Vulnerability Evaluation Framework
for Geologic Sequestration of Carbon
Dioxide
July 10, 2008
U.S. Environmental Protection Agency
EPA430-R-08-009

-------
For Further Information:

Anhar Karimjee
Climate Change Division, Office of Atmospheric Programs
U.S. Environmental Protection Agency
202-343-9260
Karimjee.anhar@epa.gov

Lisa Bacanskas
Climate Change Division, Office of Atmospheric Programs
U.S. Environmental Protection Agency
202-343-9758
Bacanskas.lisa@epa.gov

Peer Reviewed Document

This report has undergone an external peer review consistent with the guidelines of the
U.S. EPA Peer Review Policy. See the Acknowledgements section for a list of reviewers.
A copy of the EPA Peer Review guidelines may be downloaded from the following web
page at http://epa.gov/osa/spc/2peerrev.htm

-------
CONTENTS

Acknowledgments	iv
Acronyms	v
1. Introduction	1
1.1 Background on Climate Change and Geologic Sequestration	1
1.2 Vulnerability Assessment Approach	2
1.3 Report Organization	3
2. Background on Geologic Sequestration	5
2.1 Geologic Settings under Consideration for Sequestration	5
2.2 Subsurface CO2 Movement and Storage Mechanisms	8
       2.2.1 Factors that Control the Rate of CO2 Movement in the Subsurface	8
       2.2.2 Physical and Geochemical Trapping Mechanisms	8
2.3 Practical Experience Relevant to Geologic Sequestration	10
       2.3.1 Existing Projects	10
       2.3.2 Natural and Industrial Storage Analogs	11
       2.3.3 Regulatory Experience	11
2.4 Chapter Summary	12
3. Vulnerability Evaluation Framework for Geologic Sequestration:
Geologic System and Attributes	13
3.1 Geologic Sequestration System and Geologic Attributes	15
       3.1.1 Confining System and Related Geologic Attributes	15
       3.1.2 Injection Zone and Related Geologic Attributes	19
       3.1.3 Carbon Dioxide Stream	22
3.2 Spatial Area of Evaluation: Geologic Sequestration Footprint	23
3.3 Chapter Summary	26
4. Vulnerability Evaluation Framework for Geologic Sequestration:
Impacts and Receptors	27
4.1 Potential Human Health and Welfare Impacts	27
       4.1.1 Human Populations	28
       4.1.2 Populations Covered by Executive Orders	30
       4.1.3 Cultural and Recreational Resources	30
       4.1.4 Economic Resources (Surface and Subsurface)	31
4.2 Potential Atmospheric Impacts	31
4.3 Potential Ecosystems Impacts	32
       4.3.1 Sensitive Species	32
       4.3.2 Legislatively Protected Species	35

-------
4.4 Potential Groundwater and Surface Water Impacts	35
       4.4.1 Water Quality	36
       4.4.2 Regional Groundwater Flow	38
       4.4.3 Protected and Sensitive Drinking Water Supplies	38

4.5 Potential Geosphere Impacts	38
4.6 Spatial Area of Evaluation: Influence of Receptors	38
       4.6.1 Carbon Dioxide Spatial Area of Evaluation	39
       4.6.2 Pressure Spatial Area of Evaluation	40
4.7 Chapter Summary	42
5. Vulnerability Evaluation Framework for Geologic
Sequestration: Key Considerations	43
5.1 Holistic Approach to Evaluating Vulnerability	43
       5.1.1 Interplay of Geologic Attributes and Receptors	43
       5.1.2 Evaluation of Vulnerability at Different Temporal Scales	44
5.2 Key Attributes in Evaluating Geologic Sequestration System Vulnerability	45
       5.2.1 Wells as Fluid-Conducting Pathways	46
       5.2.2 Faults and Fractures as Fluid-Conducting Pathways	48
       5.2.3 Pressure-Induced Physical Effects	49
5.3 Chapter Summary	50
6. Monitoring and Mitigation	51
6.1 Monitoring	51
       6.1.1 Purposes of Monitoring	51
       6.1.2 Monitoring Technologies	52
       6.1.3 Timeframe Implications for Monitoring	53
6.2 Mitigation	54
6.3 Chapter Summary	55
7. Summary and Next Steps	56
Bibliography	57
Glossary	69
Appendix A. Properties of Carbon Dioxide (CO2)	76
Appendix B. Comparison of Attributes:  VEF,
IPIECA, and IPCC                                             ..78

-------
Figures and Table
Figure 2.1. Examples of Geologic Formations Being Considered for GS	6
Figure 2.2. Effectiveness of Trapping Mechanisms Over Time	9
Figure 3.1. VEF Conceptual Model	13
Figure 3.2. Confining System Evaluation	18
Figure 3.3. Injection Zone (IZ) Evaluation	21
Figure 3.4. Carbon Dioxide Stream Evaluation	23
Figure 3.5. Geologic Sequestration (GS) Footprint Delineation	24
Figure 4.1. Human Health and Welfare Evaluation	28
Figure 4.2. Atmosphere Evaluation	32
Figure 4.3. Ecological Receptors Evaluation	33
Figure 4.4. Groundwater and Surface Water Evaluation	36
Figure 4.5. Carbon Dioxide Spatial Area of Evaluation	40
Figure 4.6. Pressure Spatial Area of Evaluation	41
Figure 5.1. Risk Profile for CO2 Storage	44
Figure 5.2. Wells Evaluation	47
Figure 5.3. Faults Evaluation	49
Figure A.I. Phase Diagram for CO2	76
Table B.I. Comparison of Attributes/Pathways Identified as Being Likely to
Affect Unanticipated Migration or Leakage from GS Projects	78

-------
ACKNOWLEDGMENTS

The U.S. Environmental Protection Agency (EPA) would like to acknowledge those individuals
whose efforts helped bring this report to fruition.

The Vulnerability Evaluation Framework (VEF) was developed by a multi-disciplinary team of
physical and social scientists who provided research, analysis, and technical skills. The effort was
led by EPA's Climate Change Division (CCD) in the Office of Atmospheric Programs, with critical
support from EPA's Drinking Water Protection Division (DWPD) in the Office of Ground Water
and Drinking Water, and Stratus Consulting Inc.

The following reviewers provided useful input: the members of the internal EPA Workgroup
on Geologic Sequestration, James J. Dooley and  Casie Davidson (Pacific Northwest National
Laboratory), Neeraj Gupta (Battelle), Jens Birkholzer (Lawrence Berkeley National Laboratory),
Rajesh Pawar (Los Alamos National Laboratory), Grant Bromhal and John Litynski (National
Energy Technology Laboratory), Jean-Philippe Nicot (University of Texas), Scott Anderson
(Environmental Defense), George Peridas (National Resource Defense Council), John Venezia
(World Resources Institute), and Kurt Waltzer (Clean Air Task Force), and the U.S. Department of
Energy's (DOE's) Carbon Sequestration Regional Partnership Program Outreach Group.

The International Energy Agency Greenhouse Gas R&D Programme provided an invaluable service
by facilitating a formal peer review of this report. Special thanks to the co-facilitators, Brendan  Beck
and Tim Dixon, and the expert reviewers: Jason Anderson, (Institute for European Environmental
Policy), Rick Chalaturnyk (University of Alberta), Sevket Durucan (Imperial College London), Tony
Espie (BP), Todd Flach (Det Norske Veritas), Wolfgang Heidug  (Shell), John Kaldi (CO2CRC),
Jonathan Pearce (British Geological Survey), and Malcolm Wilson (University of Regina).

-------
ACRONYMS
 TDS
 irgon
Climate Change Division
 "arbon Dioxide Capture and Storage
Methane
Cyanide
Carbon Monoxide
Carbon Dioxide
U.S. Department of Energy
Drinking Water Protection Division
Enhanced Oil Recovery
U.S. Environmental Protection Agency
Iron
Greenhouse Gas
Geologic Sequestration
Hydrogen
Hydrogen Sulfide
Mercury
International Energy Agency
Intergovernmental Panel on Climate  Change
Injection Zone
Milligrams per Liter
Nitrogen
  ational Energy Technology Laboratory
Nitrogen Oxide
Nitrate
National Oceanic and Atmospheric Administration
Oxygen
Parts Per Million
Safe Drinking Water Act
Sulfur Dioxide
Sulfate
Total Dissolved Solids
  nderground Injection Control
Underground Source of Drinking Water
U.S. Geological Survey
Vulnerability Evaluation Framework

-------
INTRODUCTION
                                                     With appropriate site selection
                                                  based on available  subsurface
                                                  information,  a  monitoring
                                                  program to detect problems,
                                                  a regulatory  system, and the
                                                  appropriate use of remediation
                                                  methods to stop  or  control C02
                                                  releases if they arise, the  local
                                                  health, safety and  environment
                                                  risks of geologic storage would
                                                  be comparable to risks  of current
                                                  activities such as natural gas
                                                  storage, [enhanced oil recovery],
                                                  and  deep underground disposal
                                                  of acid gas. r>
                                                                -IPCC  (2005)
Geologic sequestration1 (GS) of carbon dioxide
(CO2), a greenhouse gas (GHG), has been identified
as one of several approaches to reduce atmospheric
concentrations of CO2, thereby contributing to
the mitigation of climate change (IPCC, 2005). A
large body of literature indicates that GS is a viable
technology that can be conducted in a safe manner
when coupled with a comprehensive approach to ensure
protection of human health and the environment (see
text box). Nonetheless, there are potential risks and
uncertainties associated with GS. To systematically
identify those conditions that could increase the
potential for adverse impacts from GS, the U.S.
Environmental Protection Agency (EPA) has developed
a Vulnerability Evaluation Framework (VEF).


1.1 Background on Climate Change
and Geologic Sequestration

In its 2007 scientific assessment, the Intergovernmental Panel on Climate Change (IPCC) concluded
that "warming of the climate system is unequivocal" (IPCC, 2007). This warming trend has been
linked to shrinking glaciers, rising sea levels, alterations in plant and animal habitats, and other
global impacts. The IPCC concluded that it is very likely that most of the increase in the average
global temperature since the mid-20th century has been caused by emissions of GHGs from human
activities. Continued GHG emissions at or above current rates will lead to further warming and very
likely to global impacts, some of which may be irreversible (IPCC, 2007).

The IPCC examined several scenarios to reduce and soon reverse increase in emissions of GHGs
and thus limit future climate change; most studies find that a range of strategies will need to be
employed. In its Special Report on Carbon Dioxide Capture and Storage, the IPCC identified CO2
capture and storage (CCS) as one of several approaches with the potential to address climate change
(IPCC, 2005). CCS is intended to mitigate climate change effects by decreasing emissions from
stationary sources such as power plants (IPCC, 2005). Although several CCS technologies have
been proposed, including ocean storage and mineral carbonation, GS  has been identified as the most
technically viable approach (IPCC, 2005). GS involves injecting captured CO2 into deep, subsurface
rock formations for long-term storage. It has been estimated that available capacity for GS in the
United States ranges from 1,300 to 3,900 gigatons of CO2, with most of the capacity in deep saline
formations (NETL, 2007). For reference, the total energy-related CO2 emissions  in the United States
in 2005 was 5.9 gigatons, with fossil fuel combustion accounting for 5.8 gigatons (U.S. EPA, 2007).
 Note, sequestration is also sometimes referred to as storage, see for example, IPCC 2005.

-------
^^^^^^^^^^^^^m
1.2 Vulnerability Assessment Approach

The vulnerability assessment incorporated in the VEF was developed to systematically identify those
conditions that could increase the potential for adverse impacts from GS, regardless of likelihood
or broad applicability. It is not a quantitative, probabilistic risk assessment tool. Vulnerability
assessment examines conditions that lead to increased or decreased susceptibility to consequences,
whereas risk assessment measures the probability and severity of consequences. It is recommended
that assessing vulnerability be an iterative process where new information is incorporated into the
evaluation as it is generated. Many of the principles, approaches, and areas of focus in the VEF,
discussed in this report, are similar to and reflect conclusions  reached in other GS assessments (see,
for example, Oldenburg et al.,  2002a, 2002b; Friedman and Nummedal, 2003; Maul et al., 2003;
Celia and Radonjic, 2004; Celia et al., 2004; Le Gallo et al., 2004; Quintessa, 2004; Walton et al.,
2004; Zhang et al., 2006).

The VEF described in this report is not designed to be a generalized site selection tool, to establish
performance standards for GS sites, or to specify data requirements. The VEF represents a first
step toward a conceptual framework designed to aid regulators, and other technical experts, in
framing key site-specific considerations and in identifying key areas that require in-depth evaluation
for project design,  site-specific risk assessment, monitoring, and management. It could serve as
a reference document for regulators responsible for approving environmental impact statements,
approving GS sites, or issuing permits for GS projects. Applying the VEF would necessitate detailed
technical information on the proposed GS site extracted from existing data sources or specifically
collected by a project operator.

Current challenges to developing a quantitative risk assessment that is  applicable across  all GS sites
are related to limited field experience and the heterogeneity of GS sites. Attempting to quantify
risks before sufficiently understanding GS systems could result in understating or overstating the
risks associated with GS. Development of a high-level quantitative risk analysis will become more
feasible as information is generated from pilot- and commercial-scale projects. Due to the inherent
heterogeneities of GS systems,  site-specific quantitative risk assessments similar to approaches taken
at the Weyburn (Canada), Gorgon (Australia), and Otway (Australia) sites provide invaluable insight
into understanding and managing potential risks. (Zhou et al., 2004; Chevron, 2005; U.S. DOE,
2007a; CO2CRC, 2008).

As with all such frameworks, the VEF is limited in that only vulnerabilities based on currently
known or understood physicochemical  or biological processes are considered. Uncertainties that go
beyond current understanding of these  fundamental processes or are related to issues of scale are
not explicitly incorporated into this vulnerability framework. Uncertainties associated with models
may also present challenges to both operators and regulators.  Acknowledging these uncertainties and
employing a strategy to manage them is a critical aspect of any assessment. Probabilistic approaches
may be applied to handle uncertainties  that arise from variability in the GS system (e.g.  description
of deep geologic storage systems, variation  in types of cement materials used in wells, uncertainty
regarding the location of existing wells, etc). Uncertainties from incomplete characterization of
the GS site may be addressed through incorporating additional information as it is generated, thus
reducing uncertainty and increasing the precision of the evaluation. It may be more difficult to
address uncertainties arising from a lack of understanding of processes involved, and from potentially
unreliable, inexplicable, or conflicting data. Approaches are being developed that can be applied to
manage uncertainties associated with GS (see Benbow et al, 2006).

-------
^^^^^^^^^^^^^m
The VEF concept was developed based on an extensive literature review as well as input from EPA's
Regional and Headquarters offices, researchers at the DOE national laboratories, experts from
academia, and members of nongovernmental organizations working on issues related to GS. The IEA
GHG R&D Programme also facilitated a peer review of this document by GS experts.

It is important to emphasize that the use of the VEF in framing site-specific considerations for site
selection does not replace or supersede other statutory or regulatory requirements for the protection
of human health and the environment. Owners and operators must obtain all necessary permits from
appropriate state and federal authorities under the Safe Drinking Water Act (SDWA) and any other
applicable statutes and regulations.
1.3 Report Organization

The goals of this report are to:

    1.  Outline key characteristics of a GS system that need to be evaluated to understand potential
       impacts to human health and the environment.

    2.  Provide approaches for conducting "first-order" assessments of GS systems to identify
       circumstances where additional study and/or monitoring may be warranted.

    3.  Identify potential mechanisms for unanticipated migration2, leakage3, and or pressure
       changes that could cause adverse impacts.

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

The report chapters address these goals as follows:

Chapter 2: For GS to be an effective climate change mitigation tool, large volumes of CO2 must
remain underground for long periods of time (hundreds, if not thousands, of years). Chapter 2
discusses the types of geologic formations that are currently being considered for GS and the types of
mechanisms that could trap CO2 underground. The VEF was developed with a focus on deep saline
formations (described in Chapter 2), but many of the  concepts apply to other geologic settings under
consideration for GS. The chapter also provides background on GS as a climate change mitigation
strategy by identifying natural and industrial analogs for GS and reviewing U.S. experience in
regulating subsurface injection.

Chapter 3: Chapter 3 describes geologic attributes that could influence (i.e.,  increase or decrease) the
vulnerability of a GS system to unanticipated migration, leakage, or pressure  changes. This chapter
also discusses the GS footprint component of the spatial area of evaluation.
2 The term "migration" refers to subsurface movement of CO2 (or other fluids) within or out of the injection zone.

3 The term "leakage" refers to the movement of CO2 (or other fluids) to the surface (for example, to the atmosphere or
oceans).

-------
^^^^^^^^^^^^^m
Chapter 4: Five impact categories and associated key receptors are described that could be affected
by unanticipated migration and leakage or pressure changes: human health and welfare, atmosphere,
ecosystems, groundwater and surface water, and the geosphere. Chapter 4 identifies approaches to
evaluate adverse impacts to receptors (human and environmental) that might occur in the event of
unanticipated migration, leakage, or pressure change, and to evaluate spatial area of evaluation with
respect to the receptors that may be impacted.

Chapter 5: The overall vulnerability of a GS site to adverse impacts is not dependent on the presence
of a single attribute or receptor, but is a combined function of the identified geologic attributes and
receptors. Chapter 5 provides a qualitative discussion of this concept of an holistic approach, the
linkage between attribute vulnerabilities and impacts, and how vulnerabilities may change over time.
This chapter also elaborates on two key attributes considered likely conduits, wells and faults/fracture
zones (both existing and pressure-induced). The discussion highlights that even for these individual
attributes, it is the interplay of multiple characteristics that will determine the level of vulnerability,
and not their simple presence.

Chapter 6: The VEF assists in identifying situations that could result in elevated vulnerability to
adverse impacts from GS and often recommends monitoring in such instances. The potential for
adverse impacts may be minimized in many cases by careful monitoring and mitigation. Chapter 6
reviews monitoring technologies that can be used to measure how much CO2 is injected, to track
the location of stored CO2, and to  detect any CO2 releases4 to the atmosphere. This chapter also
discusses potential mitigation actions in the event of leakage, unanticipated migration, or pressure
changes.

Chapter 7: Chapter 7 provides a summary of the VEF, its structure, development, purpose, and
potential applications.  The chapter also describes next steps that may be taken to further develop,
refine, and validate the VEF.
 Release is another term used for leakage.

-------
BACKGROUND ON GEOLOGIC
SEQUESTRATION
The value of GS as a climate mitigation tool is, in part, contingent on CO2 remaining stored
underground for a long period of time. Long-term storage could be accomplished by injecting
CO2 into appropriate geologic formations with effective trapping mechanisms that will not be
compromised over the storage period. It has been estimated that CO2 storage will need to function
for at least several hundred years (Chalaturnyk and Gunter, 2004; IPCC, 2005; Holloway et al.,
2007). While a desirable timeframe for effective sequestration of CO2 may be as much as thousands
of years, effective storage of CO2 for even several hundred years may provide valuable flexibility in
reducing CO2 emissions and addressing climate change impacts.

This chapter describes likely formations where CO2 might be injected and the mechanisms that can
trap CO2 underground. It reviews natural and industrial analogs and related field experience that
are often cited to support the view that GS can be an effective climate change mitigation technology
(Benson et al., 2002; Heinrich et al.,  2003; IEA, 2004; IPCC, 2005, 2007; Dooley et al., 2006;
MIT, 2007). U.S. regulation of subsurface injection of fluids also provides relevant experience that is
summarized here.

The chapter is organized into the following sections:

       Section 2.1 outlines the kinds of geologic formations being considered for GS, including
       deep saline formations, depleted oil and gas reservoirs, coal seams, and other geologic
       formations.

       Section 2.2 explains the various mechanisms and properties of CO2 and geologic formations
       that control the underground movement and trapping of CO2.

       Section 2.3 examines practical experience relevant to sequestration, including existing GS
       projects, natural and industrial analogs for GS, and relevant EPA regulatory experience.

       Section 2.4 summarizes the chapter.
2.1 Geologic Settings under Consideration for Sequestration

The behavior of CO2 in the subsurface and potential vulnerabilities of a GS project are functions
of both the type of geologic formation into which CO2 would be injected and the prior uses of the
geologic setting, if any. Figure 2.1 is a general illustration of how GS could be implemented in a
variety of geological settings. The VEF was developed mainly to evaluate deep saline formations, but
many of the concepts also apply to other geologic settings.

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

       Deep saline formations: In these sedimentary formations, the pore space between the
       formation rock is filled with water containing elevated concentrations of dissolved salts
       (brines). These formations are being considered for GS because they form very large basins,
       are located at significant depth (generally below 800-1,000 meters), and typically are not
       considered viable sources of potable  groundwater because of their salinity and depth.

-------
 ackground on geologic sequestrati
   Overview of Geological Storage Options
   1 Depleted oil and gas reservoirs
   2 Use of CO, in enhanced oil and gas recovery
   3 Deep saline formations  (a) offshore (b) onshore
   4 Use of COj in enhanced coal bed methane recovery
Produced oil or gas
Injected CO
Stored CO,
  _2km
Figure 2.1. Examples of Geologic Formations Being Considered for GS. Source: CO2CRC, 2007

       Deep saline formations are believed to have the greatest potential storage capacity and
       are more widespread than other storage options (Dooley et al., 2006; NETL, 2007), and
       therefore are the primary focus of the VEF. Stacked reservoirs, in which multiple formations
       are overlain vertically, separated by lower permeability rock, may be considered particularly
       advantageous for GS. For a given surface area, such settings would provide greater vertical
       storage space, and the multiple layering may provide additional CO2 structural and
       stratigraphic traps (see section 2.2 for a discussion of CO2 trapping mechanisms). In
       addition, because of their depth, deep saline formations are penetrated by few wells or other
       artificial penetrations that could serve as pathways for CO2. Furthermore, the pressure
       and temperatures typically encountered at such depths are sufficient to maintain CO2 as a
       supercritical fluid; and supercritical CO2 requires much less storage space than it does in
       gaseous form. Finally, there is currently little competing demand for the resources contained
       in these formations, including saline  pore waters (IPCC, 2005; Dooley et  al., 2006).
       However, advances in desalination  technologies and increasing water demand in certain
       regions could lead to increased competition in the future. A challenge in this GS option is
       the displacement of large volumes of water to create the pore space for CO2. The Sleipner
       Project in the North Sea is an example of a commercial operation in which captured CO2 is
       injected into a saline formation.

       Depleted oil and gas reservoirs: These formations have effectively stored  oil and natural gas
       for hundreds of thousands to millions of years before human extraction (Benson et al.,  2002;
       IPCC, 2005; Haszeldine, 2006). As a result, there is reason to believe these same formations

-------
ckground on geologic sequestrati*
    could effectively store injected CO2. Although a wealth of geologic data are available on
    the characteristics of these reservoirs, they are also known to have many abandoned oil
    and gas production and exploration wells that could be conduits for CO2 to escape from
    the subsurface. Chapter 5 contains more information on the potential for such wells to be
    unanticipated migration or leakage pathways.

    Enhanced oil/gas recovery: CO2 is currently injected in some U.S. oil fields to enhance
    oil production. Because these  formations effectively stored oil  for hundreds of thousands to
    millions of years, it is believed that they can be used to store injected CO2 for long periods
    of time. However, enhanced oil recovery (EOR) operations are not currently designed
    to maximize CO2 storage but rather to increase production of oil (Advance Resources
    International, 2006). The Weyburn site in Saskatchewan, Canada, is an example of an
    EOR operation using CO2 that is captured and shipped via pipeline from a commercial
    coal gasification plant in Beulah,  North Dakota. Geologic sequestration in EOR fields faces
    the same issues associated with depleted oil and gas reservoirs. Specifically, EOR fields have
    substantial geological information, but are known to have active and abandoned oil and
    gas production and exploration wells that could be unanticipated migration and leakage
    pathways (Celia et al., 2004; Heller, 2005).

    Coal seams: Coal seams are being considered for GS because of coal's high affinity for CO2
    (Haszeldine, 2006). Micropores in the coal matrix, made more accessible through coal seam
    fractures, can adsorb gases, including CO2 and methane (CH4). However, the fine cleats that
    create coal micropermeability can become plugged as CO2 replaces CH4, thereby restricting
    flow and causing localized matrix swelling. Matrix swelling around an injection wellbore may
    reduce permeability and injectivity and, if reduced injectivity is not overcome, may lead to
    reduced effective storage capacity.

    Methane is a GHG that can contribute to climate change effects if discharged into the
    atmosphere (IPCC, 2007). While there are few examples in practice, the CH4 released by
    the adsorption of CO2 could be recovered for commercial use. Coal seams are sometimes
    underground sources of drinking water (USDWs), and may therefore be subject to additional
    requirements. Though coal seams are considered an option for CO2 storage, considerable
    research and development is still  needed to understand coal seam CO2 sequestration.

    Other geologic settings: Other rock types such as basalts and oil or gas rich shale, geologic
    repositories such as salt caverns, and abandoned mines may also be considered for GS, but
    are  not the subject of current focus. Each of these settings has  advantages and disadvantages
    with regard to its potential to  effectively store CO2 based on its specific geologic
    characteristics. For example, basalt has the disadvantage of low porosity, permeability, and
    fractures that may result in the unanticipated movement of CO2 out of the injection zone;
    but has the advantage that CO2 could be permanently trapped in mineral form through
    chemical reactions of the CO2 with silicates in the  basalt to form carbonate minerals (IPCC,
    2005). Additionally, these settings may not accommodate the  anticipated scale of GS.

-------
   ckground  on geologic sequestrati*
2.2 Subsurface CO2 Movement and Storage Mechanisms

Knowledge of subsurface movement and trapping of CO2 assists in understanding the potential for
CO2 to migrate out of the injection zone.

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

The rate of CO2 fluid flow depends on the properties of CO2 and other fluid phases present in the
injection formation, properties of the formation itself, and physical and geochemical interactions
that may occur in the subsurface. The primary fluid transport mechanisms that control the rate of
movement of CO2 in the subsurface include:

      Fluid flow caused by injection-induced pressure gradients (where higher gradients result in
       faster flow rates).

      Fluid flow caused by existing hydraulic gradients in the injection formation.

      Buoyancy-driven flow caused by the density differences between CO2 and the formation
       fluids (which may result in  upward migration of CO2).

      Dispersion and fingering caused by formation heterogeneities and viscosity contrasts between
       CO2 and the formation fluid(s) (CO2 is less viscous than water and will preferentially "slide"
       over saline waters and channel into high permeability zones).

      Diffusion (this has a relatively minor effect).

