### TECHNICAL SUPPORT DOCUMENT FOR INJECTION AND GEOLOGIC SEQUESTRATION OF CARBON DIOXIDE PROPOSED RULE FOR MANDATORY REPORTING OF GREENHOUSE GASES



# MONITORING PLANS FOR GEOLOGIC SEQUESTRATION

March2010 U.S. Environmental Protection Agencyi



### TECHNICAL SUPPORT DOCUMENT: 

# MONITORING PLANS FOR GEOLOGIC SEQUESTRATION

Office of Air and Radiation U.S. Environmental Protection Agency Developed with support from Stratus Consulting, Incii 

# CONTENTS 1. Introduction ................................ ................................ ................................ ........................... 1 1.1 Geologic Sequestra tion: 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 Se questration System

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

................................ ................................ ................................ ................ 4 2.3 Subsurface CO 2 Movement

................................ ................................ ................................ .. 5 2.4 Trapping Mechanisms

................................ ................................ ................................ .......... 6 2.5 Potential CO 2 Migration and Leakage Pathways

................................ ................................ .. 7 2.6 Monitoring for Accounting and Reporting on Geologic Sequestration

................................ ... 8 2.7 Other Monitoring Pur poses ................................ ................................ ................................ .. 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 Evaluatin g 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 CO 2 Emissi ons ................................ ................................ ................................ ................... 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 Eva luation

................................ ................................ ................................ 16 Figure 4. VEF Faults Evaluation

................................ ................................ ................................ 17 Figure 5. VEF Injection Zone Evaluation

................................ ................................ ................... 19 Figure 6. VEF GS Footprint Delineation

................................ ................................ .................... 21

# 1. INTRODUCTION

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### 1.1 Geologic Sequestration: A Climate Change Mitigation Technology The Intergovernmental Panel on Climate Change (IPCC) ha s identified carbon dioxide capture and sequestration (CCS) as one of several approaches to reduce emissions of carbon dioxide (CO 2 ) to mitigate climate change (IPCC, 2007). While various CCS technologies have been explored, including ocean storage and min eral carbonation, geologic sequestration (GS) has been identified as the most technically viable approach (IPCC, 2005). GS is the long

- term containment of CO 2 in subsurface geologic formations. . The CO 2 is trapped for long periods of time as a result of bo th physical and geochemical mechanisms. The climate change mitigation benefits of GS depend on the long

- term isolation of injected CO 2from the atmosphere. 

### 1.2 Importance of Monitoring While experience from existing projects and natural and industrial analogs to GS demonstrates that CO 2 can be safely sequestered in geologic formations, there is the potential for unanticipated migration and leakage of injected CO 2 . Because of the potential for such migration and/or leakage, monitoring is a necessary compo nent of GS projects. Monitoring of GS projects can be used to show that the injected CO 2 is sequestered in the subsurface, to verify the quantity of CO 2 that has been injected, to detect leakage or unanticipated subsurface movement of CO 2 , 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 CO 2 was released. Monitoring can also serve other purposes, including informing and evaluating the success of mitig ation actions, detecting induced microseismicity, and assessing potential environmental and human health impacts of leaked CO 2 . Thus, monitoring is essential to ensuring the long

- term accountability and success of GS projects. 

### 1.3 Purpose of this Technica l 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 theinjected CO 2 is sequestered in the subsurface and of quantifying amounts of CO 2leaked, 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. Oneexampleof potential subsurface impacts include pressure - induced migration of saline waters into overlying aquifers which may serve asintroduction

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undergr ound 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 CO 2to the atmosphere, which is the focus of a mon itoring, 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 humanhealth 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 consist ent 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 go als 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 ke y components. This TSD addresses these goals in the following sections:

 Section 2

–

Need for Monitoring at Geologic Sequestration Sites . Successful GS depends on CO 2 trapping mechanisms. Potential leakage and migration pathways could potentially compromis e the ability of a geologic formation to sequester the injected CO 2 . Because of this potential for leakage and unanticipated migration, accurate accounting and verification 1 of the injected CO 2 are essential to assessing the performance of GS projects. Thi s 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 CO 2 , and the potential pathways and mechanisms that may result in leakage and migration of CO 2from 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 backgro und and information on: 1) tailoring the

