Other

resources include the International Energy Agency Greenhouse Gas (IEA GHG) Programme’s

Monitoring Technique Selection Tool, which is a Web-based tool that describes monitoring

technologies and suggests techniques for different monitoring purposes based on user-defined

conditions and criteria (IEA GHG, 2009).

3.3.1 Monitoring Techniques to Ensure Accurate Accounting and Reporting of CO2

Emissions

Monitoring can be used to confirm that injected CO2 remains underground, to detect any leakage

if it occurs, and thus to accurately measure the injected CO2 for accounting and reporting

purposes. A variety of monitoring techniques can ultimately contribute to ensure accurate

accounting and reporting of CO2 emissions. Technologies for detecting and quantifying CO2

leakage are discussed in more detail in the subpart RR General TSD (see docket EPA-HQ-OAR2009-0926).

 Monitoring techniques for establishing a baseline. The monitoring techniques used to

establish a baseline are largely dependent upon site-specific conditions. Examples of some

of the technologies that may be applied include seismic imaging; wellhead and formation

pressure monitoring techniques; temperature and fluid composition measurement techniques;

electrical measurements of subsurface conductivity/resistance; atmospheric and soil gas

monitoring technologies; and land surface deformation monitoring technologies (Benson

et al., 2004; WRI, 2008; Bacon et al, 2009; Johnson, 2009).
key components of a robust monitoring plan

28

 Monitoring techniques for ensuring injection controls. Monitoring technologies that could be

used to ensure that injection controls are handled appropriately include wellhead and

formation pressure gauges, core logging, and wellbore annulus pressure measurements

(Benson et al., 2004; IPCC, 2005; Benson, 2007a; Freifeld et al., 2009; NETL, 2009).

 Monitoring techniques for detecting unanticipated migration of CO2 and leakage.

Unanticipated migration may be detected using many of the same techniques described

below for confirming the location and quantity of injected CO2, including seismic surveys,

electrical and electromagnetic methods, and fluid and mineral sampling. Several monitoring

techniques can be used to detect leakage, including sampling air using eddy covariance,

infrared and other techniques; sampling soil gas with soil gas probes; using tracers (small

quantities of a chemical compound or isotope added to trace flow patterns); monitoring for

land surface deformation; measuring productivity of local flora and fauna; and sampling

overlying hydrologic systems (Chabora and Benson, 2009; Darby et al., 2009; Jones et al.,

2009; Schwarz et al., 2009).

 Monitoring techniques for confirming the location and quantity of injected CO2. The

movement and fate of injected CO2 are influenced by injection-related factors, properties of

the CO2, and properties of the GS formations. Thus, many different subsurface parameters

may need to be measured to assess the location and quantity of the injected CO2. Both

existing pressure gradients and those induced by injection can influence CO2 movement.

Techniques that measure the injection rate and formation pressure gradients can thus help

monitor CO2 in the subsurface. Other examples of techniques that may be used to confirm the

location and quantity of injected CO2 include seismic surveys; electrical and electromagnetic

methods, such as electrical resistance tomography; gamma ray, resistivity and other types of

logging; and fluid and mineral sampling methods (Benson et al., 2004; Benson, 2007a;

Bachu and Bennion, 2009; Bachu et al., 2009; NETL, 2009).

 Monitoring techniques for providing oversight of targeted locations and site features. In the

site characterization phase, monitoring techniques can be selected to target attributes to

protect against increased vulnerabilities (Benson et al., 2004). Monitoring during the

injection, post-injection, and site closure phases can help identify existing or newly

developed vulnerabilities and inform the application of additional, targeted monitoring

techniques. The specific type of monitoring technique to be used will depend upon the

particular vulnerability that is being addressed, and could include many of the types of

techniques listed above, such as seismic surveys, tracers, borehole logs, pressure

measurements at the wellhead and in the formation, formation fluid sampling, surface water

sampling, and air and soil gas sampling.

3.4 Ensuring a Flexible and Responsive Plan

Monitoring plans will need to be dynamic, flexible, adaptable, and responsive to ongoing

monitoring results. For example, data gathered during the site characterization or pre-injection

phase will likely be used as inputs to models and for model calibration (Bacon et al., 2009).

Monitoring data gathered during the injection phase can be used to refine the models, if needed,

and to modify predictions about CO2 subsurface migration. If unanticipated migration or leakage
key components of a robust monitoring plan

29

is detected, the monitoring plan likely will need to be altered so that monitoring takes place

where the CO2 has come to be located. The frequency of measurements and types of

technologies employed can also change, for example, based on the new trajectory of the CO2.

The need for flexibility and responsiveness in monitoring plans cannot be overemphasized.

Geologic systems are complex, and their properties can be highly spatially variable over large

and small distances. Even at well-characterized pilot and research GS sites such as Sleipner

(North Sea), Frio (Texas), and Weyburn (Saskatchewan), model simulations based on site

characterization data were not able to predict CO2 migration in the subsurface and accurately

identify the ultimate location of the injected CO2. For example, at Sleipner, the lateral

dimensions of the CO2 plume were much smaller than predicted by pre-injection model

simulations. Compared to predictions, the plume spread out less laterally and more vertically

because of the presence of unrecognized discontinuous silt layers within the injection zone

(Johnson and Nitao, 2003). At Frio, the CO2 migrated much more quickly in the subsurface than

predicted. Researchers at the site suspect that this was due to an uncharacterized high

permeability zone in the subsurface (Doughty et al., 2001; Hovorka et al., 2005; Kharaka et al.,

2009). At Weyburn, modeled predictions of the location and shape of the CO2 plume were

partially incorrect, because of a series of faults at the site that were not included in the model

simulations (Friedmann, 2003).