Properties of the injection zone formation(s) that affect the rate of CO2 movement include its
permeability, thickness,  and heterogeneity. A higher permeability results in faster CO2 migration,
and a greater thickness means that a greater total volume of CO2 can migrate at the given rate.
Geologic heterogeneities also can control CO2 flow. For example, zones of high permeability such as
a sand lens or an open fracture can  act as conduits that allow CO2 to move much faster than would
be expected based on the bulk properties of the rock. In contrast, low permeability zones such as
shale can slow down or even stop flow. Some of these physical and geochemical processes also  can
affect the movement of CO2 in the subsurface by retarding CO2 flow and acting as CO2 trapping
mechanisms, as  discussed below.

2.2.2 Physical and Geochemical Trapping Mechanisms

Geologic sequestration of CO2 occurs through a combination of structural and stratigraphic
trapping, residual CO2 trapping, solubility trapping, mineral trapping, and preferential adsorption
trapping. These  mechanisms are functions of the physical and chemical properties of CO2  (see
Appendix A for  a summary of these properties) and the geologic formations into which the
CO2 is injected. Figure 2.2 illustrates the relative effectiveness of the different mechanisms (with
the exception of preferential adsorption trapping) in trapping CO2 over time. Impermeable
physical barriers are considered to be the most effective physical trap in the near term. Although
mineralization is the most permanent trapping mechanism, it occurs relatively slowly compared to
the others. The various trapping mechanisms, based on the discussion in the IPCC Special Report on
Carbon Dioxide Capture and Storage (IPCC, 2005), are as follows.

-------
ckground on geologic sequestrati*
    Structural and stratigraphic
    trapping occurs when injected
    CO2 rises within the storage
    formation because of its relative
    buoyancy and/or the applied
    injection pressure and then
    reaches a physical barrier that
    inhibits further upward migration.
    The physical barrier could be a
    stratigraphic trap (a formation or
    a group of formations that act as a
    permeability and capillary barrier
    which impedes or prevents upward
    migration of supercritical CO2)
    or a structural trap such as one
    formed by folded or faulted rocks
    (IPCC, 2005).
100
                                          10,000
              10       100      1,000
              Time since infection stops (years)
Figure 2.2. Effectiveness of Trapping Mechanisms Over
Time. Source: IPCC, 2005, Figure 5.9.
    Residual CO2 trapping occurs
    when CO2 moving through the
    formation (s) is retained in the pore
    space by capillary forces. This retention, also called capillary trapping, may range from 15%
    to 25% for typical storage formations (Holtz, 2002), but could exceed 25%, depending upon
    the porosity and permeability of the formation(s). When the degree of capillary trapping
    is high and CO2 is injected at the bottom of a thick formation or group of formations,
    capillary trapping can prevent CO2 from reaching overlying structural and stratigraphic traps
    (IPCC, 2005; Kumar et al., 2005).

    Solubility trapping occurs when injected CO2 contacts a fluid formation (e.g., saline water)
    and dissolves into the fluid (also known as dissolution trapping). CO2 that is dissolved in a
    formation fluid is not buoyant; it is instead trapped within the formation fluid. The density
    of formation fluids with dissolved CO2 increases, so formation fluids containing dissolved
    CO2 may sink within the formations (GEO-SEQ Project Team, 2004; Streit and Watson,
    2004; IPCC, 2005), though this effect may be limited by geologic heterogeneities (Lindeberg
    and Wessel-berg, 1997; Ennis-King and Paterson, 2003).

    Mineral trapping occurs when the injected CO2 reacts with the formation waters or
    formation rocks, or both, to form carbon-containing minerals such as carbonates. Although
    some of the injected CO2 may react relatively quickly to form solid mineral phases, it is
    generally believed that converting all the injected CO2 into solid minerals could take several
    thousand years. Nevertheless, the permanence of mineral trapping makes it a desirable
    feature for long-term storage (Wilson, 2004; IPCC, 2005).

    Preferential adsorption trapping occurs when coal and certain organic-rich shales have a high
    affinity for CO2, meaning that CO2 can be adsorbed to the coal and shale surfaces. Coal may
    contain up to 25 cubic meters of CH4 per metric ton of coal (IPCC, 2006). Because coal
    has a greater affinity for CO2 than CH4,  CO2 injected into coal seams can displace CH4, be

-------
          ckground on geologic sequestrati*
              adsorbed and trapped, and the released CH4 could be recovered for commercial use, though
              application of this concept is still in early developmental stages. Carbon dioxide will remain
              trapped in coal and certain organic-rich shales under stable pressure and temperature (IPCC,
              2005; Haszeldine, 2006).


       2.3 Practical Experience Relevant to Geologic Sequestration

       Existing project experience, natural and industrial analogs, research and current regulatory experience
       with underground injection contribute to the understanding of CO2 behavior in geologic systems.

       2.3.1 Existing Projects

       Ongoing GS projects show that CO2 can be successfully injected and sequestered in geologic
       formations (see, for example,  IEA, 2004). Currently operating commercial CCS sites include the
       Sleipner Project in the North Sea (Norway), the Weyburn Enhanced Oil Recovery Project (Canada),
       and the In Salah Gas Formation (Algeria). These projects provide valuable field experience which will
       help improve understanding of GS systems, CO2 behavior, and storage mechanisms. For example,
       they provide information that different geologic formations have the injectivity, containment, and
       storage effectiveness  needed for long-term sequestration. These projects have practically applied site
       selection tools and monitoring techniques and programs. The existing projects have also highlighted
       the importance of establishing a baseline as a part of an effective monitoring program (see chapter
       6), and have provided insight into the use of models to help predict the behavior of CO2 in the
       subsurface. However, it should be noted that these sites have been operating for only a relatively
       short period of time (up to a decade), and hence do not demonstrate the efficacy of GS over the
       longer required storage time periods of hundreds to thousands of years.

       A new set of commercial GS projects will be implemented in the very near future, including the
       Gorgon Joint Venture (Barrow Island, Australia). In addition, U.S. field experiments, including
       the Frio Brine Experiment (Texas) and regional projects supported by DOE's Regional Carbon
       Sequestration Partnerships Program, will contribute valuable information about GS in coming years
       (see U.S. DOE [2007b] for a summary of this program). For a more comprehensive list of current
       and planned GS projects,  see  NETL's CO2 Storage Web site5.

       Existing projects indicate that GS is  a viable technology. However, commercial-scale deployment
       of GS will involve substantially larger volumes of CO2, and individual GS projects will need to
       inject greater volumes of CO2 than current DOE pilot projects and other international sites.
       Commercial-scale GS projects will encompass areas that may be miles in diameter (as opposed to
       the small fraction of a mile encompassed by most pilot projects). Therefore, current analogs may
       not demonstrate the full range of scenarios that are likely to be encountered in commercial-scale
       deployment. Commercial-scale projects may be more likely to:

              Encounter geologic heterogeneities that may serve as unanticipated migration and leakage
              pathways, including faults and fractures and other geologic features such as high permeability
              sand lenses or "pinches" in the confining system.

              Intersect potential anthropogenic pathways such as unplugged wells.


10     5 http://www.netl.doe.gov/technologies/carbon seq/core rd/world projects.html

-------
          ckground  on geologic sequestrati*
              Experience adverse pressure effects that can cause fracturing or regional effects on
              groundwater flow.

              Encounter basin-wide effects, and influences of neighboring projects.

       It is also noteworthy that pilot demonstration projects are generally designed to exhibit the viability
       of the technology (e.g., minimal unanticipated migration of CO2). Sites are chosen for these projects
       because they are anticipated to successfully contain the smaller volumes of CO2, and hence may
       not portray the full range of geologic and anthropogenic features that could be encountered in
       commercial-scale deployment (Friedmann, 2003).  However, pilot projects can nevertheless provide
       useful information, and some of their inherent limitations may be overcome by evaluating multiple
       projects implemented across a variety of geologic settings.

       2.3.2 Natural and Industrial Storage Analogs

       Natural and industrial systems that have stored or are storing CO2 and other fluids (e.g., liquids
       and gases) may also provide insights into the feasibility of GS. Although, as noted in the previous
       section, the quantity of CO2 that may need to be injected for GS may be much greater than these
       analogs, the evaluation  of these analogs has improved the understanding of storage mechanisms and
       processes.

       Carbon dioxide accumulates underground naturally in a variety of geologic settings. For example,
       200 million metric tons of naturally occurring CO2 have remained trapped in the Pisgah Anticline in
       central Mississippi, northeast of the Jackson Dome, for more than 65 million years with no  evidence
       of unanticipated migration or leakage (IPCC, 2005). Such natural analogs provide information
       about the ideal conditions for long term storage.

       Industrial practices of injecting and storing fluids underground may also serve as analogs for GS. The
       oil and gas industry, for example, has been storing natural gas in underground reservoirs for nearly
       100 years (IPCC,  2005). Experience from natural gas storage operations suggests that it is possible to
       store gases effectively in the subsurface. However, there are examples of gas escaping through wells,
       faults, and fracture zones (Perry, 2005).  Furthermore, these sites are generally used for temporary
       storage and hence only provide insight, but not a demonstration of, the long-term feasibility of
       underground storage of fluids and gases. These sites also provide valuable evidence that confining
       systems can be exposed to repeated stress cycling (i.e. depressurizing and pressurizing) without
       adverse effects on  seal integrity.

       The oil and gas industry also has more than 35 years of experience in site characterization and
       injection of CO2 through enhanced product recovery projects (Benson et al., 2002;  Heinrich et al.,
       2003; IPIECA, 2007). EOR projects contribute substantial knowledge about the design of CO2
       injection wells and technologies for handling, injecting, and monitoring injected supercritical CO2.
       However, such projects are designed to maximize oil production rather than provide storage of CO2
       for long periods.

       2.3.3 Regulatory Experience

       Federal and State regulations protecting underground sources of drinking water under SDWA
       address the injection of fluids into the subsurface (including liquids, gases and semisolids). These
       regulations are designed to ensure that injected fluids do not endanger USDWs and address siting,
11

-------
          ckground  on geologic sequestrati*
       well construction, monitoring and site closure. For example, the program regulates the injection of
       CO2 for enhanced oil and gas recovery. The EPA Underground Injection Control (UIC) regulations
       reflect a great deal of technical expertise by operators and regulators on relevant geological issues and
       well construction and operations associated with injection. The UIC Program has been successfully
       regulating the injection of billions of gallons of fluids annually into tens  of thousands of injection
       wells for more than 30 years (Benson et al., 2002). This regulatory experience will provide useful
       insight for GS projects and is seen as a clear indication that GS projects will be addressed through an
       established and effective regulatory system. For more information, please visit
       http://www.epa.gov/safewater/uic/wells_sequestration.html.
       2.4 Chapter Summary

       This chapter described the geologic formations and trapping mechanisms necessary for the effective
       storage of CO2. It also examined the feasibility of GS as a climate mitigation technology by
       reviewing natural and industrial analogs as well as EPA's current regulatory experience in subsurface
       injection. The next chapter provides an overview of the VEF and discusses the geological attributes
       that could result in vulnerabilities to adverse impacts.
12

-------
       VULNERABILITY EVALUATION FRAMEWORK FOR
       GEOLOGIC SEQUESTRATION: GEOLOGIC
       SYSTEMS AND ATTRIBUTES
       The VEF identifies attributes of GS systems that may lead to increased vulnerability to adverse
       impacts, identifies potential impact categories, and provides a series of decision-support flowcharts
       that are organized, systematic approaches to assess the attributes and impacts. These attributes and
       impact categories were carefully selected by EPA as the key factors of GS systems to be included in a
       vulnerability evaluation, through the process described in Chapter  1 of literature review, consultation
       with experts, and professional knowledge. The conceptual approach to the VEF shown in Figure 3.1
       has the following components:

             The GS system first is characterized in terms of the injected CO2 stream, the confining
              system, the injection zone, and a series of geologic attributes that could influence (i.e.,
              increase or decrease) the vulnerability of the GS system to unanticipated migration, leakage,
              and undesirable pressure changes (first column).
                                                                                    Human populations
                                                                                    Populations covered
                                                                                    by Executive Orders
              Capillary entry
                pressure
                                                                     Human
                                                                   health/welfare
                                                                                    Cultural/recreational
                                                                                       resources
                                                                                    Economic resources
                                                Unanticipated
                                                migration and
                                               leakage (of CO2
                                               and other fluids)
Confining
 system
              Faults/fracture
                 zones
                              Geochemical
                               processes
                                                                      Sensitive species
                                                                                      Legislatively
                                                                                    protected species
                             Geomechamcal
                              processes
                                                                                       Regional
                                                                                      groundwater
                                                                                         flow
             Physical capacity
                                                                 Groundwater and
                                                                   surface water
                                                                     Protected/sensitive
                                                                       drinking water
                                                                         supplies
                                                 Pressure
                                                 changes
                            Geochemical and
                             geo mechanical
                              processes
                OLOGIC SEQUESTRATION SYSTE
                 AND GEOLOGIC ATTRIBUTES
                                        iLAREA
                                  OF EVALUATION
POTENTIAL IMPACT CATEGORIES
      AND RECEPTORS
13
       Figure 3.1. VEF Conceptual Model

-------
14
         merability evaluation framework for geologic sequestration: geologic systems and attributes
              An approach is then provided for defining the spatial area that should be evaluated for
              adverse impacts associated with unanticipated migration, leakage, or undesirable pressure
              changes (middle column).

              Potential impact categories and associated key receptors are then identified, including human
              health and welfare, the atmosphere, ecosystems, groundwater and surface water, and the
              geosphere (last column).

       Though not explicitly shown in Figure 3.1, the VEF further recognizes that pressure changes in the
       injection zone and overlying geosphere may be linked to unanticipated migration or leakage pathways
       and the associated impacts depicted in Figure 3.1, through processes like induced fracturing, fault
       reactivation, and the exceedence of capillary entry pressure. These concepts are also discussed in
       this chapter. Furthermore, the VEF also recognizes that impacts to the geosphere, groundwater and
       surface water, ecosystems, and the atmosphere may also impact human health and welfare.

       This chapter is an overview of the geologic sequestration system, the geologic attributes that have
       been identified as affecting vulnerability, and the related spatial area that would be evaluated for
       unanticipated migration, leakage, and pressure changes. These are represented in the first two
       columns of the conceptual framework shown in Figure 3.1. Chapter 4 discusses the potential adverse
       impacts of GS and potentially affected receptors, the last column of the VEF conceptual framework.

       For the purposes of the VEF and this report,  a binary classification of low and elevated vulnerability
       are qualitatively defined  as follows:

              Low vulnerability: Adverse impacts are not expected to be associated with the attribute or
              receptor under evaluation.

              Elevated vulnerability: Particular attention should be paid to the attribute or receptor under
              evaluation. In some cases, adverse impacts may occur if actions are not taken to further
              examine and/or manage the vulnerability associated with the attribute or receptor. Examples
              of actions that may be taken include corrective action at wells, targeted monitoring, and the
              development of mitigation plans.

       The metrics and binary classification schemes for each  attribute in this chapter can be used to
       qualitatively evaluate the level of vulnerability associated with each attribute. The classification
       schemes indicate whether vulnerability is expected to be low or elevated on a qualitative basis.
       Elevated vulnerability associated with a single attribute does not imply that overall vulnerability
       associated with the GS site is elevated. In some cases, there may be actions that could be taken to
       minimize vulnerability (e.g., targeting questionable wells for corrective action). These instances
       are indicated where  applicable in the accompanying decision-support flowcharts that have been
       developed for the components of the GS system.

       The flowcharts represent first-order evaluation approaches and, as such, should be used only
       with additional information and analysis to support more comprehensive risk assessment and/or
       decision-making. In the  future, binary classification schemes could be further developed to reflect
       a more refined, multiscaled classification scheme, as warranted by available information,  data, and
       expert opinion. Future steps could also include developing the decision-support flowcharts into an
       integrated evaluation tool that has a more quantitative and numerical basis, presented in a user-
       friendly format.

-------
         merability evaluation framework for geologic sequestration:  geologic systems and attributes
       This chapter contains the following sections:

              Section 3.1 describes the GS system and geologic attributes that may affect unanticipated
              migration, leakage, and pressure changes.

              Section 3.2 discusses the spatial area of evaluation.

              Section 3.3 summarizes the chapter.


       3.1 Geologic Sequestration System and Geologic Attributes

       Descriptions of geological attributes that can influence the vulnerability of a GS system to
       unanticipated migration, leakage, or pressure changes include a variety of metrics and classification
       schemes. The metrics and schemes can be used to determine whether there is low or elevated
       vulnerability to unanticipated fluid migration, leakage, and/or pressure changes associated with
       each attribute.  The attributes presented here are organized according to those that are relevant to
       the confining system and those that are relevant to  the injection zone. The section also describes
       evaluation processes (decision-support flowcharts) for the confining system, the injection zone, and
       the CO2 stream.

       3.1.1 Confining System and Related Geologic Attributes

       A confining system is defined as a geologic  formation, group of formations (e.g., shale or siltstone),
       or part of a formation (sometimes referred to as an aquitard) that is composed of impermeable or
       distinctly less permeable material which acts as a barrier to the upward flow of fluids  stratigraphically
       overlying the injection zone. Geologic attributes of the confining system identified as influencing the
       potential for unanticipated migration and leakage include the following.

              Lateral extent. Lateral extent is defined in the VEF as the surface area of the confining
              system that overlies the GS  footprint. This attribute can be evaluated using a metric of total
              surface area measured in appropriate units such as square miles. Elevated vulnerability is
              associated with a confining  system with a lateral extent that is less than the GS footprint.

              Capillary entry pressure. The capillary entry pressure is defined in the VEF  as the added
              pressure that is needed across the interface of two immiscible fluid phases (e.g., supercritical
              CO2 and water or brine) for CO2 to enter the confining system. Appropriate evaluation
              metrics for determining sufficient capillary entry pressure include CO2 column height
              and injection pressure (Harrington  and Horseman,  1999). Elevated vulnerability may be
              associated with the exceedence of the confining system capillary entry pressure.

              Permeability. Permeability refers to the ability of a geologic material to allow transmission of
              fluid through pore spaces. Appropriate metrics for evaluating permeability include the darcy
              unit. Elevated vulnerability may be associated with geologic materials with a permeability
              greater than clay, shale, or siltstone.

              Travel  time. Travel time refers to the interval of time that is required for a fluid (e.g., CO2
              or brine)  to migrate across the thickness of the confining system. Factors that will influence
              travel time include the confining system thickness, permeability, diffusion, retardation (as a
15

-------
         merability evaluation framework for geologic sequestration: geologic systems and attributes
              result of sorption or desorption), and geochemical reactions. Thus, all other parameters being
              equal, GS systems at greater depth will have longer associated travel times. The metric for
              travel time is a unit of time such as years. Travel times that compromise the integrity of the
              project are considered to result in elevated vulnerability.

              Wells. Wells (and other artificial penetrations such as boreholes) may serve as conduits for
              fluid movement and hence could result in elevated vulnerability to adverse impacts. Numerous
              metrics have been identified as relevant to the evaluation of vulnerability associated with
              wells, including well depth, well integrity (including construction materials, and seal and plug
              materials), and spatial density of well occurrence. The level of vulnerability associated with wells
              can be evaluated by considering all of these metrics. Wells and faults have been identified as
              one of the most likely unanticipated migration and leakage pathways in GS systems; as such,
              they are discussed in greater detail in Chapter 5 of this report, which also includes an evaluation
              approach.

              Faults/fracture zones. Faults are breaks in the Earth's crust that occur when the crustal
              rock is either compressed or pulled apart. A fracture is any local separation or discontinuity
              plane in a geologic formation that divides the rock into two or more pieces. Fractures are
              commonly caused by stress exceeding the rock strength. For the purposes of this report,
              fractures are defined as distinct from faults by their smaller scale. Faults may serve as
              either barriers or conduits to fluid flow (Omre et al, 1994; Lewicki et al., 2006; Wilkens
              and Naruk, 2007). Numerous metrics may be appropriate for evaluating faults and/or
              fracture zones, including density, stratigraphic position, connectivity, sealing/conductive
              (transmissive), stress level, orientation, and fault reactivation pressure (multiplied by a safety
              factor). The level of vulnerability associated with faults and fractures can be evaluated by
              considering all of these metrics. Chapter 5 discusses faults and fracture zones in greater detail.

              Geochemical processes. Geochemical processes are chemical reactions that may cause
              alterations  in mineral phases. A number of different geochemical processes could influence
              the confining system. Acidity caused by the reaction of CO2 with water may partially dissolve
              confining zone geologic materials, which could have the unfavorable effect of opening fluid
              migration pathways within the confining zone. Geochemical reactions could also have
              favorable effects, such as the formation of mineral phases as result of the reaction of CO2
              with the geologic material of the confining system and/or formation waters that could help
              to improve the seal of the confining system, by plugging pores and fractures (Johnson et al.,
              2005). Appropriate metrics for evaluating geochemical processes include dissolution rates,
              buffering capacity, molar volume, and pH level. Mineralogy and pH that favor the  formation
              of conduits in the  confining system through dissolution and/or decreases in molar volume
              increase vulnerability; those that do not favor the formation of conduits through dissolution
              and/or increases in molar volume decrease vulnerability.

              Tectonic activity. Technically active settings may be more likely to have transmissive faults
              and/or fractures, and  may be unsuitable for GS (IPCC, 2005). Seismic activity can be used as
              a measure for tectonic activity. Seismic activity is defined as the shifting of the Earth's surface
              due to changes at depth, and it may cause seismicity or earthquakes. An appropriate metric
              for evaluating tectonic activity is the seismic hazard rating. Areas with seismic hazard ratings
              that indicate  the potential for seismicity to cause adverse impacts are considered to  have
              elevated vulnerability (see USGS, 2007).
16

-------
         merability evaluation framework for geologic sequestration: geologic systems  and attributes
              Tectonic activity also covers volcanism, which is the process by which magma and gases are
              transferred from the Earth's interior to the surface. Hotspots are shallow areas of molten rock
              below the surface that persist long enough to leave a record of uplift and volcanic activity.
              Distance to volcanic activity or hotspots serves as a metric for vulnerability, and the presence
              of active volcanoes or hotspots within the spatial area of evaluation may indicate whether
              there is low or elevated vulnerability.

              Geomechanical processes. These are processes that may result in alterations of the structural
              integrity of geologic material. Appropriate evaluation metrics for this attribute include
              fracture pressure, fracture/fault reactivation pressure, and orientation of the fracture or fault
              relative to the orientation of the principal regional stress regime. If the fracture pressure
              and the fracture/fault reactivation pressure (multiplied by a safety factor) are exceeded,
              vulnerability is considered to be elevated. It should be noted that geomechanical processes
              occur at a continuum of scales, and potential impacts, such as deformation of geologic
              formations, can occur without necessarily adversely affecting the integrity of the confining
              system.

       Confining System Evaluation Process

       An evaluation process that can be used to examine the confining system based on the attributes
       described above is shown in Figure 3.2. In the flowchart, elevated vulnerability reflects a
       determination that the confining  system is inadequate and may increase the potential for adverse
       impacts,  and low vulnerability indicates that the confining system is anticipated to be adequate for
       the proposed project. Elevated or low vulnerability determinations in the VEF refer to a specific
       attribute, system, or impact being evaluated and not to the GS site as a whole. Chapter 5 discusses
       key considerations in assessing the overall vulnerability of a GS site.

       The confining system evaluation provides an approach for assessing and reducing vulnerability
       through:

              Establishing that a confining system is present over the necessary lateral extent. To
              ensure the  confining system acts as an effective barrier to fluid flow, it is important that the
              geologic formations (rock layers) of the  confining system are sufficiently laterally extensive
              and continuous to cover the entire area  affected by the CO2 injection. This includes the area
              occupied by the CO2 and a potentially larger area affected by pressure changes associated
              with injection6. The continuity of the barrier can be maintained, despite pinch-outs or
              other discontinuities, if such features in one rock layer are blocked by overlying layers
              of the confining system. In some cases, there may also be a confining system underlying
              the injection zone that serves as a lower barrier to the GS system. If the lateral extent is
              insufficient, it may be possible to alter operational conditions to improve site suitability, for
              example, by injecting into multiple formations of the injection zone, thereby reducing the
              surface  area of the CO2 plume.

              Evaluating the physical properties of the confining system to determine if it provides
              adequate confinement of fluids under the proposed operating conditions. Attributes
              of a confining system that will help to prevent the upward movement of fluids include


       6 Deep saline formations that are laterally unconstrained may in particular have larger areas affected by pressure, in contrast for
YJ     example, to depleted oil and gas fields, where the extent of pressure changes may be limited by the geologic structures of the system.

-------
           merability evaluation framework for geologic sequestration: geologic systems and  attributes
                       O  Determine the necessary lateral extent of the confining system
                                                          Site
                                                        suitability
                                                        may be
                                                   improved by altering
                                                       operational
                                                       conditions?
    Confining
 system present
over the projected
    footprint?
                         Yes
                         Evaluate the physical properties of the confining
                         system:
                         Overlying the CC>2 plume:
                          Permeability
                          Capillary entry pressure
                          Susceptibility to geochemical
                          degradation	
                               Overlying the area
                               affected by pressure
                               changes:
                                Permeability
                                  ForN
                              'theCCV
                               footprint:
                             Projected CO2
                          column height and/or
                           injection pressure
                         vexceed the capillary/
                                 entry
                              pressure?.
                             No
Confining system evaluation
approach:

O Is the confining system present
over the necessary lateral extent?

 Do the physical properties of the
confining system provide adequate
confinement of fluids under the
given  operating conditions?

 Is the confining system integrity
interrupted by "natural" (faults) or
anthropogenic (wells) features or
compromised by operational
conditions?
                          Yes
                                     Evaluate fluid travel time
                                     through the confining system
                        Evaluate confining system integrity
                        Evaluate wells and faults using
                             evaluation flowcharts
                                             Travel time
                                            compromises
                                             the project?
                      Evaluations determine that confining
                         system integrity is interrupted
                                          Elevated vulnerability
                     Well and fault evaluations indicate that the integrity of the confining system is adequate, or
                      adequate with application of corrective action and/or targeted development of monitoring
                                                     and mitigation plans
                                           onfimng
                                          system is
                                        susceptible to
                                      induced fracturing
                                       under proposed
                                         operational
                                         conditions?
                                              Operating
                                           conditions can be
                                               altered?
                                                                Elevated vulnerability
        Figure 3.2. Confining System Evaluation
18

-------
         merability evaluation framework for geologic sequestration:  geologic systems and attributes
              its capillary entry pressure7. A high capillary entry pressure will make it difficult for CO2
              and other fluids to enter the confining system. Travel time, a function of the permeability
              and thickness of the confining system, may also contribute to an effective barrier. A long
              travel time would ensure that if any CO2 or other fluids entered the confining system, they
              would travel  only minimal distances within the confining system over the timeframes that
              are relevant to GS systems. A low permeability will contribute to a long travel time. A low
              susceptibility to dissolution and other geochemical degradation processes will also contribute
              to an effective confining system, thus avoiding thinning of the barrier and the opening of
              potential fluid pathways.