1 For the purposes of GS, CO 2accounting is the practice of quantifying and recording the volume of CO 2that is injected into underground formations. Verification refers to the aggregate actions that are taken to ensure the reliability of the information re corded when accounting for injected CO 2 . 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 pha ses 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 MONITORI NG AT GEOLOGIC SEQUESTRATION SITES

4 Experience from existing projects and natural and industrial analogs to GS demonstrate that CO 2 can be sequestered in geologic formations. In addition, a GS site that is selected and managed pro perly can sequester CO 2safely and securely over time.. Monitoring is needed to confirm and quantify the injected CO 2 , detect leakage and adverse impacts to subsurface and surface receptors , 2and enable accounting and reporting of CO 2sequestration. 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 informatio n 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 CO 2 , CO 2 trapping mechanisms, and the ways in which CO 2can 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 CO 2 is injected, an overlying confining system, and the injected CO 2 stream. An injection zone is the geologic formation (with sufficient areal extent, thickness, porosity, and permeability) into which CO 2is injected and in which the CO 2is intended to remain sequestered. The confining system is the geologic formation, group of formations, or section of a formation that serves as the low

permeability barrier to upward migration of injected CO 2 . The CO 2 stream consists of the CO 2 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 CO 2 sequestration capacity of deep saline formations i s 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

2For 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

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and migration of CO 2from the subsurface compared to o ther 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 g as 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 CO 2 . Currently, CO 2is injected into some of these formations to enhance oil recovery. Enhanced oil recovery (EOR) involves injecting CO 2 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 inje cted CO 2 .
 Coal seams. Unminable (deep or thin) coal seams are a potential formation for GS because CO 2has a high affinity for coal (higher than that of methane) and will adsorb to the micropores (cleats) within the coal seams. The technical feasibility o f 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 geolog ic formations or settings could sequester injected CO 2 , 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 CO 2 . Basalts, for ex ample, can be an effective rock type for sequestering CO 2 because of the formation of carbonates from CO 2 interacting with minerals in the basalt. Basalts may not be ideal for sequestration, however, because they typically have low porosity and permeabilit y, and may have fractures that can pose a risk for CO 2 migration or leakage. 

### 2.3 Subsurface CO 2 

### Movement Several factors control how CO 2moves in the subsurface. The rate of CO 2fluid 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 CO 2and formation fluids (known as buoyancy

- driven flow), and formation heterogeneities and viscosity contrasts that result in dispersion or fingering of CO 2 . In addition, diffusion can affect CO 2 fluid flow, but it is typically less important than the above mechanisms that affect the flow of CO 2 in the subsurface. Key properties of the injection zone that affect the rate of CO 2fluid flow include permeability, thickness, and heterogeneity of the injection zone formation. If an injection zone has high permeability, CO 2 will migrate faster through the formation. Thicker injection zones also allow a larger volume of injected CO 2 to migrate. Heterogeneities such as faults, fractures, and bedding planes may result in preferential flow pathways allowing CO 2to migrate.need for monitoring at geologic sequestration sites

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### 2.4 Trapping Mechanisms Sequestration of injected CO 2 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 s olubility 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 p rocesses in which injected CO 2is 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 refer s to situations where injected CO 2 , 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 capill ary trapping, is a physical process that occurs when CO 2migrates 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 fo rmation, but generally fall between 15% and 25% of injected CO 2 (Holtz, 2002). Residual trapping can be an effective means of preventing injected CO 2 from reaching structural and stratigraphic trapping formations (IPCC, 2005; Kumar et al., 2005).

 Solubili ty trapping . Solubility trapping, also know as dissolution trapping, occurs when injected CO 2 dissolves into formation waters. The dissolved CO 2will travel with the water rather than buoyantly rising to the top of the formation as a separate phase. In addition, when CO 2 dissolves in water, the water’s density increases, meaning waters that contain dissolved CO 2 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 CO 2 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 CO 2 , mineral trapping for large amounts of injected CO 2 can take millennia (Wilson, 2004; IPCC, 2005).