At each of the above sites, none of the unanticipated migration resulted in leakage, and the CO2

remained sequestered in the intended injection zone. Thus, despite uncertainties in the ability to

predict the exact location and shape of the CO2 plume in the subsurface, there is nonetheless a

high degree of confidence that CO2 can be sequestered in the subsurface and that GS can be

successfully implemented (Benson et al., 2002; IPCC, 2005). All of these examples, however,

reinforce the need for flexibility and responsiveness in robust monitoring plans so that

monitoring can be adjusted as needs and conditions change, to enable quantification of the

injected CO2, and to detect leakage if it occurs.

An important strength of tailored plans is that they can be focused on site-specific features of

elevated vulnerability or risk. Elevated vulnerabilities can be detected initially during site

characterization or subsequently during monitoring. As monitoring data are collected and

knowledge and understanding of the geologic system improve, understanding of the level of risk

can be improved, resulting in a greater or lesser need for targeted monitoring. Generally, the

overall level of risk of adverse impacts due to GS is expected to decline over the lifetime of a

project, which is anticipated to result in less intensive monitoring needs over time (Benson et al.,

2002; Chalaturnyk and Gunter, 2004; Pawar et al., 2006; Benson, 2007b; Birkholzer et al.,

2007).

Recognizing the potentially long injection life of a GS project, modifications to the plan can

occur in the future based on improvements to current technologies and development of new

technologies as advances in CCS research occur. Operational changes can also occur during the

injection phase; decisions can be made in future years to change the quantity or rate of CO2

injection, and these operational decisions are also likely to require changes to the monitoring

plan. The flexibility needed because of the long lifetimes of GS operations can be explicitly

incorporated into monitoring plans. Monitoring planning is likely to be undertaken with a greater

degree of certainty for the near future, but become more uncertain as monitoring needs are
key components of a robust monitoring plan

30

projected decades into the future. Thus, monitoring plans may be explicit for the near future,

detailing specific techniques, frequencies, and locations of measurements. In contrast, it may be

more appropriate and realistic for monitoring plans to explain decision processes that will be

used in later stages of the project rather than attempting to define specific monitoring

technologies, frequencies, or locations.

In summary, some of the key reasons why monitoring plans need to be flexible include:

 GS systems are spatially heterogeneous and complex, and it may be difficult to perform the

initial site characterization at a level necessary to accurately predict CO2 behavior over the

large spatial scales of GS projects (see examples from Sleipner, Weyburn, and other pilot

projects discussed above). Thus, it is anticipated that monitoring plans will likely need to be

adaptable to respond to measurement results obtained during the injection phase.

 It may be necessary to change the location, frequency, and monitoring methods if

unanticipated migration of the CO2 in the subsurface is detected during monitoring.

 Iterative plans provide the ability to calibrate, validate, and refine models and alter

monitoring based on modeling results.

 New site features with elevated vulnerability of leakage or other adverse impacts may be

detected during monitoring. New receptors may also be identified, or site receptors may

change over time — species may become endangered, human populations may shift, and

other economic resources may be targeted for extraction. Thus, it is anticipated that

monitoring plans will likely need to be flexible to incorporate potential changes in site

conditions.

 The level of vulnerability or risk associated with targeted features may change over time

(increasing or decreasing), as informed by the interpretation of monitoring results and

modeling simulations.

 Over the operational lifetime of a project, technologies may change and operational

parameters may change. Thus, monitoring plans may need to change to reflect these

modifications and advances in technology.

Given the need for flexibility and responsiveness in monitoring plans, robust monitoring plans

may need to detail explicitly the procedures or steps for modifying and adapting monitoring

based on monitoring results. For example, general contingency planning could be included for

identified site uncertainties (e.g., uncertainty regarding the stability of a fault under operational

conditions). The contingency plan could list uncertainties, identify monitoring strategies that

could be put in place if needed, and list mitigation options under different scenarios.

3.5 Summary

This section identified the key components of a robust monitoring plan, and provided

information that may inform the development of a monitoring plan that contains these key

components, which include: 1) tailoring monitoring plans to site-specific conditions; 2) taking
key components of a robust monitoring plan

31

into account the different stages of a GS project; 3) identifying and selecting appropriate

monitoring techniques for different monitoring purposes; and 4) ensuring a flexible, responsive

plan. Section 4 presents a summary of the information presented in this TSD.
4. SUMMARY

32

GS has been identified as one of several approaches to reduce the concentration of atmospheric

CO2, thereby contributing to the mitigation of climate change (IPCC, 2007). However, due to the

presence of potential pathways that may result in unanticipated migration and/or leakage of CO2,

monitoring must be a key element of GS projects. Monitoring can confirm that the injected CO2

is sequestered in underground geologic formations and has not been released to the atmosphere,

verify the quantity of CO2 that has been injected, detect leakage or unanticipated subsurface

movement of CO2, and provide an early warning of any unanticipated migration or leakage. If

leakage does occur, monitoring can be used to help determine how much CO2 was released.
33

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