### This comment letter response contains two parts:
(1) The original submittal from the commenter. (2) A data base printout of EPA’s response to each point raised by the commenter.

Notes about the response letters:

• The response letters were generated by a data base used to track the content of all public comment submittals and ensure that the Agency responds to each. Each comment letter

was assigned a Letter ID that corresponds to the last four digits of the docket number of the original comment (as posted on www.regulations.gov).

• Each response letter includes: comment text; an EPA

‐ assigned comment code; and an individual response. These are described and explained below.

• The data base contains Comment text from the letters submitted by commenters. Many hard copy letters were scanned into electronic format for inclusion in the data base. As a result, there may be some typographical errors attributable to optical character recognition issues. However, all original submittals were reviewed in the process of drafting responses to ensure that the comment content was understood and appropriately addressed.

• The content of each comment letter was divided into topic areas and assigned a unique Comment ID. Additionally, one or more Comment Codes (i.e., unique topic area ID) was assigned to each discrete point raised by a commenter. This information was added to the data base. To inform final rulemaking decisions, all comments and information received on a given topic were reviewed by technical experts in each rule area (e.g., on construction requirements).

• Following decisions on the final rule requirements, Individual responses were drafted by topic area to ensure completeness and consistency of the responses with the final rule and other responses to similar comments. Each discrete point made by a commenter was given a unique response.

• Upon completion of comment responses, all information was entered into the data base. Each letter response is a compilation of each comment and its individual response, compiled in the order of the original comment letter. As a result of this process, there is repetition in the response text. The Agency believes this approach ensures completeness and attention to each comment letter that was submitted.

• All comments in a topic area were reviewed, evaluated, and responded to concurrently. Responses on each topic area are compiled and are also available in the rulemaking docket.

## Docket ID No

. EPA-HQ-OW-2008-0390

NO 

# V 

# " U ZOOU

Comments on EPA proposed rule for 40 CFR Parts 144 and 146, Federal Requirements under Underground Injection Control (UIC), Program for Carbon Dioxide (C02) Geologic

Sequestration (GS) Wells

Comments Submitted by :
Christopher Murray, PhD, LG', Geostatistics & Geology Mark D

. White, PE, PhD, Computational Modeling Signe Wurstner, LG, LHG2, Hydrogeologist Mark Rockhold, PhD, Hydrogeologist Charlotte Sullivan, PhD, Geology & Geophysics Pete McGrail, PhD, Geochemistry Frank Spane, PhD, LG, LHG, Hydrologist Diana Bacon, PhD, LG, LHG, Hydrologist Ellyn Murphy3, PhD, Hydrologist



### Pacific Northwest National Laboratory

November 14, 2008 

### Nov 19 2oo$,

1 LG is Licensed Geologist in the state of Washington

. 2 LHG is Licensed Hydrogeologist in the state of Washington

. 3 To whom questions and correspondence should be directed : ellyn .murphy(j~pnl .gov ; (509) 375-5914 ; Pacific Northwest National Laboratory, PO Box 999, MSIN K9-02, Richland, WA, 99352p

. 2

Docket ID No

. EPA-HQ-OW-2008-0390


### Summary of Recommended Changes to EPA proposed rule

for 40 CFR Parts 144 and 146

This response provides recommended changes to the EPA proposed rule on underground injection control on C02 for geologic sequestration (GS)

. Staff scientists at Pacific Northwest National Laboratory (PNNL) have extensive experience in research, development and deployment of GS

. A dozen GS sites throughout the U .S

. have been modeled by our staff through projects supported by the Department of Energy and industrial partnerships

. This modeling supports site selection and permitting activities

. We have also been involved in comprehensive GS projects, from conceptualization to site selection to design to deployment, including long-term monitoring

. Based on this experience, we are well positioned to offer constructive comments for the proposed EPA rules for permitting GS sites

.
Our recommendations are summarized below

. Detailed comments to support each of the recommendations directly followthe summary

. Multiple Confining Layers Could be Required at the Discretion of the Director

. . . . . . . . . Page 3 Subsection (b) of § 146 .83 of the proposed regulations, which grants the Director discretion to require the presence of multiple confining layers, does not promote sound policy and should be removed from the proposed regulation

.
Area of Monitoring and Area of Review

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 4 EPA should define two regulatory regions surrounding the geologic sequestration project and provide specific definitions for those regions : 1) Area of Monitoring (AoM) and 2) Area of Review (AoR) ; where the AoM is used to define the spatial distribution of monitoring wells and the AoR is used to define the limit of corrective actions

Area of Review

Reevaluation

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 6 Comparisons of monitoring data with model results should be made and reported annually, especially in the initial years of operation, to ensure that the site is behaving as expected or predicted

. Using Multiphase Computational Models to Delineate the AoR

. . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 7 We support the use of sophisticated, multiphase, computational models designed for site-specific conditions for AoR delineation, as specified by EPA in the proposed regulations

. Uncertainty Associated with Modeling COZ Movement and Reservoir Pressure

. . . . . . . Page 8 A formal process for uncertainty assessment of the C02 plume footprint should be incorporated in the Proposed Rule

. This uncertainty assessment could be based on a bounding approach, a sensitivity analysis, Monte Carlo simulation or other method, provided the range of probable circumstances are considered

.
Validation of Multiphase Computational Models

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 10 EPA should establish a suite of test problems with specified parameters that could be used by the applicant to demonstrate the suitability of the computational code to be used for analysis of the proposed site

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Stratigraphic Restriction of Saline and Other Reservoirs to Below the Lowermost USDW

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Page 12 GS should be permitted in the following : 1) reservoirs containing >I 0, 000 mg/L TDS, 2) where the formation water is <10,000 mg/L but nonpotable for other reasons, or 3) formations exempted as USDWs by the state

. We do not support the proposed restriction of GS to stratigraphic zones belowthe lowermost USDW

. Proposed Mechanical Integrity Testing Requirement

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 15 External MIT should be performed every 5 years, as is now typically done for other classes of UIC wells, using geophysical wire-line logging surveys

.p

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### Multiple Confining Layers Could be Required at the Discretion of the Director

Proposed Regulation

p

. 43536, c

. 3 The proposed rules for Class VI wells identified by EPA include a requirement for a confining zone with suitable properties to contain the injected C02 (§ 146 .83(a))

. In addition, under subsection (b) of the proposed regulation, owners or operators may be required to identify and characterize multiple confining layers, at the discretion of the Director (§ 146

.83(b))

.
Discussion The rationale for inclusion of an optional requirement for multiple confining layers is not provided

. There does not appear to be any clear need for that additional requirement, which go beyond what is currently required for Class I UIC wells that inject hazardous wastes

. The proposed regulation could lead to significant problems in the future, but more importantly, this requirement is not needed

. If subsection (b) of § 146 .83 were removed, and if the Director or his staff find that the confining system identified by an operator does not appear to be reliable, they
can reject the application and/or require that the operator provide information on additional confining layers

. The description of the requirements for a confining system given in part (2) of subsection (a) would cover this situation, and would allow protection of USDW's and the public without creating the potential for confusion that would arise with the option outlined in subsection (b)

.
Previous experience and research have demonstrated that a single well-characterized confining system can provide more than adequate protection by preventing the upward migration of CO2

. For example, time-lapse seismic imaging at the Sleipner Project demonstrates "the caprock is an effective seal that prevents COZ migration out of the storage formation" (IPCC 2005)

. Examination of recent surveys of the safety of underground COZ sequestration found no suggestion that multiple confining systems are needed or should be required (e .g ., IPCC 2005 ; Benson and Surles 2006)

. In addition, the technical support document provided by EPA for the proposed rule changes (Vulnerability Evaluation Framework for Geologic Sequestration of Carbon Dioxide, EPA430-R-08-009) does not provide any support for requirement of multiple confining layers

.
The proposed regulations do not specify or in any way limit the conditions under which multiple confining layers might be required