 Preferential adsorption . Injected CO 2 can preferentially adsorb to coal seams or organic

- rich shales when injected into these types of geologic formations. The adsorption of CO 2 will displace methane bonded to the coal surface; this methane could potentially be recovered for commercial processes (enhanced coalbed methane). Adsorbed CO 2wil l remain immobilized in these formations under stable temperatures and pressures (IPCC, 2005; Haszeldine, 2006).need for monitoring at geologic sequestration sites

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### 2.5 Potential CO 2 

### Migration and Leakage Pathways As described in Section 2.4, several different physical and geochemical processes can trap in jected CO 2 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 CO 2to the atmosphere, it is important to consider processes an d pathways which could ultimately compromise the ability of a project to sequester CO 2 , and result in the need for additional monitoring to quantify the injected CO 2and to detect potential leakage. Processes and pathways that could lead to leakage are des cribed 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 CO 2leakage 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 fau lts in CO 2migration 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). The se factors govern whether faults and fractures form barriers or conduits to CO 2 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 re

a ctivated faults depends on the presence of receptors, the presence of additional fluid

conducting pathways, and the reopening or resealing of faults by geochemical processes (dissolution or precipitation) (IPCC, 2005).

 Wells. Wells (and other artificial p enetrations such as boreholes) have been identified as one of the most probable conduits for the escape of CO 2from the injection zone (Gasda et al., 2004; Benson, 2005; IPCC, 2005; Carey et al., 2007). Improperly sealed and plugged wells from previous ope rations 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 th e injection zone. Thus, a combination of wells, faults, or other geological discontinuities can potentially serve as a pathway for migration and leakage. Aproject ’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 CO 2into a formation will usually result in changes in subsurface pressure. If injection pressure exceeds fracture pressure when injecting CO 2 , it can indu ce 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 charact erization, 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, CO 2can be sequestered safely in subsurface geologic formations, but the potential exists f

or CO 2to 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 CO 2 is necessary to determine whether injected CO 2is being releasedto the atmosphere. Monitoring can be used to confirm that injected CO 2 remains in the intended formation, to detect leakage if it occurs, and thus to accurately account for the injected CO 2 . M onitoring site characteristics, project operations, and CO 2beh avior 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 thesedifferent purposes is provided in Section 3. Specific purposes for monitoring for accounting and reporting could include:

 Establishing baseline conditions . Because CO 2is ubiquitous in the environment and concentrations vary over space and time (e.g., diu rnally, seasonally, annually), determining background levels of CO 2 and understanding natural fluctuations is necessary to discern whether detected CO 2 is attributable to leakage or to preexisting sources. It is also important to establish baseline conditi ons before injection because changes in the parameters before and after injection and over time assist in tracking the injected CO 2 .
 Ensuring injection controls
. Monitoring the condition of the injection well, the injection rate, and wellhead and formatio n pressure is important to verify the amount of CO 2injected and to avoid leakage.

 Confirming the quantity of injected CO 2 . Determiningthe amount (mass) of CO 2 injected is important for accounting and verification purposes. Detecting unanticipated migratio n and leakage
. Monitoring in the subsurface can be used to detect unanticipated migration of CO 2 in the subsurface. Monitoring at the surface can be used to help confirm that leakage has not occurred. If CO 2has leaked, surface monitoring can help quantifythe 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 andsite 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 CO 2condu its, which can result in leakage. 

### 2.7 Other Monitoring Purposes In addition to monitoring for accounting and reporting of injected CO 2 , monitoring can also be used for the following purposes:need for monitoring at geologic sequestration sites

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 Informing mitigation actions . Mitigation refers to actions taken to prevent CO 2 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 su ch as well plugging and cementing can prevent CO 2 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 prev ent CO 2migration 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 CO 2 injection operations under certain conditions (e.g., if certain pressure thresholds are exceeded). Monitoring can help recognize such activity and mitigate impacts before theybecome severe.

 Assessing environmental and human health impacts of leakage if they occur . If CO 2leaks to the surface or to subsurface resources, adverse impacts to the environment and human health can vary depending on the concentration of the CO 2and the exposure. Monitoring techniques can help assess the severity of adverse impacts by providing information on the amount of leaked CO 2 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 CO 2 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 assu rance 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 th e 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 l eakage. 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 re sponsible 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 resul tsneed 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 CO 2can 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 CO 2migration and/or leakage. Measures can be taken to reduce the potential for migration and leakage of in jected CO 2 . Monitoring can be used to confirm the location, quantity, and physical status of injected CO 2 , and to detect any leakage if it occurs. Monitoring plans are important because they can ensure that the collected monitoring information and data arereliable, 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, f our 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 Tailorin g 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 andpotential 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 CO 2mig ration 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 tailore d approach mayenable the most cost

effective 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 monit oring 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 process es 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 site

specific considerations that might requi re in

- depth evaluation for GS project design and site

specific 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 whichare 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 Ev aluating Geologic Attributes Several geologic attributes may affect a particular GS site’s vulnerability to CO 2migration and leakage and unanticipated pressure changes which may induce leakage. These attributes, which are described in further detail in EP A’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