              Evaluating the integrity of the confining system. Another important factor that may limit
              the ability of the confining system to act as a barrier to fluid flow includes the presence of
              fluid-conducting perforations such as  unsealed or unplugged wells, and transmissive faults
              and fractures (both natural and those induced/reactivated by injection). Vulnerabilities
              associated with wells and faults may be managed through altering operational conditions
              such as injecting at a lower rate at a greater number of wells to reduce injection pressure. The
              integrity of the confining system could also be interrupted by other high permeability zones
              such as sand lenses, and other discontinuities, such as pinchouts in the confining system
              formation(s). Targeted monitoring can also detect unanticipated migration and leakage and
              pressure changes, and inform the development of mitigation actions and plans. Mitigation
              actions, such as corrective action for wells, are  discussed in more depth in Chapter 6.

       3.1.2 Injection Zone and Related Geologic Attributes

       The injection zone is a geologic formation, group of formations, or part of a formation of sufficient
       areal extent, thickness, porosity, and permeability to accommodate CO2 injection volume and
       injection rate. The injection zone is characterized by several geologic attributes identified to have the
       potential to influence pressure changes  and are described below.

              Physical capacity. Physical capacity is defined as  the volume within a geologic formation
              that is available to accept CO2.  Injected CO2 will occupy the pore space between the
              grains of the  rock that makes up the injection zone by displacing the fluids that are
              already occupying these pores. If storage capacity  is found to be insufficient for the
              proposed operation due to pressure constraints capacity could be increased by dewatering.
              Appropriate metrics for evaluating this attribute include thickness, surface area, effective (or
              interconnected) porosity, CO2 density (CO2 density generally increases with depth, because
              density increases with pressure,  and pressure generally increases with depth), and residual
              water saturation also referred to as irreducible water saturation. Induced pressure changes
              can also affect physical capacity. For example, if a geologic system is constrained laterally,
              the pressure buildup associated with the CO2 injection may restrict how much CO2 can be
              stored without exceeding fracture pressure, sometimes referred to as effective pore volume
              vs. absolute pore volume (Zhou, et al., 2008).  Other non-technical considerations could
              also potentially affect storage capacity, including economic considerations such as competing
              demands for the injection zone.

              Injectivity. Injectivity characterizes the ease with  which fluid can be injected into a
              geological formation. It will be  influenced by both properties of the injection zone and

, q     7 This is true for CO2 and other nonaqueous displaced fluids. However, in the case of brines, there is no capillary entry pressure to
       overcome because the fluid in the confining system and the brine are both aqueous.

-------
         merability evaluation framework for geologic sequestration: geologic systems and attributes
               operational factors. The properties of the injection zone that control injectivity include
               injection zone thickness, permeability, and pressure within the injection zone8. Though the
               VEF is focused on the geologic system, operational factors that influence injectivity, and
               which may be altered to reduce vulnerability, are discussed below in the description of the
               injection zone evaluation process.

              Geochemical and geomechanical processes. The geochemical and geomechanical processes
               may influence vulnerability to pressure changes by affecting injection zone capacity to store
               CO2 and injectivity.

                  Geochemical processes. Geochemical processes are the chemical reactions that may
                  cause alterations in mineral phases. A number of different geochemical processes could
                  influence the injection zone. Dissolution of CO2 into the formation waters (e.g.,  brine)
                  will reduce the volume of CO2 that is stored as a supercritical fluid, thus reducing needed
                  storage volume (Doughty et al., 2001). Acidity caused by the reaction of CO2 with  water
                  may partially dissolve injection zone geologic materials, which could improve porosity
                  and injectivity. Geochemical processes may also have unfavorable impacts. For example,
                  new mineral phases may form as a result of the reaction of CO2 with the geologic
                  materials and/or formation waters. These new minerals may partially plug pores and
                  thereby have the unfavorable impact  of reducing permeability and porosity (Knauss et al.,
                  2003).

                  Appropriate metrics for evaluating the geochemical processes that could lead to pressure
                  changes include dissolution rates, buffering capacity, molar volume, and pH level.
                  Unfavorable geochemical processes are defined as involving pH and mineralogy that
                  favor precipitation of minerals and/or increases in molar volume, resulting in elevated
                  vulnerability. Favorable geochemical processes, those with low vulnerability, are defined
                  as  involving pH and mineralogy that favor increased injection zone porosity through
                  dissolution and/or decreases in molar volume.

                  Geomechanical processes. These are processes that may result  in alterations in the
                  structural integrity of a geologic material. If pressure in the injection zone is increased
                  because of CO2 injection, the injection zone geologic material may be deformed,  and in
                  the extreme, crack or fracture, or faults might be reactivated. Fracturing of the injection
                  zone is intentionally used to increase  production in oil and gas  operations, and whether
                  or not  this method should be used for GS could be considered  based on further study of
                  the applicability of this technique to  GS and site-specific factors. As a result, appropriate
                  metrics and thresholds for injectivity may be site-specific.

       Injection Zone Evaluation Process

       An evaluation process that can be used to examine the injection zone based on the attributes just
       described is shown in Figure 3.3. As indicated in the flowchart, the vulnerability to adverse impacts
       associated with the injection zone of a GS system may be considered low if the storage capacity is
       adequate, the  injectivity is sufficient, and geochemical and/or geomechanical processes produce
       favorable conditions for injection and storage. In particular, this evaluation provides an approach for
       assessing and reducing vulnerability through:
20     pressure.
        To introduce CO2 into the injection zone, the downhole injection pressure must be higher than the injection zone formation(s) fluid

-------
          merability evaluation framework for geologic sequestration: geologic systems and attributes
               Determining that the injection zone capacity is sufficient. The physical capacity of the
               injection zone is proportional to the total pore volume of the injection zone. Total pore
               volume can be determined by multiplying the injection zone thickness by the injection
               zone surface area and by the fraction of the injection zone that is taken up by pores  (i.e., its
               porosity). However, only a fraction of this total pore volume will actually be available for
               CO2 storage.

               Numerous factors will influence the physical capacity of the injection zone. Carbon
               dioxide will enter only interconnected pores. Most of the pores in rocks such as sandstones
               are interconnected, but some fraction of the pore volume will be "dead space" and not
               interconnected. Carbon dioxide will displace only a fraction of the water in the pores,
               because of residual water saturation. Furthermore, CO2 is not likely to occupy the entire
               thickness of the injection zone because of its buoyancy and heterogeneities within the
               injection zone. Being relatively buoyant, it will tend to rise upward in the injection zone and
               spread out laterally under the confining system or under lower permeability lenses within the
               injection zone. Vertical features such as sealed faults or other discontinuities may also limit
               the pore volume that is accessible to CO2, and may put pressure constraints on the system
               that could also limit  the physical capacity.
              Elevated
             vulnerability
                                                              Elevated vulnerability due to
                                                              inadequate storage capacity
    Sufficient
     physical
    capacity?
                                       Sufficient
                                    injectivity under
                                     the proposed
                                       operating
                                      conditions?
                               Operating
                             conditions can
                              be altered to
                               improve
                               injectivity?
      Geo-
    chemical
    and geo-
   mechanical
properties facilitate
storage under the
  proposed oper-
    ating con-
     ditions?
                                         Low
                                     vulnerability
 Elevated
vulnerability
21
       Figure 3.3. Injection Zone (IZ) Evaluation

-------
         merability evaluation framework for geologic sequestration: geologic systems and attributes
              Properties of CO2 that will influence the amount of space it occupies include its density and
              ability to dissolve in water. The space needed to store a given amount of CO2 will decrease
              with increasing density. Density increases with increases in pressure and depth and decreases
              with increases in temperature. Dissolution will be greater at higher temperatures and
              pressures, and less dissolution occurs at higher salinities.

              Determining that the injectivity is sufficient. As described above, injectivity is the rate at
              which CO2 injection can be sustained over the duration of injection, and is a function of
              properties of the injection zone and operational conditions. The former include injection
              zone thickness, permeability, and pressure within the injection zone. Operational factors that
              may influence injectivity include the length and orientation  of the injection well. Injectivity
              may be increased by maximizing the slotted length of the injection wells (i.e., the part of the
              well that is open and allows passage of CO2 into the injection zone), installing horizontal
              wells that are slotted over much longer reaches than their vertical counterparts, or increasing
              the number of wells used for injecting CO2.

              Evaluating the geochemical and geomechanical properties of the injection zone. By
              virtue of their impact on porosity, geochemical and geomechanical processes may also affect
              the physical capacity of the injection zone.  Depending on whether there is net dissolution or
              precipitation of minerals associated with the injection of CO2, injectivity may be improved
              or decreased correspondingly. Impurities in the CO2 stream  may also react with the injection
              zone rock and formation waters to form new minerals that may locally decrease porosity.
              Fracturing  is intentionally used to increase  production  in oil and gas operations, and
              intentional fracturing of the injection zone could improve permeability and hence injectivity
              of the injection zone. However, the appropriateness of this technique for GS applications
              may need further study.

              Initial estimates of physical capacity can be made using equations such as those in the NETL
              Carbon Sequestration Atlas (NETL, 2007), or the approaches developed by Bachu et al.
              (2007) and Brennan and Burruss (2003). More refined evaluations of the attributes of the
              confining system, the physical capacity, injectivity, and geochemical and geomechanical
              processes within the injection zone are likely to rely on use of numerical modeling and site
              characterization data (Xu et al., 2003; Pruess et al., 2004; Celia et al., 2005; IPCC,  2005).

       3.1.3 Carbon Dioxide Stream

       The CO2 stream and its characteristics may affect geologic attributes within the GS system and
       ultimately contribute to increased vulnerability to adverse impacts. A captured CO2 stream from
       a power plant or industrial source would probably not be pure CO2. The specific impurities and
       their concentrations in the CO2 stream will differ depending on the fuel source, the capture process,
       constraints (i.e., concentration limits) associated with the mode of conveyance to the injection site
       (e.g., pipeline), and injection concentration limits. For example, trace amounts of sulfur dioxide
       (SO2), nitrogen oxide (NO), hydrogen sulfide (H2S), hydrogen (H2), carbon monoxide (CO),
       methane (CH4), nitrogen (N2), argon (Ar), mercury (Hg), cyanide (Cn), and oxygen (O2) could
       be found in a captured CO2 stream (IPCC, 2005; U.S. DOE,  2007c). Some of these may be
       of potential concern because of their toxicity (e.g.,  Cn, Hg), others because of their potential to
       accelerate corrosion processes (e.g., H2S), and still others simply because they may increase the total
       amount of needed  storage space (e.g., Ar). Under different capture scenarios, the volumes of these
22

-------
         merability evaluation framework for geologic sequestration: geologic systems and attributes
       impurities can vary significantly. For example, the concentration of impurities as a percentage of total
       CO2 volume for post-combustion capture at coal-fired power generation plants is approximately
       0.01%. In contrast, the concentration of impurities for precombustion capture at gas-fired power
       generation plants is approximately 4.4% (IPCC, 2005).

       Given the potential presence of these impurities, characterization of the CO2 stream is very
       important. Impurities could create more acidic conditions, possibly accelerating the formation
       of fluid-conducting pathways through the corrosion of well seal materials and the dissolution of
       geologic materials in the subsurface. Impurities could also impact storage capacity and well integrity.
       The CO2 Stream Evaluation, depicted in Figure 3.4, outlines key considerations for evaluating
       CO2 stream impurities. Though not a focus of the VEF, it is acknowledged that impurities may
       also impact surface infrastructure (see, for example, Rhudy 2004). Chapter 4 discusses potential
       receptors that might be affected by those impurities, and these receptors include human populations
       in general, populations covered by Executive Orders, economic and cultural/recreational resources,
       legislatively protected species and other sensitive species, and groundwater and surface water. The
       CO2 Stream Evaluation will help identify where impurity-specific targeted monitoring could indicate
       the need for mitigation actions and help reduce the potential for adverse impacts.
                           Characterize the CO2 stream: identify impurities
             Evaluate the potential effects of impurities on confining system and injection zone
               attributes, including wells, faults/fracture zones, and geochemical processes
                If there is a high level of uncertainty or heterogeneity, consider identifying
                   impact categories that may be affected by CO2 stream impurities
                Evaluate level of vulnerability, as appropriate, consider corrective actions
                and/or targeted monitoring and mitigation plans that are impurity-specific
       Figure 3.4. Carbon Dioxide Stream Evaluation

       3.2 Spatial Area of Evaluation: Geologic Sequestration Footprint

       Determining the full geographical extent of a GS system is essential for site characterization and for
       establishing the spatial area to be monitored for potential adverse impacts. The size and shape of
       the CO2 subsurface plume and associated pressure front will depend on the total volume of CO2
       injected over the duration of the injection, operational factors, and the geologic characteristics of the
       confining system and injection zone, including their geometry, heterogeneities, and other geologic
       features. The size of the CO2 plume will increase during the period of injection, which might last
       25 to 50 years. Because CO2 is relatively buoyant and has  lower viscosity than water and brine, the
       plume may also continue to change position and shape for some time after injection ceases. However,
23

-------
          merability evaluation framework for geologic sequestration: geologic systems  and attributes
       secondary storage mechanisms, including dissolution, capillary, and mineral trapping, are anticipated
       to eventually minimize the size and immobilize the CO2 plume (see Chapter 2 for a discussion of
       secondary storage mechanisms).

       The pressure front9 associated with the injected CO2 may extend significantly farther than the
       CO2 plume itself. This is particularly true in saline aquifers, where the geographical area affected by
       elevated pressure may be several orders of magnitude larger than the area occupied by CO2 (Nicot et
       al., 2006; Birkholzer et al., 2007). As discussed in Chapter  5, the pressure front may dissipate after
       injection stops, or may remain elevated, depending on the lateral boundaries of the system.

       For the purposes of this VEF, the spatial area encompassed  by the CO2 plume and associated
       pressure front is termed the GS footprint. Figure 3.5 provides different geologic scenarios that might
       be encountered, and approaches to delineate the footprint for each scenario. Estimates of the size of
       the footprint associated with the total amount of CO2 to be injected over the lifetime of a project are
       anticipated to be an element of the initial evaluation of the adequacy of a GS system. This footprint
       may also serve as the basis for defining the area where baseline conditions are established prior to
       injection at a GS site (see chapter 6 for more discussion on monitoring and baseline). Delineation
       of the footprint may also be performed at multiple stages of the project, as the footprint expands
       and changes shape during injection and due to buoyancy driven flow, and to predict the effect of
       secondary trapping mechanisms. In most instances, modeling in combination with site-specific data
       will most likely be used to delineate the GS footprint. Data collected during monitoring can be used
       to refine and calibrate  models and confirm the location and dimensions of the CO2 plume and the
       pressure front.
>

r ^
Scenario 1 : Horizontal
geologic formations
;
Delineate the GS footprint.
Possible footprint scenarios
include:
I
r -\
Scenario 2: Large regional
trap or series of traps



f >
Scenario 3: Sloped
geologic formations
;
The aerial extent of the
footprint may be initially
estimated using generic
approaches and then
refined using an
appropriate model, and
site-specific data.
>
The GS footprint may be
defined by delineating the
surface area of the traps.
Modeling may be used to
determine if the storage
capacity of the trap(s) will
be sufficient.
r ^

r

r
Scenario 4: Complex
geology
>
Difficult to predict the GS
footprint; modeling may be
used to determine CO2
migration up-dip.
r >

f
Difficult to predict the GS
footprint; modeling may be
used; site may require
additional data collection or
may be inappropriate due to
higher levels of uncertainty.
f >
f
Footprint delineation may occur at different stages of a project. Site characterization data may be used to refine footprint
and calibrate models.
24
Figure 3.5. Geologic Sequestration (GS) Footprint Delineation

9 The pressure front can be defined by the pressure differential that is significant enough to cause adverse impacts to overlying receptors
(e.g., fluid displacement into an overlying aquifer).

-------
         merability evaluation framework for geologic sequestration: geologic systems and attributes
       The spatial area to be evaluated for potential adverse impacts is dependent on geologic characteristics
       and potential receptors. Depending on the particulars of a GS system, the area to be evaluated may
       be larger than the GS footprint itself. Chapter 4 describes receptors and how they may influence the
       spatial area of evaluation.

       All of the attributes of the confining system and injection zone should be considered within the
       GS footprint. However, it is particularly important to understand the pathways that could serve as
       conduits for CO2 and other fluids, and  to understand the causes and changes in subsurface pressure.
       These pathways are introduced below and described in further detail in Chapter 5. Appendix B
       summarizes and compares some VEF geologic attributes with critical CO2 unanticipated migration
       and leakage pathways or characteristics that would affect the suitability of a proposed storage site
       identified by the IPCC (2006) and IPIECA (2007).

       Migration and Leakage Pathways

       To understand the potential for  unanticipated migration and leakage, it is necessary to identify
       pathways that could allow the movement of CO2 and fluids displaced by the CO2 (e.g., brines)
       and other geologically stored fluids (e.g., oil and CH4). The IPCC (2005) identifies three principal
       pathways:

             The pore system of the confining system if it is of low permeability, or around the lateral
              extent of the confining system if it is insufficient.

             Anthropogenic artificial  penetrations such as abandoned wells.

             Openings in the confining system, such as fractures or faults, which may be natural or
              induced by pressure changes.

       Pressure Changes

       Injecting CO2 into geologic formations will, in most cases, cause subsurface changes in pressure.
       Increased pressure can lead to the unanticipated migration of fluids either through existing pathways
       or induced fracturing. It can also have impacts on overlying receptors, even if the CO2 remains
       contained within the injection zone. The applied pressure could force CO2 and other fluids through
       existing conduits, such as abandoned wells and faults. Induced fracturing and fault reactivation
       can also occur if injection pressure exceeds formation pressures. Even if the CO2 and other fluids
       remain contained, pressure changes associated with the injection may still cause adverse impacts to
       groundwater and surface water, including causing changes in groundwater flow direction and water
       table levels, and pressure-induced displacement of brine and other fluids through the pore  structure
       of the rock into overlying aquifers. The  nature of pressure changes in the subsurface associated with
       injection will be determined by both geologic attributes of the GS system and operational  factors.
       These effects can be minimized through an understanding  of the relevant geologic attributes, careful
       site characterization, careful operation of GS systems,  and  monitoring.
25

-------
         merability evaluation framework for geologic sequestration: geologic systems and attributes
       3.3 Chapter Summary

       This chapter presented an overview of the geologic sequestration system, which comprises the
       confining system, the injection zone, and the CO2 stream. Geologic attributes that may influence the
       vulnerability to unanticipated migration, leakage, and pressure changes and evaluation processes for
       the confining system, the injection zone, and the CO2 stream were described. Finally, the spatial area
       that may be evaluated for unanticipated migration, leakage, and pressure changes was discussed, and
       a flowchart that depicts GS footprint delineation scenarios was presented. Chapter 4 discusses the
       potential adverse impacts of GS and potentially affected receptors.
26

-------
      VULNERABILITY EVALUATION FRAMEW
      FOR GEOLOGIC SEQUESTRATION: IMPACTS
      AND RECEPTORS
       Unanticipated migration, leakage, and changes in subsurface pressure could potentially cause adverse
       impacts to human health and welfare, the atmosphere, ecosystems, groundwater and surface water, or
       the geosphere. Furthermore, impacts to the latter four categories could in turn impact human health
       and welfare. For example, groundwater contaminated via unanticipated migration could have adverse
       impacts to human health. Adverse impacts to forests or fisheries or other harvested natural resources
       could also result in adverse economic (welfare) impacts to humans.

       The vulnerability of a GS system to these adverse impacts is a function of both the presence of the
       key receptors  in the impact categories and the levels of exposure. A number of factors affect exposure,
       including but not limited to the concentration and volume of the release, the rate of release (i.e.,
       slow vs. sudden), the proximity of the release to the receptor, and wind or wave dispersion. Impacts
       are also affected by whether the release is acute but limited (in time or spatial extent) or chronic.
       Unanticipated migration of CO2 from the injection zone will not necessarily result in leakage and/or
       subsequent adverse impacts. A qualitative discussion of the links between geologic attributes and
       potential impacts and receptors is provided in Chapter 5.

       The VEF includes decision-support flowcharts for evaluating the impact categories, which can help
       identify key receptors and applicable qualitative exposure thresholds. Additionally, the VEF may be
       further developed to quantitatively account for exposure and threshold levels.

       This chapter focuses on the last component of the VEF conceptual model: potential impact
       categories and key receptors associated with each category (last  column of Figure 3.1). Building
       on the GS footprint described in Chapter 3 the  spatial area of evaluation is expanded to take the
       receptors that may be impacted into account. This chapter includes the following sections:

            Section 4.1 discusses potential human health and welfare receptors  and impacts.

            Section 4.2 presents potential impacts to the atmosphere.

            Section 4.3 covers potential ecosystems receptors  and impacts.

            Section 4.4 presents potential groundwater and surface  water receptors and impacts.

            Section 4.5 considers potential impacts to the geosphere.

            Section 4.6 examines how receptors may influence the area of evaluation.

            Section 4.7 summarizes the chapter.
27
       4.1 Potential Human Health and Welfare Impacts

       Adverse health effects caused by high levels of CO2 can range from minor, reversible effects to
       mortality, depending on the concentration of CO2 and the length of the exposure. Release of CO2
       may also adversely impact recreational and economic resources by restricting access or use or by
       changing the quantity and quality of the resource.

-------
          merability evaluation  framework for geologic sequestration: impacts and  receptors
       The Human Health and Welfare Evaluation, depicted in Figure 4.1, outlines the components of
       human health and welfare impacts for the purposes of the VEF. Key receptors that could be adversely
       impacted in the event of unanticipated migration and leakage and the associated potential impacts
       are identified. Suggestions are made for assessing and managing vulnerability of these receptors to
       adverse impacts.

       Adverse health effects from the release of high levels of CO2 (and other fluids) could be experienced
       by the general human population  and sensitive subpopulations, which,  for the purposes of the
       VEF, are identified as those subpopulations covered by Executive Orders. In addition, cultural,
       recreational, and economic resources  (human welfare) could be negatively affected by CO2.  Potential
       impacts to these key receptors are  detailed below.

       4.1.1 Human Populations

       The vulnerability of a human population to the release of CO2 (and other fluids) is affected by the
       population's size and sensitivity to CO2 (and other fluids that may leak from the GS system), and  the
       proximity to and concentration of the release.

       Carbon dioxide is a naturally occurring gas present in ambient air at a concentration of roughly
       0.04% [i.e., 380 parts per million (ppm)].  However, exposure at much  higher concentrations can
       have a variety of impacts on human health10. These impacts result when concentrations of CO2 or
       other constituents in the sequestered  stream exceed toxicity threshold concentrations, and can range
       from mild discomfort to more permanent effects, including death (Benson et al., 2002).
                                    Identify human health and welfare receptors
         General human
          populations
                       Populations covered by executive orders
                   Cultural/
                  recreational
                   resources
Economic
resources
                         Environmental
                          Justice (EJ)
                          populations
                                     Tribal
                                   populations
    Children
                          Evaluate and
                          address any
                        disproportionate
                            impacts
                                   Consult and
                                  coordinate with
                                      tribal
                                  governments
Assess potential
adverse impacts
   to children
               Evaluate the potential to exceed relevant thresholds and consider developing monitoring and
                                          mitigation plans as appropriate
28
       Figure 4.1. Human Health and Welfare Evaluation
10 Projected increases in CO2 concentrations from anthropogenic emissions range from 41 to 158 % above present levels or 535
to 983 ppm by 2100 (Meehl et al., 2007). Such increases would result in atmospheric CO2 concentrations of 0.054 to 0.098 % by
volume in 2100, which is well below published thresholds for adverse health effects.

-------
   merability evaluation framework for geologic sequestration: impacts and receptors
Short-term (acute) exposure to CO2 levels at or below 3% causes only temporary and reversible
health effects such as increased breathing rate, mild headache, and sweating. When CO2 levels
exceed 3%, breathing rate increases substantially (e.g., from 11 liters per minute at 3% to 26
liters per minute at 5%), hearing and vision may become impaired, and headaches can occur.
Symptoms become more severe as CO2 levels exceed 5% (Benson et al., 2002; CEC, 2007). At high
concentrations, CO2 can be fatal to humans following a relatively short exposure, because CO2
displaces oxygen, causing suffocation (Benson et al., 2002; Oldenburg et al., 2002a; Rice, 2003;
Rice and Rhudy, 2004). When CO2 levels reach 10%, unconsciousness can occur after one to several
minutes of exposure, and at levels exceeding 15%, unconsciousness occurs in less than one minute.
When CO2 levels reach 30%, death occurs within a few minutes (Benson et al., 2002).

Measuring the effects of long-term (chronic)  exposure to elevated CO2 levels is more complicated,
but studies have found no evidence of any adverse health impacts from chronic exposure to levels
below 1% (IPCC, 2005). Furthermore, there is evidence that exposure to air with up to a 3%
CO2 concentration can result in physiological adaptation with no negative long-term health effects
(Benson et al., 2002).

Small (low-flux), continuous leaks are less likely to result in adverse impacts unless there are
topographic lows where CO2 could accumulate (Oldenburg and Unger, 2004; Shipton et al., 2005)
and a lack of dispersion based on wind patterns (Heller, 2005). Anthropogenic features may also
accumulate CO2, such as basements and other underground structures, including mines. There is a
greater potential for adverse affects to human health associated with higher flux leaks such as those
observed at natural analogs  like Mammoth Mountain in California, and Latera and Ciampino
in Italy (Chiodini and Frondini, 2001; Eichhubl et al.,  2005; NASCENT, 2005; Shipton et al.,
2005). However, Holloway et al.  (2007) report that natural analogs underline the significance of
understanding the nature of a release and subsequent dispersion, rather than focusing exclusively on
the volume of CO2 released; for example, the impacts of large sudden releases may be minimized
by atmospheric dispersion.  Furthermore, large sudden releases are anticipated to be unlikely if GS
systems are appropriately characterized, designed, and monitored (Benson et  al., 2002; IPCC, 2005;
Lewicki,  2006;  Holloway et al., 2007). Hence, events such as the 1986 fatal occurrence at Lake
Nyos, Cameroon, in which 0.24 million metric tons of CO2 (Benson et al., 2002) were suddenly
released,  are highly unlikely to occur in association with GS11.

Impurities in the CO2 stream (introduced in Chapter 3) may independently pose a health risk to
humans,  but these health risks are not currently well characterized. For example, H2S is of particular
significance because of its toxicity. Therefore, the release of stored CO2 with H2S to the atmosphere
could have greater health and safety impacts than the release of pure CO2.