. Thus, there is no guidance for imposition of the rule, and no limitations on the discretion of the Director

. We believe including this additional discretionary requirement as an option in the regulations is troublesome because there then is a good possibility that it could become the default requirement

. If that were the case, the requirement of multiple confining layers would lead to potential disqualification of excellent storage sites, and additional costs for characterization

.
An additional problemwith the optional requirement of multiple confining layers is one of perception

. We know

of no precedent within current UIC regulations, other applicable U .S

. environmental law, or internationally, for the requirement of multiple confining layers to contain injected fluids

. Given the lack of precedent, the inclusion of the requirement for multiple confining layers for COZ in the proposed regulations for Class VI wells could lead to the incorrect
3p

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perception by the public that injection and storage of COZ is more dangerous to the public than subsurface injection of hazardous wastes

. This perception of high risk, which does not appear to be justified by previous studies of the risks of subsurface COZ sequestration (e .g ., IPCC 2005 ; Benson and Surles 2006) could lead to decreased use of carbon sequestration as an option to limit global climate change, and therefore to decreased potential for full use of U .S

. energy resources

.
PNNL Recommendation For the reasons outlined above, we believe that subsection (b) of § 146 .83 of the proposed regulations, which grants the Director discretion to require the presence of multiple confining layers, does not promote sound policy and should be removed from the proposed regulation

. If EPA chooses to retain the option described in subsection (b), then we feel that the discretion of the Director should be limited, and there should be specification of the types of conditions under which multiple confining layers might be required

.
References Benson SM and T Surles

. 2006

. Carbon Dioxide Capture and Storage : An Overview with Emphasis on Capture and Storage in Deep Geological Formations, The Proceedings Special Issue, Institute of Electrical and Electronics Engineers (IEEE), Vol

. 94, No 10, October 2006, DOI 10 .1109/PROC .2006 .883716

.
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

.



### Area of Monitoring (AoM) and Area of Review (AoR)

Proposed Regulation p

. 43493, c

. 2 Area of Review (AoR) : The region surrounding the geologic sequestration project that may be impacted by the injection activity

. The area of review is based on computational modeling that accounts for the physical and chemical properties of all phases of the injected carbon dioxide stream

.
The proposed regulation uses the Area of Review (AoR) for defining both corrective actions and monitoring requirements :

p

. 43499, c

. 1 Area of Review (AoR) and Corrective Action : The Agency requires examination of both the vertical and horizontal extent of the area that will potentially be influenced by injection and storage activities and identification of all artificial penetrations in the
area that may act as conduits for fluid movement into USDWs (e .g ., active and abandoned wells) and, as needed, perform corrective action to these open wells (i .e ., artificial penetrations)

.p

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p

. 43539, c

. 3 (2) The monitoring frequency and spatial distribution of monitoring wells must be based on baseline geochemical data that has been collected under §146 .82(a)(6) and any modeling results in the area of review evaluation required by § 146 .84(b) ;

We agree with the concept of defining bounding areas for corrective actions and monitoring activities, but the proposed regulation uses a single area, Area of Review (AoR), for both and fails to address how

to quantify this area

. The proposed regulation defines the Area of Review (AoR) as being the region that may be impacted by the injection activity, without defining or quantifying impact

. To be useful the bounding areas for corrective actions and monitoring activities must have specific definitions

. Under deep geologic sequestration conditions the injected fluid (i .e ., supercritical C02) has low compressibility, the displaced fluid (i .e ., brine) has low compressibility, and the reservoir formation (e .g ., sandstone) generally has lowcompressibility, all of which suggest that the impact (i .e ., pressure change) of the injection activity could be detected at great distances fromthe injection well(s)

. Injection of COZ into deep saline formations is driven by differences between the well injection pressure and formation pressure

. To avoid inducing fractures or opening existing fractures in the formation and caprock, well injection pressure will be limited to some fraction below the fracturing pressure

. As COz migrates into the formation the formation pressure increases around the injection well

. The pressure distribution in the formation around the injection well depends on the formation properties, structure, and injection rate

. Injection formations will be required to be confined vertically and/or horizontally to prevent the vertical migration of injected fluids and saline reservoir fluids into USDWs

.
Confined formations (e .g ., a caprock overlying a horizontally extensive formation) allow the formation fluid to be displaced laterally by the injected fluid

. Injecting COz results in elevated pressures both inside and beyond the C02 plume

. Elevated pressures within the COZ plume have the potential to force COZ and formation fluid through permeable wells, fractures, or faults that penetrate the formation

. Elevated pressures outside the COZ plume have the potential to force formation fluid through permeable wells, fractures or faults that penetrate the formation

. The higher the formation pressure is above the ambient formation pressure, the greater the potential for COZ and formation fluid to leak through permeable wells, fractures, and faults

. The lateral pressure distribution is a function of the injection rate and the lateral distribution of formation permeability

. When C02 injection is halted, elevated pressures decay over time as the injected and formation fluids migrate away fromthe injection well(s)

. The proposed regulations define an AoR and Corrective Action as being an area within which artificial penetrations may act as conduits for fluid movement into USDWs (e .g ., active and abandoned wells) and, as needed, require the applicant to perform corrective action to seal these open wells

. The potential for fluid movement into USDWs from the injection of COZ into the injection formation is correlated with the area of elevated formation pressure, which is usually larger than the area that contains the injected COZ mass

. For confined formations, this area is defined by the lateral pressure distribution around the injection well(s)

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PNNL Recommendation EPA should define two regulatory regions surrounding the geologic sequestration project and provide specific definitions for those regions ; 1) Area of Monitoring (AoM) and 2) Area of Review ; where the AoM

is used to define the spatial distribution of monitoring wells and the
AoR is used to define the limit of corrective actions :

Area of Monitoring (AoM) : The region surrounding the geologic sequestration project that contains 95% of the carbon dioxide mass

. The area of monitoring is based on monitoring data and computational modeling that accounts for the physical and chemical properties of all phases of the injected carbon dioxide stream

. In the absence of monitoring data, the area of monitoring is based on computational modeling

.

Area of Review (AoR) : The region surrounding the geologic sequestration reservoir that has or is indicated to develop pressures that are greater than 10% different from the original hydrostatic formation fluid pressure, but no less than the Area of Monitoring

. The area of review is based on monitoring data and computational modeling that accounts for the physical and chemical properties of all phases of the injected carbon dioxide stream

. In the absence of monitoring data, the

area of review

is based on computational modeling

.

The proposed definitions of AoM and AoR have several important components : " The AoM

has a mass-based definition

. " The AoR has a pressure-differential-based definition

. " The AoR is never smaller than the AoM

. " The AoR and AoM may be asymmetrical around the injection well(s)

. " The AoR and AoM may vary over time

. " The carbon dioxide mass should be determined using a combination of monitoring data and computational modeling

. " Pressure differentials are based on equilibrium hydrostatic conditions

.



### Area of ReviewReevaluation

Proposed Regulation

p

. 43507, c

. 3 When monitoring data differ significantly from modeled predictions or when there

ar e appreciable operational changes (e .g ., rates) reevaluation may be mandated prior to the minimum fixed frequency

. At no time would area of review reevaluations occur less frequently than every 10 years

.

Discussion The conclusion reached by EPA that the AoR should be reevaluated over time based on comparisons of incoming monitoring and site characterization data with model predictions is very important

. However, the definition of "significantly" needs to be clarified in the regulation,
otherwise interpretation will be a constant point of confusion

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Based on PNNL's experience in modeling GS projects, comparison of monitoring data with model prediction, especially in the initial years of operation, would be extremely valuable in assessing our understanding of the reservoir behavior

.
Models of COZ injection into deep formations are sensitive to estimates of permeability in the formation

. Permeability can vary over orders of magnitude in a formation, and this can have a large impact on estimates of pressure response, and hence the AoR

. As the size of the injected C02 plume grows with time, it will encounter areas of the formation that may have different permeability than was measured at the injection well, and the AoR may vary over time

.