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The specific geologic setting may influence the relative importance of the geologic attri butes 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 CO 2is dissolving in the formation fluid and geochemical reactions are progressing as expected.
Attributes Related to the GS Confinin g 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 CO 2 . Based on Figure 1. Vulnerability Evaluation Framework Conceptual Model

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

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U.S. EPA (2008), the following are geologic attributes of the c onfining 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 sp atial extent of the CO 2 plume and its associated pressure front. The potential for CO 2 migration and leakage can increase if the lateral extent of the confining system does not extend beyond the footprint of the CO 2 plume and its pressure front.

 Capillaryentry pressure . Capillary entry pressure refers to the additional pressure needed across the interface of two immiscible fluid phases (e.g., CO 2and formation water) for CO 2 to enter the confining system. The risk of CO 2migration and leakage increases if the injection pressure exceeds the capillary entry pressure. The capillary entry pressure of a geologic formation can be evaluated using CO 2 column height and injection pressure (Harrington and Horseman, 1999).

 Permeability . Permeability refers to the eas e with which fluids flow through the pore spaces of a geologic material. A GS site can have increased vulnerability to CO 2migration and leakage if the rocks of the confining system have high permeability. Clay, shale, or siltstone have low permeability an d 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 sys tem’s thickness, permeability, diffusion, ability to retard CO 2migration, and other geochemical reactions that could slow or accelerate CO 2 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 CO 2 . Artificial penetrations are considered one of the most likely pathways of CO 2 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 therock 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 CO 2 . For e xample, increased acidity in geologic formations from CO 2 dissolved in formation fluids can dissolve minerals, thereby decreasing CO 2 travel times and increasing vulnerability to migration and leakage. In contrast, other geochemical reactions between CO 2 , 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 processeskey components of a robust monitoring plan

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can be evaluated us ing 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 CO 2migration 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 CO 2migration and leakage, wi th 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 CO 2migration 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 confin ing 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 considerationwhen developing a robust monitoring plan that is tailored to site

- specific conditions.key components of a robust monitoring plan

15

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

17

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 CO 2were identified by U.S. EPA (2008) andare described here. These attributes have implications for a site’s vulnerability to CO 2 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 CO 2 . Once CO 2is injected into a formation, it will reside in the pore space of the formation, disp lacing fluids previously held within the pore space. Factors that impact physical capacity include thickness, surface area, effective (or interconnected) porosity, CO 2density, 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 theinjectivity 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 p otential geological sequestration site by altering injection zone capacity. Geochemical processes can make the injection zone more or less favorable for GS. For example, CO 2 - rich waters can dissolve minerals, which can increase the porosity and storage cap acity 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 (Knau ss 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 th e 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 site

specific and will need to be determined prior to injection (Streit and Hillis, 2003; Streit et al., 2005). Figure5 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

19

3.1.2 Identifying and Evaluating Potential Receptors and Impact Categories Although the pri mary objective of this technical support document is to identify monitoring needs for CO 2 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 add ition, unanticipated CO 2 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 CO 2can result in a variety o f adverse health effects, because CO 2 is an asphyxiant. The vulnerability of human populations to migration and leakage of CO 2depends on proximity to the leakage area, population density, the length of exposure, and the concentration of the release. Expos ure to CO 2can 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 Injec tion Zone Evaluation

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

20

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 CO 2 can also negatively impact cultural and recreational resources and places of archaeological importance by making human use unsafe or degrading the a ctual resource (e.g., increased CO 2in 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 CO 2from 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 CO 2 . Sensitive species are organisms that, because of either their physi ology or their behavior, are highly vulnerable to adverse impacts associated with CO 2migration or leakage. Sensitive species include soil