To mitigate adverse impacts to the general population, it may be necessary to identify proximate
populations and develop monitoring plans  that reference appropriate exposure thresholds.
Appropriate thresholds can  be defined as the lowest concentration identified by a regulatory agency
as causing adverse health effects in humans. Regulatory agencies that  prescribe short-term and
chronic exposure thresholds for humans include the National Institute for Occupational Safety and
Health and the Occupational Safety and Health Administration.
11 A Lake Nyos type event is even less likely to occur with GS, because it occurred as a result of a set of relatively unique conditions
associated with the accumulation of CO2 at the bottom, and turnover of, a deep stratified lake, and there are very few deep, stratified
lakes in the United States.

-------
30
         merability evaluation framework for geologic sequestration: impacts and receptors
       4.1.2 Populations Covered by Executive Orders

       Additionally, it may be necessary to give special consideration to the potential for adverse impacts
       to populations covered by Executive Orders. Although these groups may experience impacts similar
       to general human populations, Executive Orders require proposed federal regulations and programs
       to specifically address potential impacts to environmental justice populations, children, and tribal
       governments; therefore they may need to be given particular attention.

              Environmental justice populations. Executive Order 12898, Federal Actions to Address
              Environmental Justice in Minority Populations and Low-Income Populations, establishes
              requirements to ensure enhanced protection from disproportionate impacts for minority
              and low-income populations.  The order directs federal agencies to identify and address
              disproportionately high adverse human health or environmental effects of its activities on
              minority and low-income populations (Executive Order 12898, 1994).

              If there is a potential for disproportionate impacts to environmental justice populations, it
              may be necessary to address these impacts by using targeted monitoring,  altering the project
              design, or using other mitigation methods.

              Tribal populations. Executive Order 13175, Consultation and Coordination with
              Indian Tribal Governments, establishes requirements to ensure enhanced protection from
              disproportionate impacts for American Indian tribal populations. This order requires
              that  federal agencies consult and coordinate with tribal governments in formulating or
              implementing policies that have substantial direct effects on tribal governments (Executive
              Order 13175,  2000). Thus, it may be necessary to consult and coordinate with potentially
              affected tribal governments.

              Children. Children are more  sensitive than adults to many substances. Therefore, safety
              thresholds for  areas occupied by children (e.g., schools, day-care centers)  may need to be
              more stringent than those set  for the general population. Executive Order 13045, Protection
              of Children from Environmental Health Risks and Safety Risks, establishes requirements to
              ensure enhanced protection from disproportionate impacts for children. The order requires
              federal agencies to prioritize identification and assessment of environmental health and  safety
              risks that may disproportionately affect children, and requires federal agencies to ensure that
              these risks are addressed  (Executive Order 13045, 1997).

       Other specific population subgroups that may need to be considered can be defined by conditions
       that lead to increased vulnerability to  CO2 exposure. These populations can include individuals with
       pulmonary disease, panic disorder patients, and patients with cerebral disease or trauma (Rice, 2003;
       Rice and Rhudy, 2004). For example, exposure to elevated levels of CO2 is an increased risk to  those
       with head trauma injuries or who have certain cerebral diseases, because CO2 can inhibit blood
       clotting. As with the general human populations, monitoring and mitigation plans can be developed
       for these subpopulations that take into account relevant exposure thresholds.

       4.1.3 Cultural and Recreational Resources

       Unanticipated migration and leakage  resulting from GS  activities could adversely affect cultural and
       recreational resources by precluding human use (e.g., due to elevated CO2 levels) or by impacting
       the  resource itself (e.g., chemical degradation of historical artifacts by reaction with CO2 or other

-------
31
         merability evaluation framework for geologic sequestration: impacts and receptors
       stream constituents). Cultural sites include places of archaeological, historic, or natural significance;
       American Indian resources; and other cultural resources, including cemeteries and paleontological
       resources (U.S. DOE, 2007c). In addition, certain recreational activities that are dependent on the
       health and welfare of ecological receptors (e.g., fish and game) could be affected if those ecological
       receptors are adversely impacted.  If cultural or recreational resources are present, it may be necessary
       to develop a monitoring and mitigation plan, based on appropriate exposure thresholds.

       4.1.4 Economic Resources (Surface and Subsurface)

       Unanticipated migration and leakage resulting from GS activities has the potential to adversely
       affect certain surface or subsurface economic resources such as forestry, agriculture, mineral resources
       (mining), and oil and gas reservoirs. Impacts resulting from unanticipated migration, leakage, or
       pressure changes from GS might  include restricted access to resources, restricted use of resources, or
       changes in the quantity and/or quality of resources.

       A GS project could also indirectly impact human welfare even in the absence of fluid migration
       and/or release, if it precludes alternative land use or  subsurface activities (e.g., resource extraction).
       This would include, for example,  future restrictions  on the use of saline formations as drinking water
       sources (if appropriate desalination technologies became available) or for the extraction of metal ions
       such as lithium. In addition, access to overlying or underlying oil and gas reservoirs and storage space
       for other substances (e.g., natural gas) could be limited because of GS operations. Further, changes
       in underground pressure caused by GS could increase seismic activity (discussed in Section 4.5) or
       change quantities and qualities of groundwater by altering some combination of flow rate and/or
       direction (discussed in Section 4.4).

       It may be necessary to address these potential adverse impacts with, for example, targeted monitoring
       using appropriate thresholds and/or alteration of the GS project design.
       4.2 Potential Atmospheric Impacts

       According to the National Oceanic and Atmospheric Administration, atmospheric concentrations
       of CO2 in 2006 totaled approximately 382 ppm (NOAA, 2007), and the current rate of increase
       in atmospheric CO2 concentrations is approximately 1.9 ppm per year (IPCC, 2007). Geologic
       sequestration is intended to mitigate local and global climate change impacts by providing long-
       term storage of CO2 emissions that would otherwise have contributed to global atmospheric CO2
       concentrations.

       In some cases,  small releases of CO2 or other GHGs (e.g., CH4) as a result of releases from GS may
       not adversely impact local environmental receptors (e.g., ecological receptors, groundwater and
       surface water, the geosphere, and human health). However, releases can reduce the  climate benefits
       of capturing CO2, thus decreasing the overall effectiveness of GS as a climate change mitigation
       strategy.

       An effective monitoring plan can help minimize adverse atmospheric impacts as a result of GS
       by ensuring that releases of CO2 or other GHGs are quickly identified and remedied. (For more
       information on GS monitoring approaches, see Chapter 6.) In addition to identifying and remedying
       releases, it is important to record and account for them. The IPCC develops and publishes guidelines
       for estimating  and inventorying national GHG emissions. The 2006 IPCC inventory guidelines

-------
          merability evaluation framework for geologic sequestration:  impacts and receptors
       for the first time included transport, injection, and GS of CO2 as sources of GHG emissions that
       should be considered in national inventories (IPCC, 2006). The EPA is currently evaluating how to
       implement the guidelines and accurately account for emissions associated with these activities.

       The purpose of the Atmosphere Evaluation, depicted in Figure 4.2, is to identify fluid-conducting
       pathways and identify approaches that can be used to evaluate their potential to conduct CO2 to the
       atmosphere. Monitoring strategies can be implemented to reduce the potential for CO2 to reach the
       atmosphere in the event of unanticipated migration.
                                          Identify and evaluate potential
                                            leakage pathways to the
                                                 atmosphere
             Identify faults and fracture
             zones and evaluate their
             potential to conduct fluids
  Identify and evaluate the
     integrity of wells
 Evaluate the lateral extent and
   adequacy of the confining
           system
             See Faults Evaluation for
             vulnerability assessment
           approach and monitoring and
             mitigation considerations
 See Wells Evaluation for
  vulnerability assessment
approach and monitoring and
  mitigation considerations
    See Confining System
  Evaluation for vulnerability
 assessment approach, and the
CO2 Spatial Area of Evaluation,
the Human Health and Welfare
 Evaluation and Ecosystems
 Evaluation for monitoring and
   mitigation considerations
       Figure 4.2. Atmosphere Evaluation
       4.3 Potential Ecosystems Impacts

       The Ecological Receptors Evaluation, Figure 4.3, is used to frame the discussion of ecosystems
       impacts in the following sections. As shown in Figure 4.3, legislatively protected species and other
       sensitive species are key ecological receptors that could be adversely impacted by unanticipated
       migration, leakage, or pressure changes resulting from GS activities. There are very few studies on
       the effects of CO2 at an ecosystem level (West et al., 2005), and adverse impacts to ecosystems from
       GS is currently an active area of research (West et al., 2006). Additional receptors not identified in
       the VEF may also be impacted by GS. As the VEF is a iterative evaluation process, these may be
       included as new information comes to light.

       4.3.1 Sensitive Species

       Sensitive species are organisms that are especially vulnerable to high CO2 concentrations. Numerous
       ecological receptors may be sensitive to exposure to CO2, other stream constituents, brine, or other
       gases that  may be released as a result of GS activities either because of their behavior (e.g., burrowing
       mammals, ground-nesting birds, reptiles that preferentially occupy low-lying areas, and soil-
       dwelling microorganisms) or because their physiology makes them unusually sensitive to increased
32
       concentrations.

-------
          merability evaluation framework for geologic sequestration: impacts and  receptors
                                         Identify ecological receptors
>
r
State or federally listed threatened or
endangered species or species of special
concern
>
r
Identify species and their habitat range or
location
I
May be necessary to demonstrate that the
endangered species and their habitat will not
be adversely impacted
>
r

\
r
Other sensitive species
i
r
Identify habitat range or location
>
r
Evaluate the potential to exceed relevant thresholds and consider developing monitoring and mitigation
plans as appropriate
33
       Figure 4.3. Ecological Receptors Evaluation

       It is important to understand the condition (e.g., stressed by other factors or not) of these receptors
       as well as their potential ranges or stationary locations. Sensitive species can serve as sentinels for
       potential impacts to a wider array of organisms. If sensitive species are protected, then a reasonable
       assumption could be made that less sensitive species within that area are protected as well. Some
       species may also be sensitive to secondary effects of CO2, including increased acidity (in aquatic
       environments and in terrestrial soils), mobilization of metals caused by increased acidity, and loss of
       food resources and habitat in the case of vegetation loss caused by CO2 (e.g., fish and other species
       that are sensitive to increased acidity and aqueous metal concentrations, and mollusks and other
       species that are reliant on precipitation of carbonates).

       It may be necessary to identify habitat range or location for each identified sensitive ecological
       receptor, and to consider targeting for monitoring and mitigation plans. Sensitive species and
       potentially relevant thresholds are summarized below.

       Soil-dwelling animals and microbes. Soil CO2 concentration varies between 0.2% and 4% (Benson
       et al., 2002) and is a function of depth, water content, soil type, and time of year (Bouma et al.,
       1997). Soil CO2 concentrations between 20% and 30% are thought to be sufficient to significantly
       alter the dynamics of ecosystems (Benson et al., 2002). There is a relatively wide range, and some
       overlap, in the CO2 concentrations causing different adverse effects in burrowing invertebrates such
       as worms and insects.  For example, depending on the specific species, behavioral changes may be
       exhibited at CO2 concentrations between 2% and 39%, paralysis at concentrations between  10%
       and 59%, and death at concentrations between 11% and 50% (Sustr and Simek, 1996; Benson et
       al., 2002). Soil-dwelling animals may begin experiencing negative physiological effects at 2% CO2,
       and concentrations of approximately 15% CO2 can be lethal (Benson et al., 2002). In another
       study examining CO2 tolerance of dung insects, 80% CO2 was the threshold for paralysis (Holter,
       1994). Microbes have a wide range of sensitivity to  CO2. For fungi, significant inhibitory effects are
       observed at 30% CO2, with concentrations of 40% CO2 generally lethal (Benson et al., 2002).

-------
         merability evaluation framework for geologic sequestration:  impacts and receptors
       Plants. Elevated atmospheric CO2 can enhance plant photosynthesis if adequate amounts of
       other nutrients are available. However, because plant roots take in O2 and expel CO2, elevated
       CO2 in the soil can inhibit root respiration. The specific physiological limits for plant roots (the
       time and concentration required for CO2 toxicity) are unclear (USGS, 2001). However, plant
       growth at a natural CO2 spring site was accelerated at concentrations up to 10%, but inhibited at
       concentrations between 30% and 55% (Vodnik et al., 2006). At Mammoth Mountain, California,
       a soil concentration of 30% caused death of all trees, regardless of age or species (Saripalli et al.,
       2002). Evaluations at landfill sites show that soil CO2 concentrations as low as 10% may inhibit root
       function, but that some species can tolerate concentrations up to 20%  CO2 without detrimental
       effects on root function (Flower et al., 1981; Ehrlich, 2002). In addition to inhibition of root
       respiration, increased CO2 concentrations may increase soil acidity. Increased soil acidity can make
       potentially toxic metals more bioavailable, which can affect the dynamics of soil ecosystems (McGee
       and Gerlach, 1998; Saripalli et al., 2002).

       Surface-dwelling animals.  Few data are available indicating the toxic thresholds of CO2 in surface-
       dwelling  animals, except for humans. Research suggests that birds may begin exhibiting behavioral
       changes or paralysis when CO2 concentrations reach 40% (U.S. Patent 5435776), and reptiles (and
       possibly amphibians) have a higher CO2 tolerance than other ground-dwelling vertebrates (Benson
       et al., 2002). Additionally, thresholds for human exposure to CO2 may be appropriate proxies for
       surface-dwelling animals (Benson et al., 2002; IPCC, 2005). As noted  above, wildlife that rely on
       vegetation for food, cover, or nesting/breeding habitat can suffer secondary, habitat-related effects if
       plants are affected adversely.

       Aquatic organisms. Adverse impacts to aquatic organisms can result if CO2 leaks into overlying
       water bodies, including lakes, streams, and oceans. High concentrations of CO2 reduce the amount
       of oxygen reaching the blood in aquatic organisms with gills, causing suffocation (Maina, 1998).
       Fish, for example, are likely to experience negative effects with a three-fold increase of ambient
       aquatic CO2 concentrations. Aquatic organisms and ecosystems are also vulnerable to changes in
       pH resulting from increased CO2. In general, a pH level of less than 5  or greater than 10 is lethal for
       most fish; low pH can cause acidification of body tissues (Benson et al., 2002; Turley et al., 2004,
       2006; Miles et al., 2007).

       Other invertebrates may be sensitive to increased CO2 as well. Miles et al. (2007) examined CO2
       tolerance of the sea urchin (Psammechinus miliaris), an ecologically important organism with  a
       sensitive pH balance, and found that a pH of 6.16 was lethal to 100%  of the test organisms after 7
       days, and a pH below 7.5 was severely detrimental to the test organisms. Negative effects included
       hypercapnia (excess CO2 in the blood) due to elevated CO2 concentrations and decreased pH in
       coleomic fluid (Miles et al., 2007). Turley et al. (2004) reported that in saltwater, a pH ranging from
       5 to 6.7 was lethal for zooplankton in 72 hours, and a pH ranging from 4.8 to 6.2 was lethal in 24
       hours. However, they also found that a pH ranging from 5.6 to 7.8 was not lethal after 96 hours.

       An increase in aquatic CO2 concentration is unlikely to significantly reduce aquatic photosynthetic
       productivity in phytoplankton that have CO2-concentrating mechanisms  (Turley et al., 2004).
       However, some organisms' photosynthetic rate is more sensitive to CO2 concentration, meaning that
       they may be more susceptible to changes in CO2 (Turley et al., 2006).

       Calcifying aquatic organisms. Calcifying organisms are dependent on dissolved carbonate in the
       water to produce their protective shells. As CO2 concentrations in water increase, pH decreases (a
34

-------
         merability evaluation framework for geologic sequestration: impacts and receptors
       process sometimes referred to as ocean acidification) and carbonate becomes less available in the
       aquatic environment. Organisms such as corals may be severely affected by acidic conditions if they
       are either unable to form shells or if the shells are dissolved by the acidic water. Turley et al. (2006)
       found reduced calcification in cultures of coccolithophore species that were in water in contact with
       atmospheric concentrations of 750 ppm CO2. Spicer et al. (2007) examined the effects of elevated
       CO2 and low pH on the velvet swimming crab (Necora puber) and found that after 16 days at a pH
       level of 6.74, the animal experienced hypercapnia. To compensate, the test animals dissolved their
       exoskeletons to make bicarbonate available in their blood (Spicer et al., 2007).

       Deep geologic ecosystems. Deep geologic ecosystems support significant communities of microbial
       life but are poorly characterized. Microbial life is ubiquitous and extends several  miles into the
       Earth's crust (as deep as 4,000 meters), and in all of the types of locations being considered for GS
       (IPCC, 2005). At extreme depths, microbes are  adapted to very high pressures and temperatures and
       can catalyze reactions that involve compounds such as hydrogen sulfide (H2S), sulfate (SO), nitrate
       (NO), iron (Fe), and CO2 to derive energy (West and Chilton, 1997; Ehrlich, 2002). The effect
       of a change in CO2 concentration at extreme depths is unknown; deep-earth organisms may not
       respond to CO2 as organisms living nearer the atmosphere do, unless the organism uses CO2 in its
       metabolism. Relatively little is known about the importance or unimportance of such organisms in
       ecosystems and carbon cycles.  Further research is needed to address this uncertainty.

       Microbial activity could also potentially impact the GS system (Quintessa, 2004). For example,
       microbes could catalyze the precipitation of minerals, thus decreasing storage capacity. Alternatively,
       they could produce organic acids, thereby enhancing the corrosion of well seals and the dissolution
       of geologic materials.

       4.3.2 Legislatively Protected Species

       State and federal government agencies use  legislation to identify certain species populations as
       threatened or endangered (i.e., as defined in the Endangered Species Act of 1973). Impacts similar
       to those listed in the previous section are possible for these species as well. Because of their increased
       sensitivity, it is important to understand the legal status of these species (e.g., endangered or
       threatened) as well as their potential ranges or stationary locations. Further, it may be necessary to
       demonstrate that an endangered species and its habitat will not be adversely impacted by evaluating
       the potential for adverse impacts and targeting the species for appropriate monitoring based on
       current and known thresholds.
       4.4 Potential Groundwater and Surface Water Impacts

       Geologic sequestration could potentially impact groundwater and surface water. EPA is mandated to
       protect the nation's waters (both USDWs and surface waters) under the SDWA and the Clean Water
       Act. USDWs could potentially be more susceptible to adverse impacts than surface waters because
       of their closer proximity to the injection zone. Although this discussion focuses on groundwater
       impacts, many of the concepts can be also be applied to evaluating surface waters. The evaluation
       depicted in Figure 4.4 identifies key receptors, including protected and/or sensitive water bodies,
       water quality, and regional groundwater flow, and identifies appropriate monitoring and mitigation
       strategies.
35

-------
         merability evaluation framework for geologic sequestration: impacts and receptors
       Groundwater and surface water may be vulnerable to adverse impacts associated with unanticipated
       migration of CO2 (and other fluids) and pressure changes, including the water quality and
       groundwater flow effects described below.

       4.4.1 Water Quality

       Geologic sequestration could impact groundwater if CO2 escapes the GS system or if brine or other
       fluids are displaced as a result of pressure changes into overlying or underlying aquifers. Impacts
       include changes such as increased salinity, increased acidity (reduced pH), and mobilization of
       metals or other impurities. The effect of GS on a water body may be indicated by a change in specific
       relevant water quality parameters. Relevant water quality parameters that can be used to evaluate
       groundwater include:

              Total dissolved solids (TDS). Migration of brine into overlying aquifers as a result of CO2
              injection could potentially endanger human health and the environment. Though there
              is not an EPA primary drinking water standard for TDS, water with TDS greater than
              500 milligrams per liter (mg/L) is not recommended for human consumption. EPA also
              considers aquifers with TDS below 10,000 mg/L to be underground sources of drinking
              water, therefore migration of brine into these sources may have potential implications if this
              threshold is exceeded.

              Buffering capacity.  Systems made of geologic materials with lower buffering capacities (e.g.,
              sandstones as opposed to limestones) may be more susceptible to acidification in the event of
              migration of CO2.
                                  Identify groundwater and surface water
                                               Presence
                                            of protected or
                                               sensitive
                                            drinking water
                                               supplies?
Consider targeted monitoring
    and mitigation plans
                                              No
1
4-
Total
dissolved
solids (TDS)
I
Evaluate relevant water quality and
I
Buffering
capacity
1
quantity
parameters

4- ^ ^ J

Mineralogy
>

f
Sorbed
contaminants
>
t

Microbial
populations
>

Regional groundwater
flow
1 >
Evaluate the potential to exceed thresholds, such as drinking water
standards, in the event of CO2 and/or brine migration and leakage, and
consider developing monitoring and mitigation plans

r
Evaluate effects of pres-
sure changes, and
consider developing
monitoring and mitigation
plans based on site-
specific thresholds
       Figure 4.4. Groundwater and Surface Water Evaluation
36

-------
  merability evaluation framework for geologic sequestration: impacts and  receptors
       Mineralogy. Migration of CO2 into overlying aquifers could cause acidification, which
       in turn may cause dissolution of metal-containing minerals and desorption of metals and
       organic species from the aquifer matrix (Jaffe and Wang, 2003; Kolak and Burruss, 2003,
       2004; Wang and Jaffe, 2004). If metal and metalloid-bearing mineral phases (e.g., metal
       oxides, sulfides, carbonates) are determined to be present, geochemical modeling may be
       necessary to evaluate whether metal aqueous concentrations will exceed drinking water
       standards in the event of migration into an overlying aquifer. The mobilization of metals and
       other contaminants within the injection zone could also potentially impact drinking water
       sources, if the metal-containing fluids leaked into an overlying aquifer, or could preclude the
       use of the injection zone as a future drinking water source without treatment.

       Sorbed contaminants. Sorbed metals, organic contaminants, and gases could be released
       under acidic conditions created by CO2 and other geochemical processes (e.g. competitive
       desorption). If impurities (e.g., organics, metals, gases) are present, geochemical modeling
       may be necessary to evaluate if aqueous concentrations will exceed drinking water standards
       in the event of migration of CO2 and other fluids into overlying aquifers.

       Microbial populations. As noted in Section 4.3.1, microbial populations may catalyze
       geochemical reactions. As such, their influence may need to be considered when determining
       aqueous concentrations of metals and other impurities.

In addition, impurities in the CO2 stream, such as H2S, SO2,  and other sulfur- and nitrogen-
containing species, can increase the acidification of groundwater compared to pure CO2.This could
lead to increased mobilization of trace amounts  of hazardous metals if a CO2 stream leak comes into
contact with groundwater (IPCC, 2005). However, the concentrations of sulfur and nitrogen bearing
gaseous constituents in the CO2 stream are likely to  be low, and hence their impacts on injection
zone rocks and groundwater may be relatively minor (Apps, 2006).

The migration of CO2 through overlying soils may also cause mobilization of contaminants in
the vadose zone. For example, the FutureGen risk assessment identifies radon mobilization as
a potential secondary effect of migration of CO2 and other impurities through soils. Radon is
a naturally occurring element in some soils and might be mobilized if CO2 and other potential
impurities diffuse through soils at high enough rates (U.S. DOE, 2007c). However, this impact may
be minimal because such elements are likely to be present only at trace concentration levels (Apps,
2006). Consideration may also need to be given to the possible entrainment  of organic species by
migrating CO2 in the case of injection into oil and gas fields. Though Apps (2006) reports that
concentrations of such organic species is likely to be low.

For surface water (including oceans), biotic water quality criteria may also need to be considered. For
example, fish and other species may be sensitive to increased concentrations of metals such as copper,
cadmium, and zinc, and increased acidic conditions  may inhibit precipitation of calcareous shells in
surface water and oceans.

Geologic sequestration that involves injection into subseabed geologic formations raises the potential
for water quality impacts in the ocean. If sufficient volumes of leaked CO2 were to come into contact
with sea water, it could locally increase the acidity of the water and potentially induce a secondary set
of water quality impacts (e.g., changes in concentrations of metals through interaction with surface
sediments). Impurities and substances added to the CO2 stream (e.g. to enable or improve capture)
could also impact marine water quality. Finally, the effects of supercritical CO2 on marine water

-------
38
         merability evaluation framework for geologic sequestration: impacts and  receptors
       quality parameters may need to be considered under certain circumstances. At great enough depths,
       the CO2 may be more dense than the surrounding seawater, and hence may tend to sink, rather than
       being dispersed through buoyancy-driven upward migration and dispersion.

       4.4.2 Regional Groundwater Flow

       Groundwater flow directions, water table levels, and the distribution of groundwater (areas where
       groundwater is gained and lost) could all be affected in shallower aquifers sensitive to pressure
       changes caused by CO2 injection (Nicot et al., 2006; Tsang et al., 2007). This could influence the
       quantity of water available for groundwater-based municipal water supplies and cause interferences
       with injection wells. Further, a rise in the water table level under a stream or lake fed by groundwater
       could result in elevated water levels, changes in flow rates, and changes in the geometry of the water
       bodies. Finally, there may be effects on intertidal zones, where displaced brine may ultimately be
       discharged into oceans.

       Groundwater flow direction, flow rate, and quantity are metrics for this receptor in the VEF, and
       the determination of elevated vulnerability thresholds for these metrics is likely to be site specific.
       Physical groundwater and surface water modeling is likely to be necessary to evaluate effects of
       pressure changes.

       4.4.3 Protected and Sensitive Drinking Water Supplies

       Protected and sensitive drinking water supplies may be particularly vulnerable to adverse impacts
       associated with migration of CO2 and other fluids and pressure changes, including the water quality
       and groundwater flow effects described above. Because of their increased vulnerability,  it is important
       to identify the presence or absence of such water supplies, and it may be necessary to consider
       targeted monitoring and  the development of mitigation plans if these water bodies are present.
       4.5 Potential Geosphere Impacts

       Changes in subsurface pressure from GS could have direct impacts on the geosphere. Subsurface
       pressure changes that cause an exceedence of the subsurface geologic formation's geomechanical
       strength could cause fracturing or reopening of faults and fracture zones (Quintessa, 2004; IPCC,
       2005). This in turn could cause unanticipated movement of CO2 and other fluids and increase the
       potential for the impacts discussed above. Other potential impacts include induced seismic activity
       such as earthquakes in the extreme case (Healey et al., 1968) and land deformation through uplift
       (Quintessa, 2004; Birkholzer et al, 2007).
       4.6 Spatial Area of Evaluation: Influence of Receptors

       Chapter 3 introduced the GS footprint, the geographical area that may be impacted by unanticipated
       migration, leakage, and pressure changes, and is the focus of site characterization and monitoring
       (column two of Figure 3.1). Under certain circumstances, it may be necessary to extend site
       characterization and monitoring beyond the GS footprint. Receptors outside the footprint might
       be adversely impacted if leakage occurs near or at the footprint boundary. For example, it might be
       appropriate to extend monitoring into the habitat of an endangered species that straddles the boundary
       of the CO2 plume. In addition, concentrating features such as topographic lows, wind patterns, or

-------
39
          merability evaluation framework for geologic sequestration: impacts and receptors
       ocean current patterns could transport CO2 beyond the boundary of the GS footprint (Bogen et al.,
       2006; U.S. DOE, 2007c). Furthermore, site characterization and monitoring for pressure effects,
       including displacement of brine into overlying aquifers, are likely to extend over an even larger area.