PNNL Recommendation We recommend that comparisons of monitoring data with model results be made and reported annually, to ensure that the site is behaving as expected or predicted

. If at any time the monitoring data or comparisons of observed and predicted pressure response suggest anomalous behavior, this could suggest a need for a reevaluation of the AoR (and AoM ; see previous comments), at the discretion of the Director

.



### Using Multiphase Computational Models to Delineate the AoR
Proposed Regulation

p

. 43506, c

. 1 Current UIC regulations require that the owner or operator define the Area of Review (AoR), within which the owner or operator must identify all penetrations (regardless of property ownership) in the confining zone and the injection zone and determine whether they have been properly completed or plugged

.
p

. 43506, c

. 2 EPA believes that predicting the complex multi-phase buoyant flow of the CO2, coinjectates, and compounds that may be mobilized due to injection requires the sophistication of multiphase computational models

. EPA proposes that the owners or operators of GS wells delineate the AoR for C02 GS sites using computational fluid flow models designed for the specific site conditions and injection regime

. Discussion The use of multiphase computational models for prediction of plume areas has been supported by several groups, including the Intergovernmental Panel on Climate Change (IPCC, (2005), which suggested the use of modeling for evaluation of the long-term distribution of COZ in the subsurface

. This modeling could be used as the basis for identification of the AoR for a given site

. The IPCC report makes clear that the models used for these efforts should be multiphase models that incorporate the important physical, chemical, and geomechanical processes present at a given site (IPCC 2005)

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Whereas a prescribed radius or simple mathematical computation, as included in the current UIC regulations, may be adequate for some UIC sites, research and experience in modeling COZ sequestration leads us to conclude that these simplified approaches are clearly not sufficient to predict the extent of an injected COZ plume

. EPA should be commended for requiring the use of multiphase computational models for delineating the AoR

. We know

that initial models of the Sleipner site suggested relatively simple and predictable migration of C02

. However, time lapse seismic monitoring of the injected plume revealed the importance of an internal shale formation within the injection reservoir that has significantly altered the direction and extent of COZ migration from those early predictions (Chadwick et al

. 2004)

. This finding emphasizes the importance of computational fluid flow modeling capable of handling complex internal heterogeneity for prediction of C02 plume geometries, which are unlikely to be cylindrical or closely predicted using the approaches that have been previously used in UIC applications

.

PNNL Recommendation We support the use of sophisticated, multiphase, computational models designed for site-specific conditions for AoR delineation, as specified by EPA in the proposed regulations

.

References R .A

. Chadwick, P

. Zweigel, U

. Gregersen, G

.A

. Kirby, S

. Holloway and P.N

. Johannessen

. 2004

. Geological reservoir characterization of a COZ storage site : the Utsira Sand, Sleipner, Northern North Sea, Energy 29 :1371-1381

.
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

.



### Uncertainty Associated with Modeling C02 Movement and Reservoir Pressure

Proposed Regulation

p

. 43506, c

. 3 In suggesting the use of computational flow models for delineating the AoR of GS sites, EPA notes their potential "to predict COZ movement in response to varying conditions or scenarios, such as changing injection rates, or the presence or absence of fractures or faults in confining layers ." However, the Proposed Rule does not explicitly discuss uncertainty assessment in the use of modeling for AoR delineation

.

Discussion Although we agree with the EPA that computational multiphase fluid flow models are appropriate for delineating the AoR of GS sites, the Proposed Rule does not adequately address how uncertainty should be managed in regards to modeling GS sites, and we consider this to be an important aspect

. By not explicitly addressing uncertainty, the Proposed Rule infers that a single deterministic model result would be appropriate for determining the AoR and/or for site review and reevaluation

. This ignores the fact that there is always a certain level of uncertainty in model

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results due to the cumulative variability in input data and uncertainty in model parameters

. The carbon sequestration roadmap produced by the National Energy Technology Laboratory (DOE 2007) notes that one of the goals of field tests conducted during the Validation Phase is to "Validate and refine current C02 reservoir models for various geologic sequestration sites ." Validation does not prove that a given model is correct, but it does provide additional constraints on the model parameters

. However, in the case of saline formation storage, history matching is generally not feasible for constraining uncertainties, due to a lack of underground data for comparison (IPCC 2005)

. Therefore the IPCC (2005) recommended that systematic parameter variation routines and statistical functions should be included in future coupled simulators to allow uncertainty estimates for numerical reservoir simulation results

. Uncertainty analysis is a valuable analytical tool and is critical for providing confidence in model results

. Several approaches can be taken to capture the uncertainty present in any site modeling exercise (e .g ., Neuman and Wierenga 2003)

. Three commonly used alternatives are Monte Carlo simulation, the bounding approach, and sensitivity analysis

. The type of uncertainty analysis appropriate to a particular problem depends on the complexity of the problem and the available data

.
Probabilistic approaches like those suggested by the IPCC (2005) and Neuman and Wierenga (2003) would provide useful assessments of the uncertainty associated with the predicted COZ plume footprint, and could be used to evaluate the risk associated with various choices of the AoR

.
PNNL Recommendation A formal process for uncertainty assessment of the C02 plume footprint should be incorporated in the Proposed Rule

. This uncertainty assessment could be based on a bounding approach, a sensitivity analysis, Monte Carlo simulation or other method, provided the range of probable circumstances are considered

. This would allowfor a better assessment of the AoR that addresses the inherent uncertainty in computational fluid flow modeling of the injection process

. References 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

. Neuman, SP, and PJ Wierenga

. 2003, A Comprehensive Strategy of Hydrogeologic Modeling and Uncertainty Analysis for Nuclear Facilities and Sites

. Technical Rpt

. NUREG/CR-6805, U

. S

. Nuclear Regulatory Commission, Washington DC

. U .S

. DOE

. 2007

. Carbon Sequestration Technology and Program Plan Roadmap

. National Energy Technology Laboratory

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### Validation of Multiphase Computational Models

Proposed Regulation

p

. 43507, c

. 1 Models used for GS sites should be based on accepted science and should be validated

. In some cases, owners or operators may choose to use proprietary models (i .e ., not available for free to the general public)

. EPA is aware that the use of proprietary codes may prevent full evaluation of model results (e .g ., NRC, 2007)

. Several popular codes in the petroleum-reservoir engineering discipline are proprietary and owners

or operators of particular sites may prefer to use these codes as they have previous experience with them, and they have been used in peer-reviewed studies to model COZ sequestration

. When using a proprietary model, owners or operators should clearly disclose the code assumptions, relevant equations, and scientific basis

.

Discussion We agree that computational models used for GS sites should be based on accepted science and
should be validated

. However, the proposed rule does not define validation

. We believe that these models must be validated in terms of their appropriateness for simulating the complexities of COZ sequestration under the conditions present at the specific site being evaluated

. The fact that a code has been validated and previously used to model COZ sequestration (even if peerreviewed) does not ensure that it is appropriate for use at all sites and under all conditions

. For example, it is possible that, for a specific site, certain physical and/or geochemical processes must
be considered that are not available in a given computational code

. Whereas that code may have reasonably and successfully modeled COz sequestration at a dissimilar site, it may not be appropriate for the site being considered

.