- dwelling animals and microbes, plants, surface

- dwelling animals, aquatic organisms, calcifying aquatic organisms, anddeep geologic ecosystems (i.e., microbes deep in the earth’s crust). These species can serve as indicators of the potential impacts of CO 2 leakage on larger ecosystems, because they will generally experience adverse impacts before other less vulnerable sp ecies. 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 becau se of its proximity to underground injection sites. Injected CO 2 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 migrateto ov erlying 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 canall 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 CO 2 injection also can impact groundwater by affecting groundwater flow direction, water ta ble 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 geo sphere impacts . Injection of CO 2into 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 CO 2to migrate or leak from the inje ction 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

21

3.1.3 Defining the GS Footprint for a Site - Spec ific 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 geolo gic attributes of the confining system and injection zone, as described earlier. It will also depend on the total amount of CO 2 injected over the lifetime of the project, and operational factors such as the injection rate and number and location of injecti on wells. The size of the plume will increase during the injection period, and because supercritical, separate

- phase CO 2 is buoyant and has lower viscosity than water and brine, the plume may also continue to change shape and position for some time after i njection stops. At some point, trapping mechanisms, including dissolution, physical, and mineral trapping, are anticipated to eventually minimize and immobilize the CO 2 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 site

specific data may be needed to delineate the footprint. The footprint can serve as the basis for defining the area where baselineconditions 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.key components of a robust monitoring plan

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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

- injecti on (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 pha se 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 A ll 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 monitori ng 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 betaken, 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 res ults. 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 outp uts, 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 s chedules 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 interpret ation 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 w ill 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 wh en developing the monitoring plan.key components of a robust monitoring plan

23

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 pha se 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: establishi ng 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 monitoring 3 ; 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. Establi shing the spatial area to be monitored during the injection phase will be based on site characterization results and on the total amount of CO 2planned to be injected over the lifetime of the project, and can also be informed by predictive model simulation s. 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 maybe needed for wells or faults to detect pathways of migration and leakage of CO 2and near p opulations to help in assessment of environmental and human health impacts
. Baseline conditions may be established for the attributes and receptors identified in Se ction 3.1. Establishing environmental baseline conditions before injection is important for several reasons. Changes over time in the parameters used to monitor CO 2can be used to track the injected CO 2 . Thus, successful monitoring requires establishing pre - injection baseline conditions over time and space. This is also important for surface measurements of CO 2that could detect leakage. B ecause CO 2is ubiquitous in the environment and concentrations vary over space and time (e.g., diurnally, seasonally, an nually), 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 CO 2 and potential unanticipated migration and leak age. Identifying impurities in the proposed CO 2stream can also aid in distinguishing between injected and ambient CO 2because such impurities may be unique to the stream, and not otherwise found in the atmosphere. Finally, because monitoring is used bothto confirm the quantity of injected CO 2and 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 (possib ly 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 vu lnerability. 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 CO 2 is injected into the subsurface. During the injection phase, the main purposes of monitoring are to confirm and quantify the amount of CO 2 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 int o the following categories:

 Ensuring injection controls

 Monitoring the location and quantity of injected CO 2

 Addressing targeted monitoring needs

 Detecting migration or leakage of CO 2

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

jection 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 tho se 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 w ell 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 overpressuri ng and, over the long term, can be used to help quantify and confirm the amount of CO 2injected during the life of the project. Monitoring the location and quantity (volume and mass) of injected CO 2is a key part of confirming that the CO 2 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 pre dictions and alter the delineated spatial area of evaluation. If significant unanticipated migration of CO 2in 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 CO 2 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 effectivene ss of physical and geochemical trapping mechanisms that can control where the CO 2comes 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 tar geted in monitoring plans if elevated potential for leakage of CO 2 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 unanticipa ted migration and leakage of CO 2is an important part of monitoring during the injection phase. Monitoring at the surface can be employed to determine if any injected CO 2 has leaked into the atmosphere. If there is unanticipated migration of CO 2 at depth, or in the near

- surface (e.g., if CO 2escapes from the confining system and migrates into groundwater aquifers), monitoring can determine if the CO 2is reaching near

- surface or surface environments. If CO 2 has leaked, surface monitoring can help to quantifythe 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 f requent. Measurements to confirm the location and quantity of injected CO 2 , 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 measurement s 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 modelingactivities may slow once models are refined and are able to adequately predict subsurface CO 2 movement, location, and quantity.