       It is also important to note that areas of evaluation are not expected to be static entities. As discussed
       in Chapter 3, the CO2 plume and pressure front will grow and change shape over the lifetime of a
       project, as will the corresponding spatial area of evaluation.

       This section introduces two evaluation processes for spatial area evaluation processes. The first focuses
       on characterization of the area that may be affected by the injected CO2, and the second focuses
       on the area that could be affected by pressure changes. Separate CO2 and pressure spatial areas of
       evaluation are delineated because of the anticipated much larger areal extent of pressure changes
       (particularly for deep saline formations). Additionally, a smaller set of potential impacts will be
       associated with pressure changes. The evaluation processes suggest approaches on how to delineate
       the geographical extent of the area affected by CO2 and pressure changes, and also recommend
       monitoring and mitigation plans. It is important to note that the geographical area affected by
       pressure may be evaluated for pressure-driven displacement of brine into overlying aquifers, as
       described below, but it will not need to be evaluated for direct adverse impacts of CO2 (and other
       impurities) on human health and welfare and ecological receptors.

       4.6.1 Carbon Dioxide Spatial Area of Evaluation

       The flowchart depicted in Figure 4.5 represents an approach for delineating and  examining the CO2
       spatial area of evaluation.

       Spatial Area of Evaluation Delineation

       The lateral extent of the CO2 spatial area  of evaluation can be delineated by first using the areal
       extent of the CO2 plume as the base and  then expanding on that base to include the spatial extent
       over which adverse impacts to physical features and receptors within a determined distance of its
       perimeter may occur. Physical features can include CO2 -concentrating features such as topographic
       lows and dispersing features such as atmospheric conditions and ocean current patterns. As
       discussed in Sections 4.1 through 4.5, potentially affected receptors can include human populations,
       populations covered by Executive Orders, cultural and recreational resources, economic resources
       (surface and subsurface), legislatively protected and other sensitive species, and groundwater and
       surface water sources.

       Identification of Receptors and Development of Monitoring and Mitigation Plans

       It can be useful to specifically locate potentially affected receptors within the expanded CO2 spatial
       area of evaluation and identify their vulnerability to adverse impacts, taking into account CO2-
       concentrating physical features and geologic attributes. Four potential scenarios highlighted in
       Figure 4.5 suggest different types of monitoring and mitigation plans. For example, if the location
       of receptors does not coincide with CO2-concentrating physical features or with elevated geologic
       attribute vulnerabilities, then generic monitoring may be implemented. However, if the location of
       receptors coincides with CO2-concentrating physical features, a receptor-specific monitoring and
       mitigation plan may be developed that takes those features into account. The scenarios highlight
       that the overall vulnerability associated with a GS system is determined based on a combination of
       multiple factors. This concept is discussed further in Chapter 5.

-------
          merability evaluation framework for geologic sequestration: impacts and receptors
                               Delineate the lateral extent of the CC>2 spatial area of
                                   evaluation, using the CO2 plume as the base
                               Identify physical features and receptors along or near
                                          the perimeter of the plume
                           If any of these physical features and receptors are present, the
                            CO2 spatial area of evaluation may need to be expanded to
                             include the area over which adverse impacts could occur
                                                   Identify^
                                                  and locate
                                                  receptors.
None
present
>,


Evaluation complete
                                  Evaluate the level of vulnerability to receptors
>
r >
Scenario 1 : Location of
receptor(s) does not
coincide with
CO2-concentrating
physical features, or
with elevated geologic
attribute vulnerabilities




r \
Scenario 2: Location
of receptor(s)
coincides with
CO2-concentrating
physical features that
may concentrate CC>2
in the event of leakage




t \
Scenario 3: Receptor
location coincides with
elevated vulnerability
based on geologic
attribute evaluation






r
Scenario 4: Receptor
location coincides with
CC>2-concentrating
physical features and
elevated geologic
vulnerability


Develop generic
monitoring plan



Develop
receptor-specific
monitoring and
mitigation plan that
takes into account
CO2- concentrating
features


Develop
receptor-specific
monitoring and
mitigation plan that
takes into account
geologic attribute
vulnerability


Vulnerability to adverse
impacts may be
elevated

       Figure 4.5. Carbon Dioxide Spatial Area of Evaluation
       4.6.2 Pressure Spatial Area of Evaluation

       The flowchart depicted in Figure 4.6 provides an approach for delineating and examining the
       pressure spatial area of evaluation.

       Spatial Area of Evaluation Delineation

       The lateral extent of the pressure spatial area of evaluation can be delineated using the GS footprint
       as the base. In this case, the boundary of the GS footprint is defined by the pressure change that
       is significant enough to  cause adverse impacts to overlying receptors (e.g., fluid displacement into
40

-------
         merability evaluation framework for geologic sequestration: impacts and receptors
       an overlying aquifer). The perimeter can then be expanded to include the spatial extent over which
       receptors within a determined distance of the perimeter of the GS footprint could be affected. As
       noted in the previous section, receptors can include human populations, populations covered by
       Executive Orders, cultural/recreational resources, economic resources (surface and subsurface),
       legislatively protected and other sensitive ecological species, and groundwater and surface water
       sources.

       Identification of Receptors and Development of Monitoring and Mitigation Plans

       Potentially affected receptors within the expanded  pressure spatial area of evaluation can then be
       considered. It is important to evaluate the potential impacts to these receptors, taking into account
       geologic attributes, and then develop monitoring and mitigation plans for receptors susceptible to
       adverse impacts associated with pressure changes.
             Delineate the lateral extent of the pressure SAE, using the GS
                                footprint as the base
             Identify receptors along or near the perimeter of the footprint
         If present, the pressure SAE may need to be expanded to include the
                spatial extent over which adverse impacts could occur
                                      Identify
                                     and locate
                                     receptors
None present
                      Evaluation complete
           Evaluate the level of vulnerability to receptors within the pressure
                   SAE taking into consideration geologic attributes
          Develop monitoring and mitigation plan for receptors susceptible to
                  adverse impacts associated with pressure changes
       Figure 4.6. Pressure Spatial Area of Evaluation
41

-------
         merability evaluation framework for geologic sequestration: impacts and receptors
       4.7 Chapter Summary

       This chapter examined categories of potential adverse impacts resulting from unanticipated
       migration, leakage, and pressure changes, and associated receptors. Impact categories include human
       health and welfare, atmosphere, ecosystems, groundwater and surface water, and the geosphere.
       The chapter then discussed the delineation of spatial areas of evaluation for site characterization
       and monitoring, taking into account the receptors discussed in the previous sections. Chapter 5
       incorporates elements from Chapters 3 and 4 into a holistic approach for vulnerability evaluation
       and highlights key geologic attributes that should be considered carefully when characterizing a
       potential GS site.
42

-------
       VULNERABILITY EVALUATION FRAMEWORK FOR
       GEOLOGIC SEQUESTRATION: KEY CONSIDERATIONS
       Adopting a holistic approach to vulnerability assessment is important because the overall
       vulnerability of a GS site is a combined function of all of the identified geologic attributes and
       receptors. In most instances, evaluating a single attribute will not provide sufficient information
       to characterize overall vulnerability. Rather, the entire site needs to be evaluated as a whole. The
       individual and combined influence of attributes on vulnerability as well as the presence, proximity,
       and sensitivity of receptors is likely to change over time. Generally, vulnerability and uncertainty will
       decrease after injection ceases and as monitoring data are incorporated into the assessment.

       Key attributes that have been identified as particularly important when evaluating the potential
       vulnerability to unanticipated migration and leakage from a GS system include wells and faults/
       fracture zones, as well as pressure changes that  may induce fracturing or reactivate faults. However,
       even for these key attributes, it is the interplay of multiple characteristics that will determine the
       level of vulnerability that may be associated with them; the simple presence of a well or a fault does
       not automatically indicate elevated vulnerability. The prioritization of vulnerabilities associated with
       receptors, geologic attributes and their characteristics is an important, but challenging endeavor
       that is beyond the current scope of the VEF. However, vulnerabilities can nevertheless be managed
       by carefully evaluating and managing these key attributes, thus reducing the likelihood of adverse
       impacts.

       Chapters 3 and 4 identified and described the geologic attributes that could contribute to
       vulnerability and the receptors that may be adversely impacted. They also presented evaluation
       processes and suggestions as to how particular vulnerabilities may be addressed. This chapter
       examines key considerations when examining the vulnerabilities of a GS site as a whole and contains
       the following sections:

             Section 5.1 discusses a holistic approach to vulnerability evaluation in terms of GS system
              characteristics and temporal scales.

             Section 5.2 provides additional detail on attributes that are key to determining GS site
              vulnerability, specifically wells, faults, and pressure changes. Evaluation flowcharts for these
              attributes are introduced as well as suggestions for reducing vulnerabilities.

             Section 5.3 provides a summary of the chapter.
43
       5.1 Holistic Approach to Evaluating Vulnerability

       The overall vulnerability to adverse impacts is determined by the combination of geologic attributes
       and receptors that are associated with a GS site. Further, since storage of CO2 is intended for a long
       time period, it is important to examine how the influence of different attributes may change over
       time.

       5.1.1 Interplay of Geologic Attributes and Receptors

       The overall vulnerability of a GS site to adverse impacts is a function of the vulnerability of a system
       to unanticipated migration, leakage, or pressure changes in combination with the vulnerability of

-------
          merability evaluation framework for geologic sequestration: key considerations
       specific receptors of concern. The highest level of vulnerability emerges when there is a high potential
       that there are unanticipated migration or leakage pathways in the presence of important vulnerable
       receptors, such as a sensitive drinking water supply. If the vulnerability to unanticipated migration
       or leakage is low (for example, the confining system is laterally extensive, has a very low permeability,
       is not known to be perforated by wells or faults, and has a low vulnerability to induced fracturing or
       other pressure-related effects) then the overall vulnerability to adverse impacts may be relatively low,
       despite the presence of sensitive receptors such as drinking water supply. Therefore, an evaluation of
       the combined presence and condition of GS attributes together with the presence and sensitivity of
       receptors is required for a clear understanding of overall vulnerabilities.

       5.1.2 Evaluation of Vulnerability at Different Temporal Scales

       The importance of different attributes for affecting the probability and severity of adverse impacts is
       likely to change over the lifetime of a GS project. Although a quantitative discussion of the relative
       contribution of different attributes and how this may change over the duration of a project is not
       possible at this time, some qualitative comments can be made. For example, during the injection
       phase, injectivity and geomechanical processes are anticipated to be key attributes in the evaluation
       of vulnerability to  adverse impacts. The mechanical integrity of the injection wells is also very
       important, particularly the ability to  withstand applied pressures. The vulnerability to induced
       fracturing is expected to be much higher during injection than post-injection. Post-injection, wells
       might continue to represent potential unanticipated migration or leakage pathways. However, the
       concern may be more for slow/small  leaks that develop through slow geochemical degradation
       pathways (thus primarily affecting atmospheric vulnerability) as opposed to high-intensity pressure
       blow-outs that could also affect human health and welfare and environmental receptors.

       In general, the overall vulnerability to adverse impacts is expected to decline over time, as illustrated
       in Figure 5.1.  This is based  on a combination of factors,  including the greater permanence of
       secondary trapping mechanisms such as dissolution, which also decreases buoyancy (see Chapter 2),
              .
              I
              Q.
               CD

               I
                   I
                                            Pressure recovery
                                            Secondary trapping mechanisms
                                            Confidence in predictive models
                 Injection
                  begins
Injection
 stops
2 x injection
   period
3 x injection
   period
n x injection
  period
            Monitor
            Model
                          Calibral
                             &
                          Validate
                           Models
            Calibrate
               &
            Validate
            Models
44
           Figure 5.1. Risk Profile for CO2 Storage. Source: Benson, 2007.

-------
          merability evaluation framework for geologic sequestration: key considerations
       pressure recovery once injection stops, and improved characterization and modeling of GS system
       attributes over time.

       In some cases, however, vulnerabilities associated with certain aspects of a GS system may not
       decrease appreciably over time. For example, in open systems where the injected CO2 and associated
       induced pressure changes do not reach the lateral boundaries of the injection zone formation(s),
       pressure is expected to drop significantly when injection ceases (Pawar et al, 2006; Birkholzer et al.,
       2007), thus decreasing the vulnerabilities to induced seismic activity and faulting and fracturing.
       However, in closed systems where pressure remains elevated at the lateral boundaries of the system
       (for example in EOR geologic settings, where  boundaries may be defined by geologic features such
       as an anticline or sealed fault), it may take much longer for pressure to dissipate. This can result
       in elevated vulnerability for a prolonged period of time after injection stops. It should be noted
       that this example re-emphasizes the necessity of evaluating the vulnerabilities associated with a GS
       system as a whole. Although pressure may remain elevated for a longer period of time in the closed
       system, this does not necessarily mean that the system has an overall elevated level of vulnerability.
       For example, EOR settings have withstood elevated pressure for geologic timescales in the past, thus
       demonstrating an effective confining system with low vulnerability to unanticipated migration or
       leakage.
       5.2 Key Attributes in Evaluating GS System Vulnerability

       Although the overall vulnerability of a GS site is determined through a holistic evaluation of the
       interactions among all elements of a GS site, certain key attributes have been identified as central to
       evaluating the vulnerability associated with a GS system. It is unlikely that any one of these single
       attributes alone will determine the overall vulnerability of a GS site. However, there are some basic
       screening considerations:

              Is the physical capacity of the injection zone sufficient to store the total CO2 volume?

              Is the confining system present and of sufficient lateral extent?

              Is the injectivity of the injection zone sufficient?

              Is tectonic activity (earthquakes, volcanoes, hotspots, etc.) not a concern?

              Are wells, faults, fracture zones not a concern?

       If one or more of these conditions is not met, there may be elevated vulnerability to unanticipated
       migration, leakage, or adverse pressure changes that render a GS  system less suitable. As described in
       the evaluation processes presented in Chapter 3, the suitability of the GS system can be improved,
       for example, by altering operational plans and injecting less, through targeted monitoring and
       mitigation, and/or corrective actions at vulnerable wells. The discussion below focuses on wells
       and faults because they have been identified as being the most likely attributes to contribute to
       unanticipated fluid migration out of the  injection zone. However, the presence of these attributes
       alone may not indicate elevated vulnerability; and it is important to examine them thoroughly to
       ascertain the level of vulnerability associated with a GS system.
45

-------
          merability evaluation framework for geologic sequestration: key considerations
       5.2.1 Wells as Fluid-Conducting Pathways

       Wells and other artificial penetrations such as boreholes have been identified as one of the most
       probable conduits for the escape of CO2 and other fluids from the injection zone (Gasda et al.,
       2004; Benson, 2005; IPCC, 2005; Carey et al., 2007). Industrial analogs indicate that while gases
       such as CH4 can be stored effectively in the subsurface, there are examples of unanticipated gas
       leakage through poorly completed or improperly plugged and abandoned wells (Gurevich et al.,
       1993; Lippmann and Benson, 2003; Perry, 2005). Hence, if not properly sealed and plugged, wells
       can provide an open conduit from depth to the surface. Even wells that are properly sealed may have
       fluid-conducting pathways along the outside of the casing, where a complex environment involving
       well cements, drilling muds, and possibly damaged rock zones can provide opportunities for fluid
       flow (Gasda et al., 2004). Further, the acid generated when CO2 contacts water could degrade well
       construction materials, possibly creating pathways for fluid flow (Scherer et al., 2005). The relative
       depth of wells is also an important consideration when evaluating their potential to serve as fluid
       pathways. Only wells that penetrate the injection zone can serve as direct leakage pathways to the
       surface.  However, well characterization should include all wells within the area anticipated to be
       affected by CO2 injection, because shallower wells could  be connected to faults or other features that
       do penetrate the injection zone. Thus, the combination of wells and faults can serve as a pathway for
       unanticipated migration and leakage.

       Experience from industrial processes underlines the importance of evaluating the potential for  active
       and abandoned wells to serve as fluid pathways and conducting a detailed GS  site characterization
       before injection (Cawley et al., 2005). As illustrated in Figure 5.2, evaluating the potential for
       wells to  act as conduits for CO2 and other fluids involves considering multiple factors, including
       determining whether wells are present, measuring their depth relative to injection zone, and
       evaluating the integrity of the well construction materials and seal.

       While operational factors are not the focus of the VEF, injection and monitoring wells may also have
       implications for the creation of fluid-conducting pathways. Principal design considerations for CO2
       injection wells include pressure, production and injection rates, and corrosion. There is substantial
       knowledge about the design  of CO2 injection wells developed for EOR operations (IPCC, 2005).
       Equipping injection wells with packers can help  isolate pressure effects to the injection interval.
       The slotted length of wells, use of horizontal wells (slotted over much longer reaches than vertical
       wells), and the number of injection wells used for injecting CO2 are all operational  factors that
       may minimize pressure. Using corrosion-resistant well construction materials can help maintain
       the integrity of the well. Numerous monitoring and mitigation techniques also exist and  could be
       applied to GS systems (Jarell et al., 2002; Skinner, 2003).

       Identifying and evaluating active and abandoned wells may be particularly challenging in certain
       geologic settings, such as  depleted oil and gas fields, where there may be numerous active and
       abandoned production and exploration wells of different ages, depths, and general condition.
       Industry and state records exist that can help locate such wells, and determine their depth and
       mechanisms of sealing and plugging. However, the completeness of such records, particularly for
       historical (pre-1950s) wells, may be limited, and may need to be supplemented by field verification.
       Nevertheless, site characterization that includes the considerations outlined in Figure 5.2 may reduce
       uncertainties and manage vulnerability by helping identify those conditions involving wells that
       may be of concern and by providing approaches to address them. For example, a well that penetrates
       the injection zone and is not sealed properly, or for which records do not exist on abandonment
46

-------
            merability  evaluation framework for geologic sequestration: key considerations
                                                        O Identify wells
                           Injection and monitoring wells should be carefully designed, operated, and monitored,
                                taking into consideration injection rate, pressure, and corrosion conditions
                                                      Locate pre-existing wells
         Wells evaluation approach:

         Olnitial well screen

          Evaluate penetration depth relative to
         the Injection Zone (IZ) and confining
         system, and connection to other conduits


          Evaluate well integrity
                                Higher cumulative uncertainty and vulner-
                                ability.  Corrective action and/or targeted
                                monitoring and mitigation plans may need
                                          to be considered.
                                                  High
                                               density o
                                                 wells?
         Evaluate individual wells
                                                                                  Well
                                                                               connecte
                                                                              to IZ through
                                                                               secondary
                                                                               conduits?
                   
                  Well
               penetrates
                 the IZ?
        Conduct field investigation
        to determine depth
     Assessment complete
        Low vulnerability.
                                                                         
                                                                      Adequate
                                                                      records of
                                                                     well integrity
                                                                        exist?
   ossible
 to conduct
    field
investigation
 to evaluate
 integrity?
 Elevated vulnerability.
  Consider corrective
 action and/or targeted
     monitoring and
    mitigation plans.
                                                                        Well
                                                                      integrity is
                                                                      adequate?
                                             Elevated vulnerability. Consider
                                            corrective action and/or targeted
                                             monitoring and mitigation plans.
                                                                      ntegrity i
                                                                      adequate
                                                                      along full
                                                                     penetration
                                                                       depth?
  Secondary
  conduits to
   receptors
    exist?
Assessment complete.
   Low vulnerability.
Assessment complete
   Low vulnerability.
                                                                     Secondary
                                                                       conduits
                                                                     provide path
                                                                    to receptors?
    Evaluate
   secondary
    conduits
                                                                                Assessment complete
                                                                                  Low vulnerability.
                                                            Elevated vulnerability. Consider
                                                            corrective action and/or targeted
                                                            monitoring and mitigation plans.
47
         Figure 5.2. Wells Evaluation

-------
          merability evaluation framework for geologic sequestration: key considerations
       methods, could be managed by targeting the well for monitoring and taking mitigative steps
       such as those described in Chapter 6. Decisions may need to be made on a site-specific basis
       regarding whether corrective action or targeted monitoring, or a combination of both, may be most
       appropriate to address conditions at a given well, in addition to the development of mitigation plans.

       5.2.2 Faults and Fractures as Fluid-Conducting Pathways

       Although faults and fractures have been identified, along with wells, as one of the most likely fluid
       conduits in GS systems, the simple presence/absence of fault in many cases will not be sufficient
       to evaluate the vulnerability to unanticipated migration and leakage. The potential for faults
       and fractures to act as fluid pathways in GS systems is a function of numerous factors, including
       stratigraphic position. For example, a fault that does not cut across the full thickness of the confining
       zone may not pose the same level of vulnerability as a fault that is continuous from the injection
       zone to the surface.

       Whether a fault is sealed or conductive will also determine its ability to allow passage of CO2 and
       other fluids; faults may serve as either barriers or conduits to fluid flow (Omre et al., 1994; Lewicki
       et al., 2006; Wilkens and Naruk, 2007). Existing faults and fractures that are sealed may remain
       stable or may be reactivated by induced pressure changes and geochemical conditions caused by CO2
       injection. The orientation and geometry of the fault relative to regional stresses will also influence
       whether it may be re-activated  (Rutqvist et al., 2007a, 2007b). New fractures could form if the
       fracture pressure of the formation(s) is exceeded (Healy et al., 1968; Gibbs et al., 1973; Raleigh et al.,
       1976; Sminchak et al., 2002; IPCC, 2005; Streit et al., 2005; Wo et al., 2005). However, currently
       available geomechanical methods can assess the stability of faults and estimate the maximum
       sustainable pore fluid pressures for CO2 storage (e.g., Streit and Hillis, 2003).

       The potential for induced fractures and re-opened faults to result in adverse impacts will depend on
       numerous additional factors, including whether they are connected to an overlying receptor, whether
       they are connected to other fluid-conducting pathways (e.g., wells), and whether or not they may
       be resealed by geochemical processes. In addition, an adverse impact will occur only if the amount
       of CO2 that is transported along the fault is sufficient to impact a receptor such  as a drinking water
       supply. For example, if a small  fault reopened that connects the injection zone to an overlying
       aquifer, the amount of CO2 transported along the fault may not adversely affect drinking water or
       other resources.

       Identifying and evaluating how faults and fractures will behave presents a particular challenge in
       the characterization of GS systems. Figure 5.3 presents some of the factors that can be considered
       when evaluating the potential for faults and fractures to act as fluid-conducting pathways. The
       figure highlights the need to identify whether  faults are continuous from the injection zone to
       secondary conduits, overlying aquifers, or other receptors. This evaluation can be strengthened if
       multiple techniques for site characterization are used, such as surface mapping, interpretation of well
       bore data, and seismic techniques. Use of seismic techniques is generally the more comprehensive
       approach,  because not all faults at injection zone depths have surface expressions allowing surface
       mapping to locate them, and the spatial discreteness of well bore data is generally insufficient
       to properly characterize  fault and fracture zones. Three-dimensional seismic data may provide
       advantages for complete fault characterization, because two-dimensional seismic data have associated
       spacing and resolution constraints. As work progresses, additional methods may be identified for
       fault characterization techniques within GS systems.
48

-------
          merability evaluation framework for geologic sequestration: key considerations
                      Evaluate if faults are
                    expected to be present
                     based on the regional
                     tectonic environment
                     Locate faults based on
                      site characterization
                           Presence
                           offault(s)
                      continuous from the
                         IZ to overlying
                          aquifer(s) or
                            other
                          receptors?
                               Evaluate potential conduits for all other identified
                               faults, considering secondary conduits, such as
                               wells
                      Evaluate potential to
                         conduct fluids
                                                          Evaluate potential of
                                                          combined pathway
                                                          to conduct fluids
        If conductive:
          elevated
         vulnerability
If sealing: evaluate
fault stability under
    proposed
    operational
conditions and set
  monitoring and
  mitigation plans
   If uncertain/
unknown: consider
   further site
 characterization
   and targeted
  monitoring and
 mitigation plans
                                                                   Secondary
                                                                   conduit(s)
                                                                   identified?
       Figure 5.3. Faults Evaluation
49
       Evaluating the stability of faults and fracture zones in GS systems also presents particular challenges.
       Faults and fractures are often associated with complex zones with altered properties, rather than
       being simple discontinuities. There may be significant uncertainty in being able to predict how they
       will behave under the conditions induced by GS. However, as described in Figure 5.1, careful design
       and monitoring can help manage vulnerabilities associated with faults and fractures. Approaches
       include using multiple site characterizing technologies and setting monitoring at a level appropriate
       to the level of uncertainty. Furthermore, the existing knowledge base is likely to improve as more
       data are acquired from field projects.


       5.2.3 Pressure-Induced Physical Effects


       Injecting CO2 into geologic formations will in most cases cause subsurface changes in pressure. As
       discussed above, induced fracturing and fault reactivation can occur if injection pressure exceeds

-------
         merability evaluation  framework for geologic sequestration: key considerations
       fracture pressures, which may in turn result in the opening of fluid migration pathways. If pressures
       are great enough, they could in extreme cases cause earthquakes (Healey et al., 1968). However, these
       effects could be minimized through an understanding of the relevant geologic attributes, careful site
       characterization, careful operation of GS systems, and monitoring. As summarized in Chapter 3,
       the geologic attributes that will influence pressure changes include physical capacity of the injection
       zone, its injectivity, and geomechanical and geochemical processes. Operational factors that influence
       pressure changes, and that can be managed, include the rate of injection, the slotted length of the
       injection well, the number of injection wells, and their orientation.