It may be appropriate for EPA to establish a suite of test problems that span the breadth of GS
reservoir types (i .e ., deep saline formations, depleted oil and gas reservoirs, faulted or fractured reservoirs, unmineable coal seams, basalts, salt domes, reactive carbonates, and shales), injection
conditions (i .e ., surface climatic extremes, injectate composition, single versus multiple wells,
pressure reduction wells, and horizontal versus vertical injection wells), and physical/ geochemical processes deemed important in modeling COz sequestration (ie

., changes of state and chemical reactions)

. We suggest that this suite of problems be defined by experts in the field of COZ sequestration modeling and code development, and that a variety of computational codes used in modeling GS sites be benchmarked using this suite

. A subset of the suite of problems may be
sufficient to validate a code for use at a particular site, however this subset must be rigorously
defined (by EPA or the State, if it has established primacy for UIC regulation) based on the conditions specific to the site in question to ensure that the model being validated is appropriate for

use at the site

. Efforts are already underway to evaluate and compare computational codes being
used to model GS sites

. For example, several code intercomparison studies have been conducted
to evaluate the capabilities and accuracy of numerical simulators for geological storage of greenhouse gases (Pruess et al ., 2004 and Law et al
., 2003)

. In addition, the Inter-Partnership Simulation and Risk Working Group within the DOE Regional Partnerships is embarking on an effort to
compare overall modeling methodologies on a jointly accessible data set

. This will be more inclusive than a code intercomparison effort as the methodologies and conceptual models used
may vary widely

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Model results that should be used to determine if a code is valid for solving a specified problem include plume area and maximumplume radius (as defined by 95% of the COZ mass) at particular time planes, injected mass over the time period of simulation, pressure distribution, and COZ phase)

. Based on the code benchmarks and expert knowledge, a tolerance must be defined that determines what constitutes acceptable results

. This suite of test problems could be modified or augmented as the state-of-the-science progresses in modeling, ensuring that any computational code being used at a particular site is current with regard to accepted scientific principles and rigor

.
In addition to validating particular computational codes using a set of standard benchmarks, the implementation of the conceptual model should also be validated by comparing model results to observed data, and by using model calibration and history matching to improve the model results

. EPA understands the importance of history-matching of model simulations with monitoring as stated on p

. 43507, c

. 1

.
PNNL Recommendation EPA should establish a suite of test problems with specified parameters that could be used by the applicant to demonstrate the suitability of the computational code to be used for analysis of the proposed site

. As computational approaches and methods improve, this suite of problems should be modified and/or updated to incorporate the state-of-the-knowledge in GS modeling

. We agree that proprietary models can be used, provided they can demonstrate they produce results within the defined acceptable tolerance, in addition to disclosing the code assumptions, relevant equations, and scientific basis

. These same requirements must, of course, be met by all computational codes being used at GS sites

.
References : Law, D .H .-S

., L .G

.H

. van der Meer and W .D

. Gunter, 2003 : Comparison of numerical simulators for greenhouse gas storage in coal beds, Part II : Flue gas injection

. Proceedings of the 6th International Conference on Greenhouse Gas Control Technologies (GHGT-6), J

. Gale and Y

. Kaya (eds .), 1-4 October 2002, Kyoto, Japan, Pergamon, v .I, 563-568

. Geologischen Gesellschaft, 149,3,449-464

.
Pruess K, J Garcia, T Kovscek, C Oldenburg, J Rutqvist, C Steefel, and T Xu

. 2002

. Intercomparison of Numerical Simulation Codes for Geologic Disposal of C02

. LBNL-51813, Lawrence Berkeley National Laboratory, Berkeley, California

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### Stratigraphic Restriction of Saline and Other Reservoirs

to Below

Lowermost USDW

Proposed Regulation for saline formations p

. 43494, c

. 2 Saline formations : Deep and geographically extensive sedimentary rock layers saturated with waters or brines that have a high total dissolved solids (TDS) content (i .e ., >10,000 mg/L TDS)

. p

. 43505, c

. 1 1

. Proposed Geologic Siting Requirements Existing UIC requirements for siting injection wells include identification of geologic formations suitable to receive the injected fluids and confine them such that they are isolated belowthe lowermost USDWs, minimizing the potential for endangerment

. p

. 43502-43503, section H on target geologic formations As with unmineable coal seams, EPA seeks comment on prohibiting injection into such formations [basalts, salt domes, and shales] if they are above the lowermost USDW

.
The EPA is soliciting comments on these issues : l

. Whether C02 injection should be allowed into an injection zone (saline sedimentary formations, unminable coal seam, basalt, salt caverns, shales, or other reservoirs) above the lowermost USDW, when the Director determines that geologic conditions exist that
will prevent fluid movement into adjacent USDWs

. 2

. Whether a designated USDW should be exempted to allow for GS, and under what conditions should these exemptions be approved

. 3

. Whether all USDWs should be exempted below a COZ injection receiving bed

. 4

. Whether the EPA should set a minimuminjection depth requirement for GS, rather than require that such injection take place below the lowermost USDW

.

Discussion Three of the most important issues here are (a) the physical location of saline formations or other types of geologic reservoirs for GS, relative to USDWs ; (b) under what circumstances the EPA
should remove one or more water-bearing zones from the list of USDWs to allow injection ; and (c) under what other conditions should the EPA allow injection above a USDW

.

In addition to injection into saline formations, EPA recognizes that basalt, unmineable coal

seams, salt domes, abandoned mines, black shales and other types of reservoirs may be promising, emerging targets

. Basalts, in particular appear to have considerable potential for GS

. Research by PNNL and others indicates that extruded basalt flow tops and flow bases may have regional, multi-gigaton capacity for COZ storage

. In addition, laboratory experiments suggest that
mineralization of large volumes of COZ may proceed within a few years of injection (McGrail et al, 2006)

.

In the proposed rule, the new Class VI wells would be allowed to inject C02 into basalts, deep
saline reservoirs, and other potential reservoirs only when they are beneath the lowermost
USDW

.

12p

. 14

Docket ID No

. EPA-HQ-OW-2008-0390

A USDW

is defined by the EPA as "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 ." This definition would include many non-potable aquifers that would be candidates for GS

.
The EPA's reliance on the standard definition of a USDW as being solely <10,000 mg/L TDS is far too restrictive

. In many states, this standard is not applied - all groundwaters are protected regardless of water quality

. In at least one state GS operations would be permitted if removing anthropogenic COZ is considered a greater public benefit than the loss of a marginal aquifer

.
The EPA is aware that restricting GS to a position below all EPA defined USDWs would severely limit GS in many areas

. Under the current proposed rule, directors may grant injection into deep, marginal USDWs that directly overlie crystalline basement rock and/or unmineable coal
seams

. In addition to these exceptions, the EPA would consider allowing operation of GS wells where receiving formations are above the lowermost USDW only where there are thousands of feet between the injection zone and the overlying and underlying USDWs

.
As currently defined by the EPA, "aquifer exemptions" permanently remove a specified aquifer from protection as a USDW

. Aquifer exemptions are currently granted for mineral or hydrocarbon exploration by Class III solution mining wells, or by Class 11 oil and gas related wells, when there is no reasonable expectation that the formation will be used as a drinking water supply

. It is important to note that the EPA has not previously considered the purpose of creating additional capacity for the emplacement of fluids as sufficient cause for granting an exemption

. Under the
new rule, it would be possible for operators to apply to change the designation of some aquifers being used for Class II injection

. These aquifers would need a new, separate exemption for GS

.
The EPA suggests the following possible alternatives that would allow injection between USDWs or above the lowest USDW :

1

. Injection approval for GS between USDWs formations is at the Director's discretion in circumstances in which it may be demonstrated that USDWs would not be endangered

. 2

. Directors may be allowed to exempt, at their discretion, all USDWs below a GS injection
zone

. (This would permanently remove these aquifers from protection as USDWs)

.
PNNL Recommendation GS should be permitted in the following : 1) reservoirs containing >10,000 mg/1L TDS, 2) where the formation water is <10,000 mg/L but is non-potable for other reasons, or 3) formations exempted as USDWs by the state

. We do not support the proposed restriction of GS to stratigraphic zones below the lowermost USDW

.
" Prohibition of C02 injection into saline formations above USDWs is overly restrictive and would severely limit GS in states such as Wyoming, where deep sedimentary formations

13p

. 15

Docket ID No

. EPA-HQ-OW-2008-0390

0

0

0

0 in much of the state have TDS lower than 10,OOOmg/1 (NETL)

. Thus almost any formation considered for GS would likely overlie a low TDS formation