 Keeping the monitoring plan flexible and responsive to initial field results. Significant changes to the monitoring plan may beneeded during the injection phase, if monitoring results reveal that actual CO 2 movement and location in the subsurface are different than predicted.key components of a robust monitoring plan

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3.2.4 Post - Injection Phase The post

- injection phase occurs when injection has stopped, and wells are pl ugged (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 CO 2 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 w ill 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 CO 2 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

- in jection 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 incorporate d 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 CO 2 . 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 mea surement 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 technolog ies to particular phases of a GS project. For example, monitoring to ensure injection controls, such as measuring CO 2injection rates, will take place only during the injection phase. In contrast, measurements for confirming the location and amount of inje cted CO 2could 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 u se 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 surfaceto detect leakage, others can serve multiple monitoring purposes. For example, seismic surveys can help locate and quantify the sequestered CO 2and also can reveal potential migration and leakage pathways of CO 2to the surface. Natural or introduced tracers can help locate the injected CO 2 , 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 provid ed 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 resou rces, 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 CO 2 Emissions Monitoring can beused to confirm that injected CO 2remains underground, to detect any leakage if it occurs, and thus to accurately measure the injected CO 2for accounting and reporting purposes. A variety of monitoring techniques can ultimately contribute to ensure accurate accounting and reporting of CO 2 emissions. Technologies for detecting and quantifying CO 2 leakage are discussed in more detail in the subpart RR General TSD ( see docket EPA

- HQ

- OAR

2009

- 0926 ).

 Monitoring techniques for establishing a baseline . The mon itoring 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 f luid 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, 200 9; 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 annu lus pressure measurements (Benson et al., 2004; IPCC, 2005; Benson, 2007a; Freifeld et al., 2009; NETL, 2009).

 Monitoring techniques for detecting unanticipated migration of CO 2 and leakage
. Unanticipated migration may be detected using many of the same te chniques described below for confirming the location and quantity of injected CO 2 , 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 pro ductivity 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 CO 2 . The mov ement and fate of injected CO 2are influenced by injection

- related factors, properties of the CO 2 , 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 C O 2 . Both existing pressure gradients and those induced by injection can influence CO 2movement. Techniques that measure the injection rate and formation pressure gradients can thus help monitor CO 2 in the subsurface. Other examples of techniques that may b e used to confirm the location and quantity of injected CO 2include 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 selectedto 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 a dditional, 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, flex ible, 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 CO 2 subsurface migration. If unanticipated migration or leakagekey 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 CO 2 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 CO 2 . The need for flexibility and responsiveness in monitoring plans cannot b e 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 (Saskatchew an), model simulations based on site characterization data were not able to predict CO 2 migration in the subsurface and accurately identify the ultimate location of the injected CO 2 . For example, at Sleipner, the lateral dimensions of the CO 2 plume were mu ch 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, 2 003). At Frio, the CO 2 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 CO 2 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 CO 2 remained sequestered in the intended injection zone. Thus, despite uncertainties in the ability to predict the exact location and shape of the CO 2plume in the subsurface, there is nonetheless a high degree of confidence that CO 2 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 monitorin g plans so that monitoring can be adjusted as needs and conditions change, to enable quantification of the injected CO 2 , and to detect leakage if it occurs. An important strength of tailored plans is that they can be focused on site

- specific features of el evated 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 CO 2 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 plan ning is likely to be undertaken with a greater degree of certainty for the near future, but become more uncertain as monitoring needs arekey components of a robust monitoring plan

30

projected decades into the future. Thus, monitoring plans may be explicit for the near future, detailing specific tech niques, 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 monitori ng 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 CO 2behavior 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 CO 2in the subsurface is detected during monitoring.

 Iterative plans pro vide 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 identi fied, 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 incorpo rate 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 oper ational 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) takingkey 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 info rmation presented in this TSD.

# 4. SUMMARY

32

GS has been identified as one of several approaches to reduce the concentration of atmospheric CO 2, 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 CO 2 , monitoring must be a key element of GS projects. Monitoring can confirm that the injected CO 2 is sequestered in underground geologic formations and has not been released to the atmosphere, verify the quantity of CO 2 that has been injected, detect leakage or unanticipated subsurface movement of CO 2 , and provide an early warning of any unanticipated migration or leakage. If leakage does occur, monitoring can be used to help determine how much CO 2was released.33 

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