       There may be greater uncertainty about evaluating pressure  effects in GS systems when examining
       the potential for pressure-induced regional scale impacts that do not involve fracturing or faulting.
       As discussed in Chapter 4, pressure changes in the injection zone could cause regional impacts on
       overlying aquifer systems, including changes in groundwater flow directions and water table levels.
       These may result in alterations in the distribution and fluxes of groundwater. This could in turn have
       other impacts, for example, changing the quantity of groundwater that is available for municipal
       drinking water supplies. There also could be pressure-induced migration of brines and other fluids
       through the pore structure of overlying formations into groundwater receptors, which may impact
       water quality. Furthermore, pressure-induced fluid displacement could result in the release of brine
       at locations where injection zone formations outcrop at the land surface. Regional pressure effects
       have been the focus of relatively few studies, and uncertainties and vulnerabilities associated with this
       subject should be addressed through additional research.
       5.3 Chapter Summary

       This chapter discussed the importance of a holistic evaluation of vulnerability associated with GS
       systems and highlighted key attributes (i.e., wells, faults, and pressure changes) that should be
       examined extensively in a vulnerability evaluation. Chapter 6 summarizes monitoring and mitigation
       strategies to help decrease or avoid the potential adverse impacts of GS summarized in Chapter 4.
50

-------
       CHAPTER 6
       MONITORING AND MITIGATION
       Careful site selection and site characterization of GS systems and accompanying spatial areas
       of evaluation can help minimize the potential for adverse impacts. However, in the event of
       unanticipated migration and leakage, early detection through monitoring and application
       of mitigation techniques can help prevent or minimize adverse impacts. Data collected from
       monitoring can also help refine our understanding of GS systems and overlying receptors, thereby
       enhancing operations while minimizing vulnerabilities.

       The VEF identifies the key attributes that are important to site selection and site characterization
       (geologic attributes and receptors described in Chapters 3 and 4), and approaches to delineate the
       spatial extent of the GS system (also described in Chapters 3 and 4). Although monitoring and
       mitigation are not the focus of the VEF, application of the VEF can inform and prioritize monitoring
       and mitigation approaches, focused on both the subsurface geologic attributes, and overlying
       receptors.

       This chapter contains the following sections:

              Section 6.1 presents a brief overview of the purposes of monitoring, different applicable
              monitoring technologies, and the timeframe for monitoring activities.

              Section 6.2 provides a brief summary of mitigation strategies in the event of unanticipated
              migration, leakage, and pressure changes.

              Section 6.3 provides a summary of the chapter.
51
       6.1  Monitoring

       6.1.1 Purposes of Monitoring

       Most of the geologic attribute evaluation processes provided in the VEF qualitatively identify those
       conditions that may lead to low or elevated vulnerability, and recommend the development of
       monitoring and mitigation plans in instances of elevated vulnerability. Monitoring can provide an
       early warning mechanism in the event of unanticipated movement of CO2 and other fluids from the
       injection zone (Oldenburg et al., 2003; IPCC, 2005). It can also be used to detect pressure changes
       in the subsurface. Early detection of unanticipated fluid migration and increases in pressure provides
       the opportunity to put into place mitigation strategies to avoid or minimize adverse impacts. The
       goals of monitoring include regularly confirming the location and containment of the injected CO2.
       Hence, the data collected during monitoring can also be used to develop a more comprehensive
       understanding of the GS system, and to calibrate and refine models of the behavior and location of
       CO2 in the subsurface. Operations may be modified and enhanced based on the interpretation of
       monitoring data.

       An important element of monitoring is establishing baseline conditions by collecting monitoring
       data prior to the injection of CO2. Measurements taken before injection will help evaluate how
       the subsurface might change in response to injection. Interpreting the  data gained through many

-------
          initoring and  mitigai
       monitoring techniques (such as the seismic measurements discussed in the next section) can be
       greatly facilitated if post- and pre-injection data can be compared.

       Monitoring can also be used to ensure proper functioning of injection wells and to optimize
       injection. Specific techniques can also be targeted at attributes and receptors of a GS site identified
       during site characterization that may be of particular interest, including abandoned wells, faults and
       fracture zones, and sensitive overlying receptors (IPCC, 2005).

       6.1.2 Monitoring Technologies

       Several types of monitoring technologies can be used to meet the purposes identified above.
       The IPCC, in its Special Report on Carbon Dioxide Capture and Storage, describes monitoring
       technologies for the different monitoring purposes:

             Location and movement of stored CO2. The location and movement of stored CO2 and
              other underground fluids and gases can be monitored using direct or indirect approaches.
              Direct approaches, which are employed at many EOR projects, involve measuring the
              volume of injected quantities of CO2 at multiple production wells. A second direct approach
              to monitoring movement of stored CO2 and other underground fluids and gases is to drill
              monitoring wells.  Monitoring wells can also be used in other assessments of subsurface
              fluid migration, including tracer tests. Injecting unique tracers (e.g., gases not found within
              the injection zone) and detecting for them at monitoring wells can help characterize CO2
              migration in the subsurface (IPCC, 2005).

              Indirect approaches for monitoring movement of stored  CO2 and other underground
              fluids and gases include using seismic and non-seismic technologies (IPCC, 2005). Seismic
              technologies measure the velocity and absorption of energy waves through rock and provide
              a picture of underground layers of rocks and reservoirs. Non-seismic techniques such as
              electrical and electromagnetic techniques measure the relative conductivity of subsurface
              layers of various solids and fluids (IPCC, 2005). A comprehensive list of technologies for
              monitoring movement of stored CO2 and other underground fluids and gases is given in
              Table 5.4 of the IPCC Special Report on Carbon Dioxide Capture and Storage (IPCC,
              2005).

             Injection rates and pressures. The oil and gas industry has a long history of monitoring
              injection rates and pressures. Technologies used for this purpose are mature and commercially
              available. Gauges at wellheads are used to measure injection rates. Injection pressure is
              typically measured at most injection wells, and downhole formation pressure measurements
              are also routine, though the latter measurements may only be  made on a periodic basis.
              Pressure gauges at injection wells are often linked to shut-off mechanisms that will slow or
              cease injection if injection pressure deviates from a determined range (IPCC, 2005).

             Well integrity. Wells are often subjected to extensive induced-pressure testing for mechanical
              integrity before use. Continued integrity of the well during injection can  be evaluated by
              monitoring pressure. The injection pressure can be continuously monitored at the wellhead
              by meters  (IPCC, 2006). The pressure  in the injection well annul us can also be monitored
              during injection to ensure the integrity of the injection well packer and casing. A decrease
              in pressure in the annulus may indicate unanticipated subsurface migration. In addition,
52

-------
          initoring and mitigai
              temperature and "noise" logs can be used to detect well failures. Such logs have historically
              been used in natural gas storage projects. The integrity of the casing material can also be
              monitored using corrosion monitoring techniques such as caliper logs (IPCC,  2005).

              Local environmental impacts. In addition to technologies for monitoring CO2 movement
              underground, several technologies are available for monitoring the local environmental
              impacts of unanticipated migration and leakage. Several reviews have been published
              on near-surface monitoring technologies that can evaluate local environmental impacts,
              including both conventional and unconventional technologies (IPCC, 2005; Benson et al.,
              2002; Oldenburg et al., 2003).

              Monitoring CO2 levels at the surface is a common practice in many occupational
              applications. Many heating, ventilation, and air conditioning systems, for example, have
              CO2 sensors. In addition, the literature on CO2 concentrations in soil and air  is extensive (as
              described in Oldenburg et al.,  2003; Miles et al., 2005) and monitoring occurs frequently
              in many areas (e.g., regular CO2 flux measurements by the U.S. Geological Survey at
              Mammoth Mountain, California) (USGS, 2001).

              Satellite-based remote sensing  and airborne imaging measurements of CO2 levels are also
              possible technologies for monitoring CO2 levels at the surface. However, these technologies
              may not be sensitive enough to detect the small CO2 fluctuations that may be  associated
              with GS. Monitoring ecosystems for exposure to CO2 is another technique for assessing
              local environmental impacts at the surface. Such monitoring often involves measuring
              the productivity and biodiversity of flora and fauna and measuring pH levels in aquatic
              ecosystems. Impacts on USDWs can be monitored by water sampling that tests levels of
              major ions and gases that may be carried by the leaked plume or mobilized by  the plume's
              constituents (IPCC, 2005). Hyperspectral imaging is also being considered for the detection
              of stressed vegetation.

       6.1.3 Timeframe Implications for Monitoring

       The long timeframe over which GS needs to be effective underlines the importance of regular
       monitoring. It is  suggested that monitoring periods for GS can be divided into three periods:
       injection and operation; storage during pressure  equilibration; and long-term storage (Chalaturnyk
       and Gunter, 2004).

       The length of the injection and operation period will depend on the capacity of the injection zone
       and the rate of injection, but is generally expected to be 25-50 years. The period of time required
       for pressure equilibration after injection stops will be site specific. For open systems, where the
       injection zone  formation is very large compared to the volume of the CO2 plume, the  time period
       for pressure to  decay back to the regional formation pressure may be 100 years or less (Birkholzer et
       al., 2007). For closed systems, pressure may remain elevated for a much longer period  of time. After
       pressure stabilization (either with a return to the regional pressure regime in the case of open systems
       or a long-term stabilization at elevated pressures  in closed systems), if unanticipated migration and
       leakage has not occurred, the GS system may be considered stable, and the formation of pressure-
       related fluid conduits no longer anticipated (Chalaturnyk and Gunter, 2004). Geochemical  changes,
       however, can continue over a longer timeframe. The integrity of the caprock or wellbore cements, for
       example, may be affected by reactions  with injected CO2 (IPCC, 2005).
53

-------
          initoring and  mitigai
       Monitoring may be more intensive in the earlier stages, with more frequent measurements and
       specific targeting of operational infrastructure. The frequency of monitoring may decrease as the
       system stabilizes post-injection, and eventually cease (Chalaturnyk and Gunter, 2004; IPCC, 2006).

       Additionally, the area requiring monitoring is also likely to change over time, expanding during the
       injection period as the CO2 plume grows. The continued mobility of CO2 post-injection, because
       of its relatively high buoyancy and lower viscosity compared to water and brine, may also result
       in a dynamic post-injection area of evaluation, necessitating adaptive monitoring. The area to be
       monitored for pressure impacts will also expand during injection and may be significantly larger than
       the area covered by the CO2 plume. Post-injection, if the pressure dissipates (e.g., in the case of open
       systems), the area requiring monitoring for pressure changes may decrease correspondingly.
       6.2 Mitigation

       Unanticipated migration, leakage, and adverse pressure changes identified through monitoring
       may be addressed in many cases with mitigation actions. For the purposes of this report, mitigation
       refers to actions taken to prevent unanticipated migration, leakage, or pressure changes that have the
       potential to cause adverse impacts. It also includes actions taken to prevent or reduce impacts once
       unanticipated migration, leakage, or pressure changes have been detected by monitoring. This report
       does not address actions taken to address impacts that have already occurred. Several important
       mitigation concepts are discussed below. Some of these approaches are routine within the oil and gas
       and other industries, while others are more experimental, and will likely require further study prior
       to implementation.

       Wells.  According to the IPCC, the development of mitigation plans for active or abandoned wells
       is particularly important, because they are known vulnerabilities (Gasda et al., 2004; IPCC, 2005;
       Perry 2005). Standard mitigation techniques, also referred to as corrective actions, exist to stop
       unanticipated migration and leakage from injection and abandoned wells. These include injecting
       a heavy mud to plug the opening. If the well is not accessible at the surface, a new well can be
       drilled to intercept the casing of the  compromised well below the ground surface and cement can be
       pumped  in from the interception well.

       Further mitigation techniques also exist for injection wells. The integrity of CO2 injection wells
       can be repaired by replacing injection tubing and packers. If the space between the casing and the
       formation borehole leaks, the casing can be perforated to allow injection of cement behind the casing
       to seal  the leak. If the well cannot be repaired, it can be plugged and abandoned using the standard
       procedures cited above, and replaced with a new injection well.

       Faults and fractures. The unanticipated migration of CO2 and other fluids along faults (and
       fractures) could be mitigated by lowering the pressure driving flow along the fault by injecting at a
       lower rate, or through more wells. Alternatively, the pressure in the injection zone could be lowered
       by removing water or other fluids, or by possibly creating a pathway to access additional formations
       within the injection zone. Further, extraction wells could be used to intersect the pathway, or a
       hydraulic barrier could be created by increasing the reservoir pressure upstream of the fault. It may
       also be appropriate to  consider ceasing injection to stabilize the project and, in the extreme case, the
       CO2 could be recovered from the formation and re-injected in a more suitable formation (IPCC,
       2005).
54

-------
          initoring  and mitigai
       Accumulation in indoor environments. Slow releases of CO2 can accumulate in confined spaces
       (e.g., basements) and cause harm to human health. These slow releases into structures can be
       eliminated using techniques designed to dilute the CO2 before it enters indoor environments, such
       as basement/substructure venting or pressurization (IPCC, 2005).

       Large releases to the atmosphere. For very large releases to the atmosphere spread over large areas,
       natural attenuation through atmospheric dispersion may be the main mitigation option. Smaller
       localized releases at wellheads or in buildings could be addressed using fans or venting (Benson and
       Hepple, 2005; IPCC, 2005).

       Accumulation in soil gas. As discussed in chapter 4, accumulation of CO2 in soil gas can be
       detrimental to ecological receptors. Removing the CO2 from the vadose zone would also prevent its
       escape to the atmosphere. CO2 can  be extracted from the vadose zone and soil gas using standard
       vapor extraction techniques that use horizontal or vertical wells to collect the CO2. Additionally,
       it may be possible to decrease or stop releases to the atmosphere from the vadose zone by the use
       of caps or gas vapor barriers. The CO2 accumulated under the cap could then be removed using
       extraction wells. Since CO2 is denser than air, it may be possible to collect it in subsurface trenches.
       The CO2 captured via these approaches could then be reinjected into the subsurface. Finally, in
       some cases, acidification of the soils from contact with  CO2 could be remediated by irrigation and
       drainage, acid-neutralizing substances such as lime could be applied to the soil (IPCC, 2005).

       Groundwater and  surface water. Fluids that reach groundwater could be captured using extraction
       wells and reinjected into the subsurface. Carbon dioxide that reaches a groundwater aquifer and
       then becomes immobile through residual trapping could be removed by flushing the aquifer with
       water, thereby enhancing CO2 dissolution, and then extracting the dissolved CO2 using groundwater
       extraction wells. Additionally, CO2 that has dissolved in shallow groundwater can similarly be
       removed with groundwater extraction wells. The mobilization of metals or other contaminants as a
       result of CO2 migration into groundwater could be addressed through pump and treat technologies,
       the creation of hydraulic barriers, or by in situ passive methods such as enhanced bioremediation
       (IPCC, 2005).

       Shallow surface  water bodies that have significant turnover (shallow lakes) or turbulence (streams)
       may be naturally attenuated, due to their relatively quick release of dissolved CO2 back into the
       atmosphere. For deep, stably stratified lakes, active systems for venting gas accumulations have been
       developed and applied (IPCC, 2005).
       6.3 Chapter Summary

       Chapter 6 summarized the importance of delineating the geographical extent of GS systems for site
       characterization and monitoring purposes, how monitoring can be used to achieve multiple goals,
       the types of monitoring technologies, and the timeframe for monitoring activities. It also described
       mitigation strategies that can be employed in the event of unanticipated migration and leakage of
       fluids in order to avoid or minimize adverse impacts.
55

-------
       The VEF approach was developed by EPA to systematically identify those conditions that may
       increase or decrease susceptibility to adverse impacts from GS. The VEF is a reference document that
       can assist regulators and other technical experts in identifying key areas for in-depth site-specific risk
       assessment, monitoring, and management. The vulnerability assessment introduced in the VEF is
       intended to be an iterative process where new information is incorporated into the evaluation as it
       is generated. It is not intended to be broadly applicable or to measure the severity of an outcome, to
       establish performance standards for GS sites, or to specify data requirements.

       The VEF identifies attributes of GS systems that may lead to increased vulnerability to adverse
       impacts, identifies potential impact categories, identifies thresholds that may indicate  low versus
       elevated vulnerability, and provides a series of decision-support flowcharts that are organized,
       systematic approaches to assess the attributes and potential impacts. Though certain attributes or
       receptors may increase the overall susceptibility of a GS site to adverse impacts, it is important
       to recognize that assessing overall vulnerability is dependent on the interplay between individual
       attributes and receptors.

       The VEF, as described in this report, represents the first step toward the development  of a conceptual
       framework to evaluate potential vulnerabilities of GS projects and as such is a work in progress.
       Numerous reviewers have encouraged further development and refinement of the VEF as well as
       demonstration of its practical applicability. Future work could also involve developing the decision-
       support flowcharts into an integrated evaluation tool that has a more quantitative and numerical
       basis. This could include refining the binary vulnerability classification (low versus elevated) into a
       multiscaled classification scheme as warranted by available information, data, and expert  opinion. In
       addition, the VEF may be validated and refined by applying it to case studies that represent a range
       of likely scenarios in which various aspects of the VEF framework can be tested. These scenarios may
       be based on real, successful demonstration  projects as well as more challenging situations where risk
       may be higher. Applying the VEF  in this manner would improve the framework and could be an
       instructive exercise for regulators, operators and other stakeholders and can provide a  means to test
       approaches developed to manage uncertainties.

       The VEF is designed to focus on vulnerabilities of GS  systems that could increase adverse impacts
       associated with GS. It could be expanded to include vulnerabilities to adverse impacts associated
       with the capture or transport of CO2, operational aspects of GS such as surface infrastructure (e.g.,
       buildings, pipelines, well construction), and specific monitoring techniques  to ensure  the integrity
       of the GS system recommended. Though not currently included in the VEF, these elements need
       to be considered because they may play an  important role in defining a proposed project's overall
       vulnerability.
56

-------
       BIBLIOGRAPHY

       Apps, J.A. 2005. The Regulatory Climate Governing the Disposal of Liquid Wastes in Deep
       Geologic Formations: A Paradigm for Regulations for the Subsurface Storage of CO2. In Results from
       the CO2 Capture Project. V 2: Geologic Storage of Carbon Dioxide with Monitoring and Verification,
       S.M. Benson (ed.). Elsevier, London, UK, 1173-1188.

       Apps, J.A. 2006. A Review of Hazardous Chemical Species Associated with CO2 Capture from Coal-Fired
       Power Plants and their Potential Fate in the CO2 Geologic Storage. LBNL-59731. Lawrence Berkeley
       National Laboratory, Berkeley, CA. February.

       Arts, R and P. Winthaegen. 2005. Monitoring Options for CO2  Storage 1001.  In Results from the
       CO2 Capture Project. V 2:  Geologic Storage of Carbon Dioxide with Monitoring and Verification, S.M.
       Benson (ed.). Elsevier,  London, UK, 1001-1014.

       Bachu, S., W.D. Gunter, and E.H. Perkins. 1994. Aquifer disposal of CO2: Hydrodynamic and
       mineral trapping. Energy Conversion and Management 3 5 (4): 269-279.

       Bachu, S., and K. Haug. 2005. In situ Characteristics of Acid-Gas Injection Operations in the
       Alberta Basin, Western Canada: Demonstration of CO2 Geological Storage. In Results from the
       CO2 Capture Project. V 2:  Geologic Storage of Carbon Dioxide with Monitoring and Verification, S.M.
       Benson (ed.). Elsevier,  London, UK, 867-876.

       Beaubien, S., G. Ciotoli, P. Coombs, M.C. Dictor, M. Krueger, S. Lombardi, J.M. Pearce, and J.M.
       West. 2008. The impact of a naturally occurring CO2 on the shallow ecosystem and soil chemistry of
       a Mediterranean pasture (Latera, Italy). International Journal of Greenhouse Gas Control. doi:10.10l6/
       j.ijggc.2008.03.005.

       Benbow, S.J., R Metcalfe, and M.J. Egan. 2006. Handling uncertainty in safety assessments for long
       term geological  storage of CO2. In Proceedings of the 8th Greenhouse Gas Technology Conference. June,
       Trondheim, Norway.

       Benson, S. 2007. Addressing Long-term Liability of Carbon Dioxide Capture and Geological
       Sequestration. World Resource Institute (WRI) Long-term liability Workshop,  June 7th, 2007.
       Washington D.C.  available: http://www.iepa.com/ETAAC/ETAAC%20Handouts%208-8-07/2007
       %20June%205%20liability%20workshop%20summary%20final%20K%20Davis.pdf

       Benson, S.M. 2005. Lessons learned from industrial and natural analogs for health,  safety and
       environmental risk assessment for geologic storage of carbon dioxide. Carbon dioxide capture for storage
       in deep geologic  formations. In Results from the CO2 Capture Project.  V2: Geologic Storage of Carbon
       Dioxide with Monitoring and Verification, S.M. Benson (ed.). Elsevier, London, UK, 1133-1141.

       Benson, S.M. 2005a. Overview of Geologic Storage of CO2. In Results from the  CO2 Capture Project.
       V 2: Geologic Storage of Carbon Dioxide with Monitoring and Verification, S.M. Benson (ed.). Elsevier,
       London, UK, 665-672.

       Benson, S. and R Hepple. 2005. Prospects for Early Detection and Options for Remediation of
       Leakage from CO2 Storage Projects. In Results from the CO2 Capture Project.  V 2: Geologic Storage of
       Carbon Dioxide with Monitoring and Verification, S.M. Benson (ed.). Elsevier, London, UK, 1189-1204.
57

-------
58
Benson, S.M., R. Hepple, J. Apps, C.F. Tsang, and M. Lippmann. 2002. Lessons Learned from
Natural and Industrial Analogues for Storage of Carbon Dioxide in Deep Geological Formations. LBNL-
51170. Lawrence Berkeley National Laboratory, Berkeley, CA.

Birkholzer, J., Q Zhou, J. Rutqvist, P. Jordan, K. Zhang, and C.-F. Tsang. 2007. Research Project
on CO2 Geological Storage and Groundwater Resources: Large-Scale Hydrological Evaluation and
Modeling of the Impact on Groundwater Systems. Annual report 2006-Sept 30, 2007. NETL.

Bogen, K.B.E., J.S. Friedmann, and F. Gouveia. 2006. Source Terms for CO2 Risk Modeling and
CIS/Simulation Based Tools for Risk Characterization. 8th Greenhouse Gas Technology Conference,
Trondheim, Norway. Session El-3.

Bouma, T.J., K.L. Nielsen, D.M. Eissenstat, and J.P. Lynch. 1997. Soil CO2 concentration does not
affect growth or root respiration in bean or citrus. Plant, Cell and Environment 20:1495-1505.

Brennan, S.T and R.C. Burruss. 2003. Specific Sequestration Volumes: A Useful Tool for CO2
Storage Capacity Assessment. USGS OFR 03-0452. Available: http://pubs.usgs.gov/of/2003/of03-452/.
Accessed 2/28/2008.

Bryant, S. and L.W. Lake. 2005. Effect of Impurities on Subsurface CO2 Storage Processes. In
Results from the CO2 Capture Project. V 2: Geologic Storage of Carbon Dioxide with Monitoring and
Verification, S.M. Benson (ed.). Elsevier, London, UK, 983-998.

Carey, J. W., M. Wigand, S.J. Chipera, G. WoldeGabriel, R. Pawar, PC. Lichtner, S.C. Wehner,
M.A. Raines, and G.D. Guthrie. 2007. Analysis and performance of oil well cement with 30 years of
CO2 exposure from SACROC unit, west Texas, USA, International Journal of Greenhouse Gas Control,
April 2007, Vol. 1, No.  1, pp 75-85.

Cawley, S., M.Y. Saunders, B. Le Gallo, S. Carpentier, G.A. Holloway, T Kirby, L. Bennison, R.
Wickens, T Wikramaratna, S.L.B. Bidstrup, M.A.E. Arkley Browne, andJ.M. Ketzer. 2005. The
NGCAS project - assessing the potential for EOR and CO2 storage at the Forties oil field, offshore
UK. In Results from the CO2 Capture Project, v.2: Geologic Storage of Carbon Dioxide with Monitoring
and Verification, S.M. Benson (ed.). Elsevier Science, London, UK, 1163-1188.

CEC. 2007. Geologic Carbon Sequestration Strategies for California.  The Assembly Bill 1925 Report to
the California Legislature. CEC-500-2007-100-SD. Available: http://www.energy.ca.gov/2007publications/
CEC-500-2007-100/CEC-500-2007-100-SD.PDF. Accessed 2/27/2008.

Celia, M.A. and M.  Radonjic. 2004. Leakage through Existing Wells: Models, Data Analysis, and
Lab Experiments. Third CMI Annual Meeting, Princeton Environmental Institute.

Celia, M.A., S. Bachu, J.M. Nordbotten, S.E. Gasda, and H.K. Dahle. 2004. Quantitative
estimation of CO2 leakage from geological storage: Analytical models, numerical models, and data
needs. In 7th International Conference on Greenhouse Gas Control Technologies. September 5-9, 2004,
Vancouver, Canada.

Celia, M.A., S. Bachu, J. M. Nordbotten, S.E. Gasda, and H. K. Dahle. 2005. Quantitative
estimation of CO2 leakage from geological storage: Analytical models, numerical models and data
needs. In Proceedings of 7th International Conference on Greenhouse Gas Control Technologies (GHGT-
7). September  5-9, 2004, Vancouver, Canada, v.I, 663-672.

-------
       Chalaturnyk, R. and W Gunter. 2004. Geological storage of CO2: Time frames, monitoring, and
       verification. In Proceedings of the 7th International Conference on Greenhouse Gas Control Technologies.
       Vol. 1:  Peer-Reviewed Papers and Plenary Presentations.

       Chiodini, G. and F. Frondini. 2001. Carbon dioxide degassing from the Albani Hills volcanic region,
       Central Italy.  Chemical Geology, Volume 177, Issues 1-2,  15 July 2001, 67-83.

       CO2CRC. 2008. Otway Project Site Selection Risk Assessment. Available: http://
       www.co2crc.com.au/otway/site.html#ra. Accessed 7/4/2008.

       Dooley, J.J., R.T. Dahowski, C.L. Davidson, M.A. Wise, N. Gupta, S.H. Kim, and E.L. Malone.
       2006. Carbon Dioxide Capture and Geologic Storage. A Technology Report from the Second Phase
       of the Global Energy Technology Strategy Program.

       Doughty, C., K. Pruess, S.M. Benson, S.D. Hovorka, PR. Knox, and C.T Green. 2001. Capacity
       investigation of brine-bearing sands of the Frio Formation for geologic sequestration of CO2. In
       Proceedings of First National Conference on Carbon Sequestration. USDOE/NETL-2001/1144, Paper
       P.32. May 14-17, 2001, Washington, D.C.  United States Department of Energy, National Energy
       Technology Laboratory.

       Ehrlich, H.L. 2002. Geomicrobiology. 4th ed. Marcel Dekker, New York.

       Eichhubl, P., P. D'Onfro, A. Aydin, J. Waters, and D.K. McCarty. 2005. Structure, petrophysics, and
       diagenesis of shale entrained along a normal fault at Black Diamon Mines, California. Implications
       for fault seal. AAPG Bulletin 89(9): 1113-1137.