. GS should be allowed into an injection zone above the lowermost USDW (whether saline or other formation) when the Director determines that geologic conditions exist that will prevent fluid movement into nearby USDWs

. Conditions that will prevent downward movement of fluids include the presence of intact low permeability beds between the receiving bed and the underlying USDW

. In consideration of the buoyant behavior of injected C02, the requirement of thousands of feet of intervening formations may be less important than the demonstration of the sealing capacity of the intervening layers

. We believe that "thousands of feet" is a vague requirement, when the more rigorous test is the demonstration of the sealing capacity of the intervening layers

. The draft rule is far too restrictive and is not based on technical risk factors

. Risk of COZ contamination to a USDW is lower for aquifers that underlie an injection reservoir due to the inherent buoyancy of the CO2

. Vertical migration can be interrupted completely by the presence of one or more low permeability layers

. Hence, a prohibition on injecting above the shallowest USDW is not based on real technical risk of a GS project

. The conditions for exemptions of aquifers should be clarified

. Aquifer exemptions should be allowed for GS ("a receiving bed for Class VI wells") when it can be demonstrated that there is no reasonable expectation that the formation will be considered for a drinking water source, and when it can be demonstrated that COZ is highly unlikely to migrate updip into a portion of the formation that may be considered for use as a USDW

. Blanket aquifer exemption of all USDWs below a GS zone is not needed if there is demonstration of a competent seal between the injection zone and the first underlying USDW

. No blanket prohibition should be introduced for any potential non-saline reservoir ; promising GS targets of basalt, unmineable coal seams, etc
., should be evaluated with methodologies similar to those for saline formations

. GS is better served by neither mandating a set minimum depth for injection nor by requiring injection take place below the lowermost USDW

. Minimum depth requirements would be unnecessarily proscriptive

. Shallow injections could be done for very small commercial producers or in cases where reservoir engineering practices (such as pre-dissolution of the COZ into formation water) are done so that no free COZ phase exists

.
References McGrail, B

. P
., H

. T

. Schaef, A

. M

. Ho, Y

. Chien, J

. J

. Dooley, and C

. L

. Davidson

. 2006

. Potential for Carbon Dioxide Sequestration in Flood Basalts, J

. Geophys

. Res

. 111, B 12201

.

NETL

. 2007

. National Carbon Sequestration Atlas

. http ://www

.netl .doe .gov/technologies/carbon seq/refshelf/atlas/index .htmlp

. 16

Docket ID No

. EPA-HQ-OW-2008-0390



### Proposed Mechanical Integrity Testing Requirement

Proposed Regulation

p

. 43513, c

. 3

. . .require owners and operators of Class VI GS project to monitor internal mechanical integrity of their injection wells by continuously monitoring injection pressure, flow rate, and injected volumes

. . .
p

. 43514, c

. 1

. . .require owners and operators of C02 wells to demonstrate injection well external mechanical integrity at least once annually

.
The EPA seeks comment on the appropriate frequency of internal and external MITs for GS injection wells

.
Discussion During the injection period the proposed rule states that internal MIT should be performed continuously by monitoring pressures (e .g ., injection and annular zones), injection rates and volumes, etc
., and external MIT should be performed annually

. Continuous monitoring of injection rates, volumes, and pressures during the injection phase seems reasonable and appropriate

. The semiannual reporting frequency of such results also seems reasonable

.
The proposed rule also states that external MIT be performed annually during the injection phase

. The rationale for this is to protect USDWs given the potential corrosive effects on injection well components (steel casing and cement) that are exposed to the GS stream and the buoyant nature of the injected fluid

.
To execute an annual external MIT, the injection well would be required to be shut down (i .e ., cease C02 injection operation) and the internal COZ injection delivery tubing systemto be physically removed fromthe injection well

. At a minimum, this would require that all GS projects, no matter how

large, to have a backup injection well or multiple injection wells to be available so as to not adversely impact or interrupt daily plant operations

.
In general, we believe that annual external MIT evaluations would be excessive and overly disruptive for normal plant operations

. This is because quantitative external MIT evaluation tests would require :

Removal of the surface wellhead and internal C02 injection delivery tubing system Performing detailed wireline geophysical surveys for cement-seal and well casing integrity (e .g ., cement-bond log), and If warranted by the results of the wireline geophysical surveys, conducting direct leakage-detection tests with installed internal test equipment (e .g ., downhole internal straddle-packer system) of suspected areas of concern

15p

. 17

Docket ID No

. EPA-HQ-OW-2008-0390

The performance of the various aforementioned elements of an external MIT and re-installation of components of the injection well system are likely to take between 2 to 5 days, depending on the characteristics of the injection well (e .g ., injection reservoir depth), complexity of the internal COZ injection well system, and whether quantitative leakage detection tests are warranted

. Because of extended time required to conduct an external MIT program, the performance of fixed-annual external MIT evaluations (outside the schedule of planned plant operation maintenance shut-downs) are believed to be excessively disruptive

.
PNNL Recommendation We recommend that an external MIT be performed on a recurrence interval of every 5 years, as is
now typically done for other classes of UIC wells

. The external MIT should rely primarily on results obtained from geophysical wire-line logging surveys, and (if warranted by wireline geophysical survey results), conducting direct leakage-detection tests of suspected areas of
concern

. If the external MIT indicates the potential for a compromise to the cement seal characteristics or well casing integrity, then additional, more quantitative tests (e .g

. Gasda et al

. 2008) and subsequent corrective actions may be warranted

.
The recommended external MIT recurrence assessment of every 5 years is predicated on no indications of injection system compromise that would be observed by changes in monitored internal MIT parameters during injection (e .g ., injection or annular zone pressure), or pressure fall-off test response following normal, temporary operational shut-down periods

. If at any time the monitoring parameters or pressure data responses indicate anomalous injection test system behavior, then this could indicate the need for an external MIT evaluation to be performed, at the discretion of the Director

.

Reference Gasda, S

. E
., J

. M

. Nordbotten, and M

. A

. Celia

. 2008

. Determining effective wellbore permeability from a field pressure test : a numerical analysis of detection limits

. Environmental Geology, 54(6), 1207-1215, doi :10.1007/s00254-007-0903-7

.

### Letter ID: 0207 Commenter: Christopher Murray Organization: Pacific Northwest National Laboratory

Letter ID: 0207 Comment ID: 22430 Letter Lines: 31 to 41

Comment Code: 1852 1.4.2.1 DOE Regional Carbon Seque stration Partnerships

Comment: This response provides recommended ch anges to the EPA proposed rule on underground injection control on CO2 fo r geologic sequestration (GS). Staff scientists at Pacific Northwest National Laboratory (PNNL) have extensive experience in research, development and deployment of GS. A dozen GS sites throughout the U.S. have been modeled by our staff through projects supported by the Department of Energy and industrial partnerships. This modeling supports site selection and permitting activities. We have also been involved in comprehensive GS projects, from conceptualization to si te selection to design to deployment, including long-term monitoring. Based on this experience, we are well positioned to of fer constructive comments for th e proposed EPA rules for permitting GS sites.
Individual Response: EPA acknowledges the value of the DOE pilot projects and resultant data and information generated by such projects. EPA has evaluated permits and reviewed operational data and information associated with the seve n RCSPs funded by the U.S. Department of Energy and coordinated through the National Energy Te chnology Laboratory (NETL). In addition to the information cited in the preamble to the pr oposed rule, EPA published a Notice of Data Availability and Request for Comment on August 31, 2009 (74 FR 44802), in which the Agency described the results of research at three DOEpartnership sites: the Escatawpa, Mississippi project; the Aneth Field project in Paradox Basin, Southeast Ut ah; and the Pump Canyon Site, New Mexico. The Agency will continue to track GS project developments and evaluate new and ongoing research and information to ensure USDW protection.

