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 -55°C (-67°F). 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 31.3°C and pressures greater than 73.9 bar
(IPCC, 2005).
Chemical Properties of CO2
Carbon dioxide is soluble in water, forming carbonate ions and acidity
[CO2 (g) +H2O => H2CO3 (aqueous) => 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 11°C (IPCC, 2005).
Characteristics of
Supercritical CO2
Supercritical CO2 is approximately
one order of magnitude less viscous
than water and oil and is highly
mobile (Oldenburg et al., 2002a;
GEO-SEQ Project Team, 2004;
Wilson, 2004). Supercritical CO2 is
less dense and more buoyant than
oil and 30% to 50% more buoyant
than saltwater (Benson et al., 2002;
GEO-SEQ Project Team, 2004;
Wilson, 2004). As a result, injected
supercritical CO2 will rise to the
top of depleted oil or deep saline
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
------- |