       Ennis-King, J. and L. Paterson. 2003. Role  of convective mixing in the long-term storage of carbon
       dioxide in deep saline formations. SPE 84344. October 5-8, SPE Annual Technical Conference and
       Exhibit, Denver, CO.

       Espie, T 2004. Understanding risk for the long term storage of CO2 in geologic formations. In 7th
       International Conference on Greenhouse Gas Control Technologies. September 5-9, Vancouver, Canada.

       Executive Order 12898. 1994. Federal Actions to Address Environmental Justice in Minority
       Populations and Low-Income Populations. February 11.  Federal Register 59(32). February 16.
       Available: http://www.epa.gov/fedrgstr/eo/eol2898.pdf.  Accessed 1/16/2008.

       Executive Order 13045. 1997. Protection of Children from Environmental Health Risks and Safety
       Risks. Available: http://www.epa.gov/fedrgstr/eo/eol3045.htm. Accessed 1/16/2008.

       Executive Order 13175. 2000. Consultation and Coordination with Indian Tribal Governments.
       Available: http://www.epa.gov/fedrgstr/eo/eol3175.htm. Accessed 1/16/2008.

       Flower, F.B., E.F. Gilman, and LA. Leone. 1981.  Landfill gas, what it does to trees and how its
       injurious effects may be prevented. Journal of Arboriculture 7(2):43-52.

       Friedmann, S.J. 2003. Thinking Big: Science and technology needs for a large-scale geological carbon
       storage experiments. Submitted December 1, 2003 to Energy. Available at http://www.tyndall.ac.uk/
       events/past_events/large_scale.pdf. Accessed July  8, 2008.
59

-------
60
       Friedmann, S.J. and D. Nummedal. 2003. Reassessing the geological risks of seal failure for saline
       aquifers and EOR projects. In 2nd Annual Conference on Carbon Sequestration. May 5-8, 2003,
       Alexandria, VA.

       Gasda, S.E., J.M. Nordbotten, and M.A. Celia, "Determining Effective Wellbore Permeability from
       a Field Pressure Test: A Numerical Analysis of Detection Limits", Environmental Geology, published
       online 18 July 2007.

       Gasda, S.E., S. Bachu, and M.A. Celia. 2004. The potential for CO2 leakage from storage sites in
       geological media: Analysis of well distribution in mature sedimentary basins. Environmental Geology
       46(6-7):707-720.

       GEO-SEQ Project Team. 2004. GEO-SEQ Best Practices Manual: Geologic Carbon Dioxide
       Sequestration: Site Evaluation to Implementation. LBNL-56623. Lawrence Berkeley National
       Laboratory, Berkeley,  CA.  September 30.

       Gibbs, J.F., J.H. Healy, C.B. Raleigh, and J. Coakley. 1973. Seismicity in the Rangely, Colorado
       area: 1962-1970. Bulletin of the Seismo logical Society of America 63:1557-1570.

       Gorgon Joint Ventures. 2005a. Draft Environmental Impact Statement/ Environment Review and
       Management Programme for the Proposed Gorgon Development: Executive Summary. 120 pages,
       available from www.gorgon.com.au. Access date 12 March 2008.

       Gorgon Joint Ventures. 2005b. Draft Environmental Impact Statement/ Environment Review and
       Management Programme for the Proposed Gorgon Development: Main Report, available from
       www.gorgon.com.au. Access date 12 March 2008.

       Grigg, RB. 2005. Long-Term CO2 Storage: Using Petroleum Industry Experience. In Results from
       the CO2 Capture Project. V 2: Geologic Storage of Carbon Dioxide with Monitoring and Verification,
       S.M. Benson (ed.). Elsevier, London, UK, 853-867.

       Gunter, W.D., E.H. Perkins, and I. Hutcheon. 2000. Aquifer disposal of acid gases: Modeling of
       water-rock reactions for trapping acid wastes. Applied Geochemistry 15:1085-1095.

       Gurevich, A.E., B.L. Endres, J.O. Robertson Jr., and G.V Chilingar. 1993. Gas migration from oil
       and gas fields and associated hazards. Journal of Petroleum Science and Engineering 9:223-238.

       Harrington, J.F. and S.T Horseman. 1999. Gas transport properties of clays and mudrocks. In
       Muds and Mudstones: Physical and Fluid Flow Properties, A.C. Aplin et al.  (eds.). Geol.  Sco. Special
       Publication 158:107-124.

       Haszeldine, R.S. 2006. Deep geological CO2 storage: Principles reviewed, and prospecting for bio-
       energy disposal sites. Mitigation and Adaptation Strategies for Global Change 11(2):377-401.

       Healy, J.H., W.W. Ruby, D.T Griggs, and C.B. Raleigh. 1968. The Denver earthquakes. Science 161:
       1301-1310.

       Heggum, G., T Weydahl,  R Mo, M. Molnvik, and A. Austegaard. 2005. CO2 Conditioning and
       Transportation. In Results from the CO2 Capture Project. V 2: Geologic Storage of Carbon Dioxide with
       Monitoring and Verification, S.M. Benson (ed.).  Elsevier, London, UK,  925-936.

-------
61
       Heinrich, J. J., H. J. Herzog, and D.M. Reiner. 2003. Environmental Assessment of Geologic
       Storage of CO2. MIT LFEE 2003-002. Massachusetts Institute of Technology Laboratory for Energy
       and the Environment. Prepared for Clean Air Task Force, Boston, MA. December.

       Heller, A. 2005. Locked in rock: Sequestering carbon dioxide underground. Science & Technology
       Review 12-19 (May).

       Hepple,  R P. 2005. Human Health and Ecological Risks of Carbon Dioxide. In Results from the
       CO2 Capture Project. V 2: Geologic Storage of Carbon Dioxide with Monitoring and Verification, S.M.
       Benson (ed.). Elsevier, London, UK, 1143-1172.

       Holloway, S., J.M. Pearce, V.L. Hards,T Ohsumi, and). Gale. 2007. Natural emissions of CO2 from
       the geosphere and their bearing on the geological storage of carbon dioxide. Energy 32:1194-1201.

       Holter, P. 1994. Tolerance of dung insects to low oxygen and high carbon dioxide concentrations.
       European Journal of Soil Biology 30(4): 187-193.

       Holtz, M.H. 2002. Residual Gas Saturation to Aquifer Influx: A Calculation Method for 3-D
       Computer Reservoir Model Construction. SPE Paper 75502. Presented at the SPE Gas Technologies
       Symposium, Calgary, Alberta, Canada. April.

       Hoversten, G.M. and E. Gasperikova. 2005. Non-Seismic Geophysical Approaches to Monitoring.
       In Results from the CO2 Capture Project.  V 2: Geologic Storage of Carbon Dioxide with Monitoring and
       Verification, S.M. Benson (ed.). Elsevier, London, UK, 1071-1112.

       IEA. 2004. IEA GHG Weyburn CO2 Monitoring and Storage Project Summary Report 2000-2004.

       IEA. 2004a Impact of impurities on CO2 Capture, Transport and Storage. IEA-GHG Report PH4/32.

       Imbus, S. 2005. Technical Highlights of the CCP Research Program on Geological Storage of
       CO2. In Results from the CO2 Capture Project. V 2: Geologic Storage of Carbon Dioxide with
       Monitoring and Verification, S.M. Benson (ed.). Elsevier, London, UK, 673-684.

       Imbus, S. and C. Christopher.  2005. Key Findings, Technology Gaps and the Path Forward. In
       Results from the CO2 Capture Project. V 2: Geologic Storage of Carbon Dioxide with Monitoring
       and Verification, S.M. Benson  (ed.). Elsevier, London, UK, 1317-1322.

       IPCC. 2005. IPCC Special Report: Carbon Dioxide Capture and Storage. Summary for Policy Makers
       and Technical Summary, B. Metz, O. Davidson, H. de Coninck, M. Loos, and L. Meyer (eds.).
       Cambridge University Press, New York.

       IPCC. 2006. 2006 IPCC Guidelines for National Greenhouse Gas Inventories: Volume 2 - Energy.
       Chapter 5 Carbon Dioxide Transport, Injection, and Geological Storage. Available: http://www.ipcc-
       nggip.iges.or.ip/public/2006gl/index.htm.

       IPCC. 2007. IPCC Fourth Assessment Report: Mitigation of Climate Change. IPCC Working Group III.
       Available: http://www.ipcc.ch/ipccreports/ar4-wg3.htm. Accessed 1/18/2008.

       IPIECA. 2007. Oil and Natural Gas Industry Guidelines for Greenhouse Gas Reduction Projects
       Part II: Carbon Capture and Geological Storage Emission Reduction Family.

-------
       Jaffe, PR. and S. Wang. 2003. Potential effect of CO2-releases from deep reservoirs on the quality
       of fresh-water aquifers. In Proceedings 6th International Conference on Greenhouse Gas Control
       Technologies, J. Gale and E. Kaya (eds.). October 2003, Kyoto Japan, 1657-1660.

       Jarrell, P.M., C.E. Fox, M.H. Stein, and S.L. Webb. 2002. Practical Aspects of CO2 Flooding. SPE
       Monograph Series No. 22, Richardson, TX.

       Johnson, J.W, J.J. Nitao, C.I. Steefel, and K.G. Knauss. 2001. Reactive transport modeling
       of geologic CO2 sequestration in saline aquifers, the influence of intra-aquifer shales and the
       relative effectiveness of structural, solubility, and mineral trapping during prograde and retrograde
       sequestration. In DOE Workshop on Carbon Sequestration Science Conference Proceedings 2001.
       Available: http://www.netl.doe.gov/.

       Johnson, J.W, J.J. Nitao, and J.P. Morris. 2005. Reactive Transport Modeling of Cap-Rock Integrity
       During Natural and Engineered CO2 Storage. In Results from the CO2 Capture Project. V 2: Geologic
       Storage of Carbon Dioxide with Monitoring and Verification, S.M. Benson (ed.). Elsevier, London, UK,
       787-814.

       Knauss, K.G., J.W Johnson, and C.I. Steefel.  2003. CO2 sequestration in the Frio Fm., TX: Evaluation
       of the impact of CO2, co-contaminant gas, aqueous fluid and reservoir rock interactions (Presentation).
       In Second Annual Conference on Carbon Sequestration. May 5-8, 2003, Alexandria, VA.

       Kolak, J.J. and R.C. Burruss. 2003. An organic geochemical assessment of CO2- coal interactions
       during sequestration.  In Second Annual Conference on Carbon Sequestration. May 5-8, 2003,
       Alexandria, VA.

       Kolak, J.J. and R.C. Burruss. 2004. A geochemical investigation into the effect of coal rank on the
       potential environmental impacts of CO2 sequestration in deep coal beds. In Third Annual Conference
       on Carbon Sequestration. May 3-6, 2004, Alexandria, VA.

       Kumar, A., M.H. Noh,  K. Sepehrnoori, G.A. Pope, S.L. Bryant, and L.W Lake. 2005. Simulating
       CO2 storage in deep saline aquifers, carbon dioxide capture for storage in deep geologic formations.
       In Results from the CO2 Capture Project, v.2: Geologic Storage of Carbon Dioxide with Monitoring and
       Verification, S.M. Benson, (ed.). Elsevier, London, UK, 977-998.

       Le Gallo, Y. , J. M. Ketzer, and B. Carpentier. 2004. Assessing the risks of geological storage of
       CO2 in mature oil fields. In Seventh International Conference on Greenhouse Gas Control Technologies.
       September 5-9, 2004, Vancouver, Canada.

       Lewicki, L.L., J. Birkholzer, and C.F. Tsang. 2006. Natural and Industrial Analogues for Leakage
       of CO2 from Storage Reservoirs: Identification of Features, Events, and Processes, and Lessons Learned.
       LBNL-59784. Available: http://repositories.cdlib.org/lbnl/LBNL-59784/. Accessed 12/31/2007.

       Lindeberg, E. and D.  Wessel-berg. 1997. Vertical convections in an aquifer column under a gas cap
       of CO2. Energy Conversion Management 38:S229-S234.

       Lippmann, M.J. and S.M. Benson. 2003. Relevance of underground natural gas storage to geologic
       sequestration of carbon  dioxide. Department of Energy's Information Bridge. Available: http://
       www.osti.gov/dublincore/ecd/servlets/purl/813565-MVm7Ve/native/813565. Accessed 2/27/2008.
62

-------
       Mace, M. J. 2007. Regulatory challenges to the implementation of carbon capture and geological
       storage within the European Union under EU and international law. International Journal of
       Greenhouse Gas Control 1:253-260.

       Maina, J.N. 1998. The Gas Exchangers: Structure, Function, and Evolution of the Respiratory Processes.
       Springer, Berlin, Germany.

       Maul, P., D. Savage, S. Benbow, R Walke, and R Bruin. 2003. Development of a FEP database
       for the geological storage of carbon dioxide. In Seventh International Conference on Greenhouse Gas
       Control Technologies. September 2004, Vancouver, Canada.

       McGee, K.A.  andT.M. Gerlach. 1998. Annual cycle of magmatic CO2 in a tree-kill soil at
       Mammoth Mountain, California: Implications for soil acidification. Geology 26.5:463-466.

       Meehl, G.A., T.E Stocker, W.D. Collins, P. Friedlingstein, A.T. Gaye, J.M. Gregory, A. Kitoh, R
       Knutti, J.M. Murphy, A. Noda, S.C.B. Raper, I.G. Watterson, A.J. Weaver and Z.-C. Zhao. 2007.
       Global Climate Projections. In: Climate Change 2007: The Physical Science Basis.  Contribution of
       Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change,
       Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller
       (eds.). Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

       Miles, N., K.  Davis, and J. Wyngaard. 2005. Detecting Leaks from CO2 Reservoirs Using
       Micrometeoro logical Methods.  Carbon Dioxide Capture for Storage in Deep Geologic Formations - Results
       from the CO2  Capture Project, Volume 2, S. Benson (ed.). Elsevier Science, London, UK, 1031-1044.

       Miles, H., S. Widdicombe, J.I. Spicer, and J. Hall-Spencer. 2007. Effects of anthropogenic seawater
       acidification on acid-base balance in the sea urchin Psammechinus miliaris. Marine Pollution Bulletin
       54:89-96.

       MIT. 2007. The Future of Coal: Options  for a Carbon-Constrained World. Massachusetts Institute
       of Technology. Available: http ://web .mit.edu/coal/. Accessed
       1/16/2008.

       NASCENT. 2005. Natural Analogues for the Storage of CO2 in the Geological Environment, IEA
       Greenhouse Gas R&D Programme. Technical Report 2005/06. Available: http://www.ieagreen.org.uk/
       2005.html.

       NETL. 2007. Carbon Sequestration Atlas of the United States and Canada. Appendix A:
       Methodology for Development of Carbon Sequestration Capacity Estimates.

       Nicot, J., S. Hovorka, and S.  Lakshminarasimhan. 2006. Impact of carbon storage on shallow
       groundwater and pressure-controlled regional capacity for brine aquifers, Abstract. Proceedings of
       AGU Fall Meeting. San Francisco, CA, December.

       Nimz, G.J. and G.B. Hudson. 2005. The  Use of Noble Gas Isotopes for Monitoring Leakage of
       Geologically Stored CO2. In Results from the CO2 Capture Project. V 2: Geologic Storage of Carbon
       Dioxide with Monitoring and Verification, S.M. Benson (ed.). Elsevier, London, UK,  1113-1130.
63

-------
       NOAA. 2007. Trends in Atmospheric Carbon Dioxide - Global. National Oceanic and Atmospheric
       Administration Earth System Research Laboratory. Available: http://www.esrl.noaa.gov/gmd/ccgg/trends/
       index.htmlffglobal. Accessed 2/27/2008.

       Oldenburg, C.M. 2005. Health, Safety, and Environmental Screening and Ranking Framework for
       Geologic CO2 Storage Site Selection. LBNL-58873. Lawrence Berkeley National Laboratory, Berkeley,
       CA. September 20.

       Oldenburg, C.M. and A.J.A. Unger. 2004. Coupled Vadose Zone and Atmospheric Surface-Layer
       Transport of Carbon Dioxide from Geologic Carbon Sequestration Sites. Vadose Zone Journal3:
       848-857.

       Oldenburg, C.M. and A.J.A. Unger. 2005. Modeling of Near-Surface Leakage and Seepage of CO2
       for Risk Characterization. In Results from the CO2 Capture Project. V 2: Geologic Storage of Carbon
       Dioxide with Monitoring and Verification, S.M. Benson (ed.). Elsevier, London, UK, 1205-1216.

       Oldenburg, C.M., J.L. Lewicki, and R.P. Hepple. 2003. Near-Surface Monitoring Strategies for
       Geologic Carbon Dioxide Storage Verification. LBNL-54089. Lawrence Berkeley National Laboratory,
       Berkeley, CA.

       Oldenburg, C.M.,TE.  McKone, RP. Hepple, andAJ.A.  Unger. 2002a. Health Risk Assessment for
       Leakage and Seepage from Geologic Carbon Sequestration Sites: Requirements and Design of a Coupled
       Model. LBNL-51131. Lawrence Berkeley National Laboratory, Berkeley, CA.

       Oldenburg, C.M., A.J.A. Unger, RP. Hepple, and P.D. Jordan. 2002b. On Leakage and Seepage from
       Geologic Carbon Sequestration Sites. LBNL-51130. Lawrence Berkeley National Laboratory,  Berkeley,
       CA.

       Omre, H., K. Solna, N. Dahl, and B. Torudbakken. 1994. Impact of fault heterogeneity in fault
       zones on fluid flow. In North Sea Oil and Gas reservoirs III, J.O. Aasen, E. Berg, A.T. Buller, O.
       Hjelmeland, R.M. Holt, J. Kleppe, and O. Torsaeter (eds.). Kluwer, Dordrecht, 185-200.

       Onstott, T C.2005. Impact of CO2 injections on Deep Subsurface Microbial Ecosystems and
       Potential Ramifications for the Surface Biosphere. In Results from the CO2 Capture Project. V 2:
       Geologic Storage of Carbon Dioxide with Monitoring and Verification, S.M. Benson (ed.). Elsevier,
       London, UK, 1217-1250.

       Pawar, R., J. Carey, S. Chipera, J. Fessenden, J. Kaszuba, G. Keating, P. Lichtner, S. Olsen, P.
       Stauffer, H. Viswanathan, H. Ziock, and G. Guthrie. 2006. Development of a framework for long-
       term performance assessment of geologic CO2 sequestration sites, Proceedings of the 8th Greenhouse
       Gas Technology Conference. June, Trondheim, Norway.

       Perry, K.E 2005. Natural gas storage industry experience and technology: Potential application
       to CO2 geological storage. In Results from the CO2 Capture Project. V 2: Geologic Storage of Carbon
       Dioxide with Monitoring and Verification, S.M. Benson (ed.). Elsevier, London, UK, 815-825.

       Pickles, W.L. and W.A.  Cover. 2005. Hyperspectral Geobotanical Remote Sensing for CO2 Storage
       Monitoring. In Results from the CO2 Capture Project. V 2: Geologic Storage of Carbon Dioxide with
       Monitoring and Verification, S.M. Benson (ed.). Elsevier, London, UK, 1045-1070.
64

-------
65
       Pruess, K., J. Garcia, T. Kovscek, C. Oldenburg, J. Rutqvist, C. Steefel, andT. Xu. 2004. Code
       intercomparison builds confidence in numerical simulation models for geologic disposal of CO2.
       Energy 2004. 29: 1431-1444, 2004, LBNL-52211.

       Quintessa. 2004. CO  FEP Database. Quintessa Ltd. Available: http://www.quintessa.org/consultancy/
       index.html?http://www.quintessa.org/consultancy/fepDatabase.html. Accessed 1/16/2008.

       Raleigh, C.B., J.D. Healy, and J.D. Bredehoeft, 1976. An experiment in earthquake control of
       Rangely, Colorado. Science 191:1230-1237.

       Rice, S.A. 2003. Health effects of acute and prolonged CO2 exposure in normal and sensitive
       populations. In Second Annual Conference on Carbon Sequestration. May 5-8.

       Rice, S.A. and R. Rhudy.  2004. Health & ecological risk assessment. In Risk Assessment Workshop.
       February 11-12, London, UK.

       Rutqvist, J., J. Birkholzer, F. Cappa, and C.-F. Tsang. 2007a. Estimating maximum sustainable
       injection pressure during geological sequestration  of CO2 using coupled fluid flow and
       geomechanical fault-slip analysis, Energy Conversion and Management. 48, 1798-1807.

       Rutqvist, J., J.T. Birkholzer, and C.-F. Tsang. 2007b. Coupled reservoir-geomechanical analysis of the
       potential for tensile and shear failure associated with CO2 injection in multilayered reservoir-caprock
       systems, International Journal of Rock Mechanics. Mining Sci., doi:10.10l6/j.ijrmms.2007.04.006.

       Saripalli, K. P., E.M. Cook, and N. Mahasenan. 2002. Risk and hazard assessment for projects
       involving the geological sequestration of CO2,  In Proceedings from 6th International Conference on
       Greenhouse Gas Technologies, Gale, J. and Kaya, Y.  (eds.). Kyoto, October 2002.

       Sass, B., Monzyk, B., S. Ricci, A. Gupta, B. Hindin, and N. Gupta. 2005. Impact of SOx and
       NOx in Flue Gas on CO2 Separation, Compression, and Pipeline Transmission. In Results from the
       CO2 Capture Project. V 2: Geologic Storage of Carbon Dioxide with Monitoring and Verification, S.M.
       Benson (ed.). Elsevier, London, UK, 955-982.

       Scherer, G.W., M.A. Celia, J.H. Prevost, S. Bachu, S. Bruant, A. Duguid, R Fuller, E. Sarah. S.E.
       Gasda, M. Radonjic, and W Vichit-Vadakan. 2005. Leakage of CO2 Through Abandoned Wells:
       Role of Corrosion of Cement. In Results from the CO2 Capture Project. V 2: Geologic Storage of Carbon
       Dioxide with Monitoring and Verification, S.M. Benson (ed.). Elsevier, London, UK, 827-850.

       Schutt, H., M. Wigand, and E. Spangenberg. 2005. Geophysical and Geochemical Effects of
       Supercritical CO2 on Sandstones. In Results from the CO2 Capture Project. V 2: Geologic Storage of
       Carbon Dioxide with Monitoring and Verification, S.M. Benson (ed.). Elsevier, London, UK, 767-786.

       Seiersten, M., and K.O. Kongshaug. 2005. Materials Selection for Capture, Compression, Transport
       and Injection of CO2. In Results from the CO2 Capture Project.  V 2: Geologic Storage of Carbon Dioxide
       with Monitoring and Verification, S.M. Benson (ed.). Elsevier, London, UK, 937-954.

       Shipton, Z.K., J.P. Evans, B. Dockrill, J.E. Heath, A. Williams, D. Kirschner, and P.T Kolesar. 2005.
       Natural leaking CO2-charged systems as analogs for failed geologic sequestration reservoirs. In The
       CO2 Capture and Storage Project (CCP) II, D.C. Thomas and S.M. Benson (eds.).  Elsevier Science,
       679-701.

-------
       Shuler, P. and Y. Tang. 2005. Atmospheric CO2 Monitoring Systems. In Results from the CO2 Capture
       Project. V 2: Geologic Storage of Carbon Dioxide with Monitoring and Verification, S.M.  Benson (ed.).
       Elsevier, London, UK, 1015-1030.

       Skinner, L. 2003. CO2 blowouts: an emerging problem: World Oil, v. 224, no. 1, p. 38-42.

       Sminchak, J., N. Gupta, C. Byrer, and P. Bergman. 2002. Issues related to seismic activity induced
       by the injection of CO2 in deep saline aquifers. Journal of Energy & Environmental Research 2:32-46.

       Spicer, J.I., A. Raffo, and S. Widdicombe.  2007. Influence of Correlated seawater  acidification
       on extracellular acid-base balance in the velvet swimming crab Necora puber. Marine Biology 151:
       1117-1125.

       Stenhouse, M. J., W. Zhou, D. Savage,  and S. Benbow. 2005. Framework methodology for long-
       term assessment of the fate of CO2 in the Weyburn field. In Results from  the CO2 Capture Project.
       V 2: Geologic Storage of Carbon Dioxide with Monitoring and Verification, S.M. Benson (ed.).
       Elsevier, London, 1251-1261.

       Stevens, S.H. 2005. Natural CO2 Fields as Analogs for Geologic CO2 Storage. In Results from the
       CO2 Capture Project. V 2: Geologic Storage of Carbon Dioxide with Monitoring and Verification,
       S.M. Benson  (ed.). Elsevier, London, UK,  687-698.

       Streit, J.E. and R.R. Hillis. 2003. Building geomechanical models for the  safe underground storage
       of carbon dioxide in porous rock. Proceedings of the 6th International Conference on Greenhouse
       Gas Control Technologies (GHGT-6), J. Gale and Y. Kaya (eds.). October 1-4, 2002,  Kyoto, Japan.
       Pergamon, Amsterdam, v.I, 495-500.

       Streit, J.E. and M.N. Watson. 2004. Estimating rates of potential CO2 loss from geological storage
       sites for risk and uncertainty analysis. In Seventh International Conference on Greenhouse Gas Control
       Technologies. Vancouver, Canada.

       Streit, J., A. Siggins, and B. Evans.  2005. Predicting and monitoring geomechanical effects of CO2
       injection, carbon dioxide capture for storage in deep geologic formations.  In Results from the CO2
       Capture Project, v. 2: Geologic Storage of Carbon Dioxide with Monitoring and Verification, S.M.
       Benson (ed.). Elsevier Science, London, UK, 751-766.

       Sustr, V AND M. Siemk. 1996.  Behavioural responses to and lethal effects of elevated carbon
       dioxide concentration in soil invertebrates. European Journal of Soil Biology,  32:149-155.

       Tsang, C-E, J. Birkholzer, and J. Rutqvist.  2007. A comparative review of hydrolologic issues
       involved  in geologic storage of CO2 and injection disposal of liquid waster. Journal of Environmental
       Geology

       Turley, C, J.C. Blackford,  S. Widdicombe, D. Lowe, P.D. Nightingale, and A.P. Rees. 2006.
       Reviewing the impact of increased atmospheric CO2 on oceanic pH and the marine ecosystem.
       In Avoiding Dangerous Climate Change, H.J. Schellnhuber (ed.). Cambridge University Press,
       Cambridge, UK, 65-70.
66

-------
       Turley, C., P. Nightingale, N. Riley, S. Widdicombe, I. Joint, C. Gallienne, D. Lowe, L. Goldson, N.
       Beaumont, P. Mariotte, S. Groom, G. Smerdon, A. Rees, J. Blackford, N. Owens, J. West, P. Land,
       and E. Woodason. 2004. Literature Review: Environmental Impacts of a Gradual of Catastrophic
       Release of CO2 into the Marine Environment Following Carbon Dioxide Capture and Storage.
       DEFRA :  MARP 30 (ME2104).