Letter ID: 0207 Comment ID: 22431 Letter Lines: 46 to 49

Comment Code: 3120 2.1.2 Need to identify additional confining and containment zones (seeking comment)

Comment: Multiple Confining Layers Could be Required at the Discretion of the Director.....Page 3 Subsection (b) of § 146.83 of the proposed regulations, which grants the Director discretion to requi re the presence of multiple confining layers, does not promote sound policy and should be re moved from the proposed regulation.
Individual Response: EPA believes that the Class VI requir ements, including the level of site characterization required under §§146.82 and 146.83, appropriately address the unique challenges posed by large-scale GS including large injection volumes, the relative buoyancy of

1

### CO2, its corrosivity in the presence of water, th e potential presence of impurities in captured CO2, and its mobility within subsurface geologic formations.
EPA retains the requirement to identify additi onal containment/confining zones, if required by the Director, as proposed (§146.83(b)). EPA acknowle dges that such additional zones may not be needed in all circumstances, and clarifies that this requirement is at the Director's discretion, based on his or her understanding of a particular site. EPA believes that, in some situations, the presence of additional confining and containm ent zones above the primary (i.e., lowermost) confining zone can improve USDW protection. These layers could further impede vertical fluid movement and allow for pressure dissipation, an d also provide additional sites for monitoring, mitigation, and remediation. The Agency's deci sion is supported by commenters representing states, water associations, and an electric utility who assert that these requirements promote a flexible, risk-based, site-specific approach to ch aracterizing and permitting a site for GS projects, and are intended to improve understanding of the project as a whole and promote better modeling of the site.

Letter ID: 0207 Comment ID: 22432 Letter Lines: 51 to 56

Comment Code: 3200 2.2 Proposed Area of Review and Corrective Action Requirements (40 CFR 146.84)

Comment: Area of Monitoring and Area of Review . . .. . . . . .. . . . . Page 4 EPA should define two regulatory regions surrounding the geol ogic sequestration project and provide speci fic definitions for thos e regions: 1) Area of Monitoring (AoM) and 2) Area of Review (AoR); where the AoM is used to define the spatial distribution of monitoring wells and the AoR is used to define the limit of corrective actions
Individual Response: EPA believes that, while the commente r offers different terminology for the AoR and the area to be monitored, the GS rule addresses the commenter's suggestions. Notably, the owner or operator is required ( under 146.90) to develop and implement a testing and monitoring plan for each GS project that mu st consider the delineated AoR that includes both the CO2 plume and associated pressure front and incorporates many of the attributes suggested by the commenter. EPA clarifies that delineation of the AoR, as required at 146.84(c), must account for the physical and chemical properties of all phase s of the injected CO2 stream, based on available site characterization, monitoring, and operational data.
Furthermore, any changes to the AoR would necess itate a review of and po tential change in the testing and monitoring plan, consistent with the commenter's a ssertion that the AoR may vary over time. The final GS regulation includes a requirement at 146.84(e) that the AoR be reevaluated at least every 5 years, and also re quires that the owner or operator periodically review the AoR and corrective action, testing and monitoring, and emergency and remedial response plans within one year of an AoR reeval uation (as well as follo wing significant changes to the facility, or when required by the Directo r). The relationship betw een the AoR reevaluation and the testing and monitoring plan should allow fo r considerations suggested by the commenter.

2

### EPA believes that by linking develo pment and review of the testi ng and monitoring plan to the AoR evaluation and reevaluation process, the GS rule, in effect, incorporates many of the commenter's considerations for an area of monitoring.

Letter ID: 0207 Comment ID: 22433 Letter Lines: 58 to 61

Comment Code: 3530 2.2.3.1 AoR reevaluation fre quency (seeking comment)

Comment: Area of Review Reevaluation. . .. . . . . . . . . . . . . . . . Page 6 Comparisons of monitoring data with mode l results should be made and reported annually, especially in the initial years of operation, to ensure that the site is behaving as expect ed or predicted.
Individual Response: EPA agrees with commenters that reevaluations of the AoR every ten years may not be sufficient to appropriately model plume and pressure front movement, establish conditions for corrective action, and ensure prot ection of USDWs from endangerment. For these reasons, the Agency changed the minimum frequency for the AoR reevaluation at §146.84(e) from 10 years to at least every 5 years in the final GS rule. EPA belie ves that this revised frequency addresses commenters' concerns about the inherent uncertainties in modeling CO2 movement, the emerging nature of GS technology, risk-based targeting of monitoring activities, and the unique nature of CO2 GS. In addition, the Director has discre tion to require more frequent AoR reevaluation if he or she has reasonto believe, based on site-specific information, that a five-year reevaluation schedule is not sufficiently protective of USDWs.

Letter ID: 0207 Comment ID: 22434 Letter Lines: 63 to 66

Comment Code: 3310 2.2.1.1 Comments in favor of requiring computational models to delineate the AoR (seeking comment)

Comment: Using Multiphase Computational Models to Delineate the AoR. . . . . . . Page 7 We support the use of sophisticated, multiphase, computational models designed for site-specific conditions for AoR de lineation, as specified by EPA in the proposed regulations.
Individual Response: EPA acknowledges the commenter’s s upport and agrees that predicting the complex multiphase, buoyant flow of the CO 2, co-injectates, and compounds that may be mobilized due to injection requires the sophistication of computational models, as required at §146.84(c)(1). Models can predict CO2 fate and tr ansport, as well as changes in formation pressure, in three dimensions, as a function of ti me in order to delineate the subsurface storage site and the AoR. Models can account for site-specific and predicted operating conditions as well as geologic heterogeneities and potential migration through fau lts, fractures, and artificial penetrations.

3

### Letter ID: 0207 Comment ID: 22435 Letter Lines: 68 to 72

Comment Code: 3300 2.2.1 Delineating the AoR/M odeling CO2 movement

Comment: Uncertainty Associated with Modeling COZ Movement and Reservoir Pressure . . . . . . . Page 8 A formal process for uncertainty assessment of the C02 plume footprint should be incorporated in the Proposed Rule. This uncertainty assessment could be based on a bounding approach, a sensitivity analysis , Monte Carlo simulation or other method, provided the range of probabl e circumstances are considered.
Individual Response: EPA acknowledges that there will be some uncertainty involved in AoR modeling. An EPA-prepared technical backgr ound document, "Computational Modeling of Underground Injection of Carbon Dioxide for De termination of Area-of-Review and Potential Risk to Underground Sources of Drinking Water" (cited as USEPA, 2008d) was provided in the regulatory docket for the proposed GS rule. This document discusses, for example, uncertainty analysis in detail. This document was provided as a background to clarify what information EPA analyzed in order to draft the modeling require ments at 146.84(c)(1), as well as what actions EPA expects owners or operators and Directors to take in this area. For example, the document states: "The requirement for an a-priori AoR delineation based on modelin g predictions stresses the need for uncertainty and sensitivity analyses for the initial prediction. Sensitivity analysis is recommended as the principal evaluation tool fo r characterizing the most and least important sources of uncertainty in environmental models."

Letter ID: 0207 Comment ID: 22436 Letter Lines: 74 to 77

Comment Code: 3330 2.2.1.3 Applicability /adequacy of computational models (seeking comment)

Comment: Validation of Multiphase Computatio nal Models . . . . . Page 10 EPA should establish a suite of test probl ems with specified parameters that could be used by the applicant to de monstrate the suitability of the computational code to be used fo r analysis of the proposed site.
Individual Response: EPA notes that the Agency will not perform independent "tests" or validation of individual modeling products. EP A believes that operators should have the flexibility to select models with which they ar e familiar and that are most appropriate to site conditions and available data types and amounts.
EPA clarifies that an EPA-prepared techni cal background document, "Computational Modeling of Underground Injection of Carbon Dioxide for De termination of Area-of-Review and Potential Risk to Underground Sources of Drinking Water" (cited as USEPA, 2008d) was provided in the regulatory docket for the proposed GS rule. This document discusses, for example, uncertainty analysis in detail. This document was provided as a background to clarify what information EPA analyzed in order to draft the modeling require ments at §146.84(c)(1), as well as what actions

4

### EPA expects owners or operators and Directors to take in this area. For example, the document states: "The requirement for an a-priori AoR delineation based on modelin g predictions stresses the need for uncertainty and sensitivity analyses for the initial prediction. Sensitivity analysis is recommended as the principal evaluation tool fo r characterizing the most and least important sources of uncertainty in environmental models."
EPA based this background document in part on guidance drafted by the National Research Council and on EPA's Council on Regulatory Envi ronmental Modeling (CREM) document. EPA refers interested commenters to the document "Guidance on the Development, Evaluation and Application of Environmental M odels," (74 FR 14551, March 31, 2009).
EPA is developing guidance on AoR and corrective action at GS Class VI s ites to assist owners or operators and Directors in evaluating requi rements and proposed AoR and corrective action plans under §146.84. The guidance describes EPA's reco mmended approaches for the data to be considered in an initial AoR delineation, how to apply computat ional modeling to delineate the AoR, performing corrective action at GS sites, and, when appropriate, completing AoR reevaluations. The guidance will be available on EPA's web site, at http://water.epa.gov/type/groundwat er/uic/wells_se questration.cfm.

Letter ID: 0207 Comment ID: 22437 Letter Lines: 79 to 83

Comment Code: 4324 2.4.4.1.2 Comments opposed to requiring injection below the lowermost USDW

Comment: Stratigraphic Restriction of Saline and Other Reservoirs to Below the Lowermost USDW. . . .Page 12 GS should be permitted in the following: 1) reservoirs containing >10,000 mg/L TDS, 2) where the formation water is reasons, or 3) formations exempted as USDWs by the state. We do not support the proposed restriction of GS to stratigraphic zones be low the lowermost USDW.
Individual Response: EPA acknowledges that there are lim ited circumstances where injection above the lowermost USDW can be performe d in a manner that protects USDWs from endangerment, as long as appropriate protections are in place. The final GS rule at §146.95 establishes a process by which owne rs or operators can seek a waiv er of the Class VI injection depth requirements for injecti on into non-USDW formations above and/or between USDWs. The final rule also establishes limited circumstances under which expansions of the areal extent of aquifer exemptions may be granted for owne rs or operators of Class II EOR/EGR wells transitioning to Class VI injec tion for GS. Additionally, the final GS rule does not categorically preclude or prohibit in jection into any type of formation.
EPA believes that these requirements will ensure USDW protection while providing flexibility to UIC program Directors and owne rs or operators who undertak e CO2 injection for GS. The Agency believes this approach: 1) responds to co ncerns about local and re gional geologic storage

5

### capacity limitations imposed by th e proposed injection depth require ments; 2) allows for a more site-specific assessment of injection depth for GS projects; 3) accommodates injection into different formation types; 4) a llows for injection of CO2 for GS into non-USDWs above and/or between USDWs when appropriate and where it can be demonstrated that USDWs will be protected; and 5) responds to concerns about the use of aqui fer exemptions for GS. Finally, EPA’s approach to addressing injection depth variability through a waiver process responds to concerns about future drinking water resource availability and the need to ensure that high quality water remains available in sufficient quantities to supply dr inking water needs.
For additional information on the waiver approa ch and aquifer exemptions for Class VI wells, please see the final rule and pr eamble, as well as EPA's responses to comments on the Notice of Data Availability and Request for Comment.

Letter ID: 0207 Comment ID: 22438 Letter Lines: 85 to 87

Comment Code: 4546 2.5.2.2 MIT frequency (seeking comment)

Comment: Proposed Mechanical Integrity Testing Requirement . . . . . . . . Page 15 External MIT should be performed every 5 years, as is now typically done for other classes of UIC wells, using geo physical wire-line logging surveys.
Individual Response: EPA disagrees that a five-year extern al MIT is appropriate. The final GS regulation retains the requirement that owners or operators of Class VI wells must demonstrate injection well external mech anical integrity at least once annually under §146.89(c). EPA believes this is justifiable for the protecti on of USDWs from endangerment given the unique nature of CO2 GS relative to other injection operations, including the pote ntial corrosive impact of CO2 in the presence of water and impurities on well materials, the potential geomechanical impacts to well integrity due to CO2 phase ch anges, the temperature and pressure effects associated with injecting supercritical CO2, and the buoyant nature of th e injected CO2. Because of the unknown long-term effects of CO2 GS, EPA chose a more cons ervative initial approach in the early stages of deployment for GS.
In addition, a comprehensive review conducte d by EPA of MIT results from 1993 to 1998 found that Class I wells injecting corrosi ve fluids fail MITs at rates two to three times higher than those that inject non-corrosive fluids (USEPA 1999). Given the potentia l for CO2 to be corrosive (e.g., in the presence of water or aqueous formation flui ds), there is a potential risk factor for MIT failure and, for this reason, EPA be lieves that the tailored MIT requi rements in the final GS rule are appropriate.
EPA believes that the MITs described in §146.89(c) of the rule an a pproved tracer log or temperature or noise logs are e ffective methods to demonstrate external MI, provide an important early warning of potential probl ems in the injection well system, and represent standard techniques that have been used successfully by the drilling industry and for the UIC Program for decades.
EPA adds that it is using an ad aptive approach to regulating GS. The Agency plans to collect and

6

### review data every six years on GS projects, including MIT data, to determine whether the appropriate amount and types of information and appropriate documentation are being collected, and to determine if modifications to the requirements are appropriate or necessary.

Letter ID: 0207 Comment ID: 22439 Letter Lines: 95 to 174

Comment Code: 3120 2.1.2 Need to identify additional confining and containment zones (seeking comment)

Comment: PROPOSED REGULATION
p. 43536, c. 3 The proposed rules for Class VI wells identifiedby EPA include a requirement for a confining zone withsuitable propertie s to contain the injected CO2 (§ 146.83(a)). In addition, under subsection (b) of the proposed regulation, owners or operators may be re quired to identify and characterize multiple confining layers, at the disc retion of the Director (§ 146.83(b)).

DISCUSSION
The rationale for inclusion of an optional requirement for multiple confining layers is not provided. There does not appe ar to be any clear need for that additional requirement, which go beyond what is currently requi red for Class I UIC wells that inject hazardous wastes. The proposed regulati on could lead to significant problems in the future, but mo re importantly, this requirement is not needed. If subsection (b) of § 146.83 we re removed, and if the Director or his staff find that the confining syst em identified by an operator does not appear to be reliable, they can reject the application and/or require that the operator provide information on additional confining layers. The description of the requirements for a confining system given in part (2) of subsection (a) would cover this situation, and would allo w protection of USDW's and the public without creating the potential for confusi on that would arise with the option outlined in subsection (b).
Previous experience and research have demonstrated that a single wellcharacterized confining system can provi de more than adequate protection by preventing the upward migration of CO2. Fo r example, time-lapse seismic imaging at the Sleipner Project demonstrates "the caprock is an effective seal that prevents CO2 migration out of the storag e formation" (IPCC 2005). Examination of recent surveys of the safety of underground CO2 sequestration found no suggestion that multiple confining systems are needed or should be required (e.g., IPCC 2005; Benson and Surles 2006). Inaddition, the technical support document provided by EPA for the proposed ru le changes (Vulnerability Evaluation Framework for Geologic Sequestration of Carbon Dioxide, EPA 430-R-08-009) does not provide any support for requirement of multiple confining layers.