       U.S. DOE. 2007a. Final Risk Assessment Report for the FutureGen Project Environmental Impact
       Statement. Available: http://www.ned.doe.gov/technologies/coalpower/futuregen/EIS/FG%20Risk%20Assessment%
       20110807.pdf

       U.S. DOE. 2007b. National Energy Technology Laboratory: Carbon Sequestration, Regional
       Carbon Sequestration Partnerships. Available: http://www.netl.doe.gov/technologies/carbon seq/partnerships/
       partnerships.html - Phase2.

       U.S. DOE. 2007c. FutureGen  Project Final Environmental Impact Statement (DOE/EIS-0394).
       Available:  http://www.netl.doe.gov/technologies/coalpower/futuregen/EIS/.

       U.S. EPA. 2007. EPA Newsroom. Announcement October 11, 2007. Available: http://yosemite.epa.gov/
       opa/admpress.nsf/fOd7b5b28db5b04985257359003f533b/84bdlefl9cOOeb7a85257371006b6a21!OpenDocument.

       USGS. 2001. USGS Fact Sheet: Invisible Gas Killing Trees at Mammoth Mountain, California.
       Available:  http://pubs.usgs.gov/fs/fsl72-96/.

       USGS. 2007. 2007 Seismic Hazard Mapping Project (2007 Preliminary). Available: http://
       earthquake.usgs.gov/research/hazmaps/products  data/2007/. Accessed 1/10/2008.

       van der Meer, L.G.H. 1995. The CO2 storage efficiency of aquifers. Energy Conversion and
       Management 36(6-9):513-518.

       Vodnik, D., D. Kastelec,  H. Pjanz, I. Macek, and B. Turk. 2006. Small-scale spatial variation in soil
       CO2 concentration in a natural carbon dioxide spring and some related plant responses. Geoderma
       133:309-319.

       Walton, F.B., J.C. Tait, D. LeNeveu, and M.I. Sheppard. 2004. Geological storage of CO2: A
       statistical approach to assessing performance and risk. In Seventh International Conference on
       Greenhouse Gas Control Technologies,  September 2004. Vancouver, Canada.

       Wang, S. and PR Jaffe. 2004.  Dissolution of a mineral phase in potable aquifers due to CO2 releases
       from deep formations: Effect of dissolution kinetics. Journal of Energy Conversion and Management
       45(18-19):2833-2848.

       Watson, M.N., RE Daniel, PR. Tingate, and C.M. Gibson-Poole. 2005. Correlated seal capacity
       enhancement in mudstones: Evidence from  the Pine Lodge natural CO2 accumulation, Otway
       Basin, Australia. In Greenhouse Gas Control Technologies, M. Wilson, T Morris, J. Gale, and K.
       Thambimuthu (eds.).  Proceedings from the  7th Greenhouse Gas Control Technologies Conference,
       Vol 2. Elsevier, 2313-2316.

       West, J.M. and P.J. Chilton. 1997. Aquifers as environments for microbiological activity. Quarterly
       Journal of Engineering Geology 30:147-154.
67

-------
       West, J. M., J.P. Pearce, M. Bentham, and P. Maul. 2005. Issue Profile: Environmental Issues and the
       Geological Storage of CO2. European Environment. 15:250 - 259.

       West, J.M., J. Pearce, M. Bentham, C. Rochelle, and P. Maul. 2006. Environmental issues, and the
       geological storage of CO2 - a European perspective. Proceedings of the 8th International Conference on
       Greenhouse Gas Control Technologies. Trondheim, Norway, June 2006.

       Wildenborg, A.F.B., A.L. Leijnse, E. Kreft, M.N. Nepveu, A.N.M. Obdam, B. Orlic, E.L. Wipfler,
       B. van der Grift, W van Kesteren, I. Gaus, I. Czernichowski-Lauriol, P. Torfs, and Wojcik, R
       2005. Risk Assessment Methodology for CO2 Storage: The Scenario Approach. In Results from the
       CO2 Capture Project. V 2: Geologic Storage of Carbon Dioxide with Monitoring and Verification, S.M.
       Benson  (ed.). Elsevier, London, UK, 1293-1316.

       Wilkens S.J. and S.J. Naruk. 2007. Quantitative analysis of slip-induced dilation with application of
       fault seal. AAPG Bulletin 91:97-113.

       Wilson, E.J. 2004. Managing the Risks of Geologic Carbon Sequestration: A Regulatory and Legal
       Analysis. PhD thesis, Carnegie Mellon University.

       Wo, S. and J.T. Liang. 2005. CO2 storage in coalbeds: CO2/N2 injection and outcrop seepage modeling,
       carbon dioxide capture for storage in deep geologic formations. In Results from the CO2 Capture Project, v.
       2: Geologic Storage of Carbon Dioxide with Monitoring and Verification, S.M. Benson (ed.). Elsevier Science,
       London, UK, 897-924.

       Wo, S., Liang, J. and L.R Myer. 2005. CO2 Storage in Coalbeds: Risk Assessment of CO2 and Methane
       Leakage. In Results from the CO2 Capture Project. V2: Geologic Storage of Carbon Dioxide with Monitoring
       and Verification, S.M. Benson (ed.). Elsevier, London, UK, 1263-1292.

       Xu, T., J.A. Apps, and K. Pruess. 2003. Reactive geochemical transport simulation to study mineral
       trapping for CO2 disposal in deep arenaceous formations. Journal of Geophysical Research 108(B2):
       2071-2084.

       Zhang, Y., C.M. Oldenburg, S. Finsterle, and G.S. Bodvarsson. 2006. System-level modeling for
       geological storage of CO2. In Proceedings: TOUGH Symposium 2006. May 15-17. Lawrence Berkeley
       National Laboratory, Berkeley, CA.

       Q. Zhou, J. Birkholzer, C.-E Tsang, and J. Rutqvist. 2008. A method for quick assessment of CO2
       storage capacity in closed and semi-closed saline formations. Lawrence Berkeley National Laboratory.
       Paper LBNL-63820.

       Zhou, W, M. Stenhouse, M. Sheppard, and E Walton. 2004. Theme 4: Long-term risk assessment
       of the storage site. In IEA-GHG Weyburn CO2 Monitoring and Storage Project Summary Report
       2000-2004, Wilson, M., and Monea, M. (ed.). 211-268. Petroleum Technology Research Centre,
       Regina,  Canada.
68

-------
       GLOSSARY
       Term
       Abandoned Well
       Adsorption
       Artificial Penetration
       Aquifer
       Borehole
       Brine
       Buffering Capacity


       Capillary Entry
       Pressure

       Capillary Trapping

       Caprock
       Carbon Dioxide
       Capture And Storage
       Closed System
       CO2 Plume
       CO2 Spatial Area Of
       Evaluation
       CO 2 Stream
A well that is no longer in use and has been closed according to
standardized procedures, which may often include placing cement or
mechanical plugs in all or part of the well.

The adhesion of molecules on the surface of a solid or a liquid.
Man-made perforations that penetrate the subsurface (e.g., wells, boreholes,
and mines).
An underground geological formation or group of formations, containing
water. Aquifers are sources of groundwater for wells and springs.
A characteristic of the geologic system that may influence the vulnerability
to adverse impacts. Geologic attributes are characteristics that can affect the
vulnerability to unanticipated migration, leakage, or pressure changes.

A hole drilled into the subsurface, typically to collect soil samples, water
samples or rock cores. A borehole may be converted to a well by installing a
vertical pipe (casing) and well screen to keep the borehole from caving.

Water containing  a high concentration of dissolved salts.
A measure of the ability of a solution to resist change in pH when acid or
base are added.

The additional pressure needed for a liquid or gas to enter a pore occupied
by a different phase. For example, for CO2 to displace water and enter the
pores  of the confining system.
Retention of CO2 in pore spaces by capillary forces.
See confining system.
A climate change mitigation strategy that involves capturing CO2 emissions
from large stationary sources, transporting the CO2 to a storage location,
and sequestering the CO2 for long periods of time.
A system where elevated pressure levels associated with the injection of CO2
do not dissipate to background levels, but remain elevated at its physical
boundaries, and may remain elevated for a long period of time after
injection stops.
The extent, in three dimensions, of an injected carbon dioxide stream.
The CO2 spatial area of evaluation is delineated based on potential adverse
impacts resulting from unanticipated migration or leakage. The CO2
spatial area of evaluation is typically larger than the CO2 plume, because
unanticipated migration or leakage of CO2 right along the boundary of the
plume may affect areas beyond the plume.

The content of the CO2 stream captured from large point sources for
injection. The CO2 stream may include trace amount of impurities in
addition to CO2.
69

-------
       Confining System
       Corrective Action
                      The confining system is a geologic formation, group of formations (e.g.,
                      shale or siltstone), or part of a formation that is composed of impermeable
                      or distinctly less permeable material stratigraphically overlying the injection
                      zone that acts as a barrier to the upward flow of fluids.

                      Use of methods to assure that wells do not serve as conduits for
                      unanticipated migration or leakage.
       Cultural/Recreational  Cultural resources include gathering places for human populations,
       Resources             including places of archaeological, historic, or natural significance;
                              American Indian resources, cemeteries and paleontological resources.
                              Recreational resources include ecological features that provide fishing,
                              hunting, and hiking to the public.
       Dip
       Dissolution Rate
                      The angle between a planar feature, such as a sedimentary bed or a fault,
                      and the horizontal plane.
                      The rate at which a substance is dissolved in a fluid.
       Dissolution Trapping  The trapping of CO2 when it contacts the fluid formation and dissolves into
                              the fluid. Also referred to as solubility trapping

                              Injection pressure at the point where CO2 exits the well and is introduced
                              to the injection zone formation(s).

Downhole Injection
Pressure
Economic Resources
       Endangerment
       Enhanced Coal Bed
       Methane Recovery

       Enhanced Gas
       Recovery
       Enhanced Oil
       Recovery
Surface and subsurface resources with economic value, including mineral
and hydrocarbon reservoirs, forests, and croplands.
The construction, operation, maintenance, conversion, plugging, or
abandonment of an injection well, or the performance of other injection
activities, by an owner or operator in a manner that allows the movement
of fluid containing any contaminant into a USDW, if the presence of
that contaminant may cause a violation of any primary drinking water
regulations or may adversely affect the health of persons.
The process of injecting a gas (e.g., CO2) into coal, where it is adsorbed to
the coal surface and methane is released. The methane can be captured and
produced for economic purposes; when CO2 is injected, it adsorbs to the
surface of the coal, where it remains sequestered.

Typically, the process of injecting a fluid (e.g., water, brine, or CO2) into
a gas bearing formation to  recover residual natural gas. The injected fluid
thins (decreases the viscosity) or displaces small amounts of extractable gas,
which is then available for  recovery.

Typically, the process of injecting a fluid (e.g., water, brine, or CO2) into
an oil bearing formation to recover residual oil. The injected fluid thins
(decreases the viscosity) or  displaces small  amounts of extractable oil, which
is then available for recovery.
70

-------
       Fault
       Flux Rate

       Formation

       Fracture
                      Faults are breaks in the Earth's crust that occur as a result of when the
                      crustal rock is either compressed or pulled apart. Faults may either serve
                      as barriers or conduits to fluid flow, depending upon whether they are
                      transmissive or sealed.
                      The rate of transfer of fluid, particles, or energy across a given area.
                      A body of rock of considerable extent with distinctive characteristics that
                      allow geologists to map, describe, and name it.
                      A separation or discontinuity plane in a geologic formation, such as a joint
                      that divides a rock segment into two or more pieces. Fractures can be caused
                      by stress exceeding the rock strength.
       Fracture/Fault
       Reactivation Pressure

       Geochemical Process

       Geologic
       Sequestration

       Geomechanical
       Processes
       GS System

       GS Footprint

       Hypercapnia
                  egory
                      The geologic formation pressure threshold that when achieved will cause re-
                      opening of a sealed fault or fracture.

                      Geochemical processes refer to chemical reactions that may cause alterations
                      in mineral phases.
                      The process of injecting captured CO2 into deep, subsurface rock
                      formations for long-term storage. This term does not apply to its capture or
                      transport.
                      Processes that may result in alterations in the structural integrity of geologic
                      material.
                      A system that is comprised of the confining system, the CO2 stream, and
                      the injection zone.
                      The areal extent of the CO2 plume and associated pressure front.

                      A physical condition involving an excessive amount of CO2 in the blood.
impa
       Injection

       Injection Zone
                      Term for classifying groups that might experience adverse impacts. Impact
                      categories include human health and welfare, atmosphere, ecological
                      receptors, surface and ground water, and the geosphere.
                               ^H
                      The subsurface discharge of fluids through a well.
                      A geologic formation, group of formations, or part of a formation of
                      sufficient areal extent, thickness, porosity, and permeability to accommodate
                      CO2 injection volume and injection rate.
                                                             ^H
                      Injectivity characterizes the ease with which fluid can be injected into a
                      geological formation.
                      The surface area of the confining system that overlies the GS footprint.
                                    ^H  ^H                  ^H       ^H
                      The movement of CO2 (or other fluids and gases) to the surface (for
                      example, to the atmosphere or oceans).
Legislatively Protected Species designated as threatened or endangered by State and federal
Species               government authorities.

       Injectivity

       Lateral Extent
       Leakage
71

-------
       Measurement,
       Monitoring, And
       Verification
                       Collectively, a comprehensive protocol for providing an accurate accounting
                       of stored CO2.
       Mechanical Integrity   The absence of fluid-conducting openings within the injection tubing,
                              casing, or packer (known as internal mechanical integrity), or outside of the
                              casing (known as external mechanical integrity).
                              Evaluations used to confirm that a well maintains internal and external
                              mechanical integrity. These tests are a means of measuring the adequacy of
                              the construction of an injection well.

                              Subsurface movement of CO2 (or other fluids) within or out of the
                              injection zone.

                              When injected CO2 reacts with the formation waters and/or formation
                              rocks to form carbon-containing minerals such as carbonates, thereby
                              effectively retaining the CO2 in the formation.

                              The chemical process  involving the reaction of CO2 with the formation
                              waters and/or formation rocks to form carbon-containing minerals such as
                              carbonates.
Mechanical Integrity
Test
Migration


Mineral Trapping



Mineralization

       Mineralogy
       Molar Volume

       Monitoring
       Open System
       Packer
                      The mineral content of a rock or geologic formation. Evaluating mineralogy
                      can help determine whether metal and metalloid-bearing mineral phases
                      (e.g., metal oxides, sulfides, carbonates) are present.

                      The volume occupied by one mole of a substance (chemical element or
                      chemical compound) at a given temperature and pressure.
                      Employing various technologies for the purposes of measuring quantities
                      of injected CO2, tracking the location and movement of injected CO2
                      and other fluids, ensuring the effectiveness of injection wells, assessing the
                      integrity of abandoned wells.
       Other Sensitive
       Receptors
                      A system in which pressure levels associated with injection of CO2 dissipate
                      to background levels before reaching the physical boundaries of the system.

                      Ecological receptors that may be sensitive to exposure to CO2, other stream
                      constituents, brine, or other fluids that may result from GS activities but are
                      not legislatively protected. These receptors may be vulnerable either because
                      of their behavior (e.g., soil-dwelling organisms) or because their physiology
                      makes them unusually sensitive to increased concentrations of released
                      substances.

                      A mechanical device set immediately above the injection zone that seals the
                      outside of the tubing to the inside of the long string casing. A packer may
                      be a simple mechanically set rubber device or a complex concentric seal
                      assembly.
72

-------
       Permeability
       Permit
       Physical Capacity

       Physical Trapping

       Plugging

       Populations Covered
       By Executive Orders


       Pore Space
       Porosity
       Precipitate
       Preferential
                       The ability of a geologic material to allow transmission of fluid through
                       pore spaces.

                       An authorization, license, or equivalent control document issued by EPA
                       or a State to operate an injection well. Permits may be individual permits
                       (covering a single well) or area permits (covering multiple wells in one area).

                       The volume within a geologic formation that is available to accept CO2.

                       When injected CO2 rises owing to its relative buoyancy or the applied
                       injection pressure, and reaches a physical barrier that inhibits further
                       upward migration.

                       The act or process of stopping the flow of water, oil or gas into or out of a
                       formation through  a borehole or well penetrating that formation.

                       A series of executive orders require proposed federal regulations and
                       programs to specifically address potential impacts to environmental justice
                       populations, children, and tribal governments.

                       Open spaces in rock or soil. In the subsurface, these are typically filled with
                       water, brine (i.e., salty fluid), or other fluids such as oil and methane.
                       A measure of the percentage of a rock that is occupied by pore space.

                       A solid separated from a solution, especially as the result of a chemical
                       reaction (i.e., the reaction of minerals within the confining system with
                       CO2 and salt ions).

                       When micropores in certain geologic formations tend to adsorb CO2 and
       Adsorption Trapping  displace other present gases to which they have a lower affinity.

       Pressure Change
                       A change in force per unit area. Pressure changes are likely to be associated
                       with the injection of CO2 into the subsurface.
Pressure Equilibration A state of balance achieved when formation pressure levels reached during
                       injection return to the original formation pressure levels.
       Pressure Spatial Area
       Of Evaluation
                       The pressure spatial area of evaluation is delineated based on the potential
                       for subsurface pressure changes that are sufficiently significant to cause
                       adverse impacts to overlying receptors (e.g., fluid displacement into an
                       overlying aquifer).
Protected Or Sensitive Drinking water supplies that are vulnerable to endangerment or
Drinking Water
Supplies
Receptor
                              contamination.

                              A surface or underground feature whose presence within the CO2 and
                              pressure spatial areas of review could affect the vulnerability of adverse
                              impacts in the event of unanticipated migration, leakage, or pressure
                              changes.
       Regional Groundwater The direction and rate of groundwater movement in the subsurface.
        'low
73
H

-------
       Release
       Remediation

Another term for leakage, the movement of CO2 (or other fluids) to the
surface (for example, to the atmosphere or oceans).

The process of correcting any source of failure to stop or control undesired
CO2 movement if it occurs.
       Residual Water
       Saturation

       Risk Assessment

       Safe Drinking Water
       Act
Retainment of water in pore space due to capillary forces.
An approach to measuring the probability and severity of consequences.

       Seismic Activity
       Seismic Technology
       Seismicity
The main federal law that ensures the quality of Americans' drinking water.
The Safe Drinking Water Act sets the framework for the Underground
Injection Control Program to control the injection of fluids. EPA and states
implement the UNDERGROUND INJECTION CONTROL Program,
which sets standards for safe injection practices and bans certain types of
injection.
Seismic activity is defined as the shifting of the Earth's surface due to
changes at depth. Increased seismic activity can lead to earthquakes.
A monitoring approach that involves measuring the velocity and absorption
of energy waves through rock and provide a picture of underground layers
of rocks and reservoirs.
The episodic occurrence of natural or human-induced earthquakes.
       Stratigraphic Position  The order and relative arrangement of a specific layer of rock that is
                              recognized as a cohesive unit based on lithology, fossil content, age, or other
                              properties.
       Strike
       Subsidence
       Supercritical Fluid

The line representing the intersection of a planar feature with the
horizontal.
Lowering, or "sinking," of geologic formations due to dissolution of
formation minerals. Dissolution of formation materials can result from
CO2 acidification of formation waters.

A fluid above its critical temperature (31.1 degrees Celsius for CO2) and
critical pressure (73.8 bar for CO2). Supercritical fluids have physical
properties intermediate to those of gases and liquids.
       Tectonic Activity       Natural activity involving structural changes to the Earth's geology.

       Total Dissolved Solids  The measurement, usually in mg/L for the amount of all inorganic and
                              organic substances suspended in liquid as molecules, ions, or granules. For
                              injection operations, TDS typically refers to the saline (i.e., salt) content of
                              water-saturated underground formations.
74

-------
       Tracer                A chemical compound or isotope added in small quantities to trace flow
                             patterns.

       Trapping Mechanism A physical or geochemical feature in the geologic system that retains
                             injected CO2, immobilizing it under thick, low-permeability seals or by
                             converting it to solid minerals.
       Travel Time           The interval of time that is required for a fluid (e.g., CO2 or brine) to
                             migrate across the thickness  of the confining system.
       Tubing
       Underground Source
       Of Drinking Water
       Uplift
       Vulnerability
       Assessment
A small-diameter pipe installed inside the casing of a well. Tubing conducts
injected fluids from the wellhead at the surface to the injection zone and
protects the long-string casing of a well from corrosion or damage by the
injected fluids.
An aquifer or portion of an aquifer that supplies any public water system or
that contains a sufficient quantity of ground water to supply a public water
system, and currently supplies drinking water for human consumption, or
that contains fewer than 10,000 mg/1 total dissolved solids and is not an
exempted aquifer.

Rising of geologic formations due to increased pore  pressure from injection.
An approach to examining conditions that lead  to increased or decreased
susceptibility to consequences.
       Wellbore
The characteristics of a source of water that determine its usefulness for a


A bored, drilled, or driven shaft or a dug hole whose depth is greater than
the largest surface dimension. Wells can be used for production, injection,
or monitoring purposes.

See borehole.
75

-------
      APPENDIX A
       Properties of Carbon Dioxide (CO2)

       This appendix provides a brief overview of the physical and chemical properties of CO2 and a
       description of the supercritical phase in which CO2 will be injected for GS. The properties discussed
       in this section are important considerations addressed in the VEF.

       Physical Properties of CO2

       Carbon dioxide typically exists as a gas at normal pressure and temperature (i.e., at the earth's
       surface). It can be processed into a solid (i.e., dry ice) at temperatures below -55C (-67F). At
       very high pressure, CO2 may exist as a solid, liquid, or a supercritical fluid. A substance may be a
       supercritical fluid at high pressure and temperature when the gaseous and liquid phases have the
       same density and cannot be further compressed with additional pressure. Carbon dioxide exists
       as a supercritical fluid at temperatures greater than 31.3C and pressures greater than 73.9 bar
       (IPCC, 2005).
       Chemical Properties of CO2

       Carbon dioxide is soluble in water, forming carbonate ions and acidity
       [CO2 (g) +H2O => H2CO3 (aqueous) => HCCV + H+ => CO32- + 2H+] (IPCC, 2005, Chapter 6).
       The solubility decreases with increasing temperature and salinity and increases with increasing
       pressure. Solid hydrates may form in aqueous solutions of CO2 at high pressure and temperatures
       below 11C (IPCC, 2005).
       Characteristics of
       Supercritical CO2

       Supercritical CO2 is approximately
       one order of magnitude less viscous
       than water and oil and is highly
       mobile (Oldenburg et al., 2002a;
       GEO-SEQ Project Team, 2004;
       Wilson, 2004). Supercritical CO2 is
       less dense and more buoyant than
       oil and 30% to 50% more buoyant
       than saltwater (Benson et al., 2002;
       GEO-SEQ Project Team, 2004;
       Wilson, 2004). As a result, injected
       supercritical CO2 will rise to the
       top of depleted oil or deep saline
                                             10000
      1000
  I
  !
  e
  CL
       100-
        10-
               solid
supercritical
   fluid
76
          200       250      300      350
                       temperature (K)

Figure A. 1. Phase Diagram for CO2
            400

-------
       ^^^^^^^^^^^^H
       formations. In contrast, supercritical CO2 is denser than natural gas, and will sink when injected
       in depleted gas reservoirs. As temperature increases, the density of supercritical CO2 decreases at a
       given pressure, but increases with increasing pressure at a given temperature. Similarly, the viscosity
       of supercritical CO2 decreases with increasing temperature at a given pressure, but increases with
       increasing pressure at a given temperature. As temperature and pressure increase with  subsurface
       depth, the density and viscosity of supercritical CO2 injected will be determined by the specific
       pressure and temperature conditions (Haszeldine, 2006). Finally, when CO2 dissolves, it shares the
       physical properties of the substrate  (e.g., saltwater) and no longer behaves independently (IPCC,
       2005).
77

-------
       APPENDIX B
        Comparison of Attributes: VEF, IPIECA, and IPCC

        The VEF geologic attributes are similar to characteristics identified in GS summaries by IPCC
        (2006) and IPIECA (2007) as shown in the table below. However, there are two discrepancies
        between the IPCC and IPIECA summaries and the VEE First, in the VEF, the seismic and volcanic
        risks in a proposed project area must be explicitly identified and considered. This potential pathway
        is not explicitly addressed in the IPCC and IPIECA guidelines, although their recommended
        consideration of faults and fractures would presumably include consideration of seismically active
        areas.  Second, the IPCC recommendations explicitly request consideration of potential methane
        displacement from GS projects. In the VEF this is not explicitly addressed by a specific attribute.
        Rather, the VEF addresses this potential release as part of the CO2 and Pressure Areas of Evaluation
        processes consideration of substances that could be released.
        Table B.I. Comparison of Attributes/Pathways Identified as Being Likely to Affect
        Unanticipated Migration or Leakage from GS Projects
        VEF geologic attributes    IPIECA storage site attributes"   IPCC potential emissions pathways and sources'1
        Lateral extent (of the
        confining system)

        Capillary entry pressure


        Permeability
Effective seal provided by
overlying caprock.
                                                    Local absence of cap rock.

                                                    Through the pore system in low permeability cap
                                                    rocks if the capillary entry pressure is exceeded or if
                                                    the CO2 is in a solution.
                       Permeable distribution is suitable  Through the pore system in low permeability cap
                       for both injection and post-      rocks if the capillary entry pressure is exceeded or if
                       injection CO2 migration.        the CO2 is in a solution.
   .r .  .           ..    New and existing wells will not                                r            r
Artificial penetrations (i.e.,              .           r .     Operational or abandoned wells or future mining of
  ,,       ,             compromise the integrity of tfie     *
wells, mines)                                          '  ' ' ---""<'
        Geomechanical processes
        or properties


        Faults/fracture zones

        Earthquakes, volcanoes,
        hot spots

        Physical capacity (of the
        injection zone)
      ...                  CO2 reservoir.
(caprock) seal.
Trapping mechanisms (capillary,
solubility, and mineralization) are  CO2/water/rock reactions degrade cap rock.
effective.
Likelihood of contacted faults
reactivating and potential for      Natural or induced faults and/or fractures.
existing fractures to re-open.
Sufficient storage capacity.
                                                    Reservoir overfilling results in release from a spill
                                                    point.
        a. Source: Bulleted list in on page 23 of IPIECA (2007).
        b. Source: Table 5.3 in IPCC (2006).
78

-------