7

### The proposed regulations do not specify or in any way limit the conditions under which multiple confining layers might be required. Thus, there is no guidance for imposition of the rule, and no limitations on the discretion of the Director. We believe including this additional discretionary requirement as an option in the regulations is trouble some because there then is a good possibility that it could become the defau lt requirement. If that were the case, the requirement of multiple confining layers would lead to potential disqualification of excellent storage sites, and additional costs for characterization.
An additional problem with the optional requ irement of multiple confining layers is one of perception. We know of no preced ent within current UIC regulations, other applicable U.S. environmental law, or internationally, for the requirement of multiple confining layers to contain injected fluids. Given the lack of precedent, the inclusion of the requirement for multiple confining layers for CO2 in the proposed regulations for Class VI wells could lead to the incorrect perception by the public that injection and storage of CO 2 is more dangerous to the public than subsurface injection of hazardous wastes. This perception of high risk, which does not appear to be justifie d by previous studies of the risks of subsurface CO2 sequestration (e.g., IPCC 2005; Benson and Surles 2006) could lead to decreased use of carbon seques tration as an option to limit global climate change, and therefore to decreased potential for full use of U.S. energy

resources.

PNNL RECOMMENDATION
For the reasons outlined above, we believe that subsection (b) of § 146.83 of the proposed regulations, which grants the Director discretion to require the presence of multiple confining layers, does not promote sound policy and should be removed from the proposed regulation. If EPA chooses to retain the option described in subsection (b), then we feel that the discretion of the Director should be limited, and there should be specification of the types of conditions under which multiple confining layers might be required.

REFERENCES

Benson SM and T Surles. 2006. Carbon Dioxide Capture and Storage: An Overview with Emphasis on Capture and Storage in Deep Geological Formations, The Proceedings Special Issue, Institute of Electrical and Electronics Engineers (IEEE), Vol. 94, No 10, October 2006, DOI 10.1109/PROC.2006.883716.
IPCC. 2005. IPCC Special Report: Carbon Dioxi de Capture and Stor age. Summary for Policy Makers and Technical Summary, B. Metz, O. Davidson, H. de Coninck, M. Loos, and L. Meyer (eds.). Cambridge University Press, NewYork.
Individual Response: EPA believes that the Class VI requir ements, including the level of site

8

### characterization required under §§146.82 and 146.83, appropriately address the unique challenges posed by large-scale GS including large injection volumes, the relative buoyancy of CO2, its corrosivity in the presence of water, th e potential presence of impurities in captured CO2, and its mobility within subsurface geologic formations. The Agency believes that in some geologic settings, the presence and identification of additional confining and containment zones above the primary (i.e., lowermost) confining zone can improve USDW protection. These formations could further impede vertical flui d movement, allow for pressure dissipation, and provide additional sites for monitoring, mitigatio n, and remediation, while also affording the greatest protection of USDWs from endangerment. While other countries may not require such additional protection, EPA believes that providing the Director th is additional discretion is an important element of assuring that GS activities will not endanger USDWs.
The Agency believes that providing the Director di scretion will allow consideration of a range of sites for GS, based on site-specific conditions and ensure USDW protection. EPA believes that requiring appropriate site ch aracterization by the owner or operator (under §146.82) and providing an appropriate de gree of flexibility to the Director to evaluate site-specific geologic data will afford the greatest protec tion of USDWs from endangerment.
EPA is developing guidance on site characterization for GS projects to assist owners or operators and Directors in evaluating permit requirements and applications under §146.82. The guidance provides recommendations regard ing characterizing the region al geology and hydrogeology of a proposed GS site as well as performing th e necessary geochemical, geomechanical, and geophysical analyses of the injection and confin ing zone. EPA expects that this guidance will assist permit writers in making an objective deci sion about requiring additional confining layers. The guidance can be found on EPA’s website at http://water.epa.gov/type/groundwat er/uic/wells_se questration.cfm.
To the comment that the geologic siting require ments could create a negative public perception, EPA believes that a demonstrati on to the public that extensive data have been collected and evaluated would likely improve confidence in the project and help address public questions and

concerns.

Letter ID: 0207 Comment ID: 22440 Letter Lines: 176 to 211

Comment Code: 3300 2.2.1 Delineating the AoR/M odeling CO2 movement

Comment: AREA OF MONITORING (AoM) AND AREA OF REVIEW (AoR)

PROPOSED REGULATION
p. 43493, c. 2 Area of Review (AoR): Th e region surrounding the geologic sequestration project that may be impact ed by the injection activity. The area of review is based on computational modeling that accounts for the physical and chemical properties of all phases of the injected carbon dioxide stream.

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### The proposed regulation uses the Area of Review (AoR) for defining both corrective actions and m onitoring requirements:
p. 43499, c. 1 Area of Review (AoR) and Co rrective Action: The Agency requires examination of both the vertical and horiz ontal extent of the area that will potentially be influenced by injection and storage activities and identification of all artificial pene trations in the area that may act as conduits for fluid movement into USDWs (e.g., active and abandoned wells) and, as needed, perform corrective action to these open wells (i.e., artificial penetrations).
p. 43539, c. 3 (2) The monitoring freque ncy and spatial distribution of monitoring wells must be based on baseli ne geochemical data that has been collected under §146.82(a)(6) and any modeling results inthe area of review evaluation required by § 146.84(b);
We agree with the concept of defining bounding areas for corrective actions and monitoring activities, but the proposed regulation uses a single area, Area of Review (AoR), for both and fails to a ddress how to quantify this area. The proposed regulation defines the Area of Revi ew (AoR) as being the region that may be impacted by the injection acti vity, without defining or quantifying impact. To be useful the bounding areas for corrective actions and monitoring activities must have specific definiti ons. Under deep geologic sequestration conditions the injected fluid (i.e., supe rcritical CO2) has low compressibility, the displaced fluid (i.e., brine) has lo w compressibility, and the reservoir formation (e.g., sandstone) generally has low compressibility, all of which suggest that the impact (i.e., pressure change) of the injec tion activity could be detected at great distances from the injection well(s).
Individual Response: EPA believes that, while the commente r offers different terminology for the AoR and the area to be monitored, the GS rule addresses the commenter's suggestions. Notably, the owner or operator is required ( under 146.90) to develop and implement a testing and monitoring plan for each GS project that mu st consider the delineated AoR that includes both the CO2 plume and associated pressure front and incorporates many of the attributes suggested by the commenter. EPA clarifies that delineation of the AoR, as required at 146.84(c), must account for the physical and chemical properties of all phase s of the injected CO2 stream, based on available site characterization, monitoring, and operational data.
Furthermore, any changes to the AoR would necess itate a review of and po tential change in the testing and monitoring plan, consistent with the commenter's a ssertion that the AoR may vary over time. The final GS regulation includes a requirement at 146.84(e) that the AoR be reevaluated at least every 5 years, and also re quires that the owner or operator periodically review the AoR and corrective action, testing and monitoring, and emergency and remedial response plans within one year of an AoR reeval uation (as well as follo wing significant changes to the facility, or when required by the Directo r). The relationship between the AoR reevaluation and the testing and monitoring plan should allow fo r considerations suggested by the commenter.

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### EPA believes that by linking develo pment and review of the testi ng and monitoring plan to the AoR evaluation and reevaluation process, the GS rule, in effect, incorporates many of the commenter's considerations for an area of monitoring.
EPA did not specifically define the term "impacted" in this context because of the wide range of potential geologic and other site-s pecific conditions that may be vary from site to site. EPA believes that each GS site will be unique and that the affected area where USDWs may be at risk should be defined based on computational m odeling that accounts for available site characterization, monitori ng, and operational data.
EPA anticipates that the delineated AoR will se rve as the foundation for identifying areas in which corrective action and monitoring associated with the site would oc cur, and that the AoR and corrective action plan and the testing and m onitoring plan would be based on the delineated AoR. EPA is developing guidance for owners or operators on developing pr oject-specific plans; the guidance will be available on EPA's web site, at http://water.epa.gov/type/groundwat er/uic/wells_se questration.cfm.
EPA believes it is appropriate to allow the owner or operator the flexibility to develop a sitespecific AoR and corrective action plan that takes into account the unique conditions of the site, subject to the approval of the Director. With in the UIC Program, impacts associated with injection would include anything that could endanger a USDW, including, but not limited to, the CO2 plume and pressure front, displaced saline fluids, co-contaminants, and mobilized minerals.

Letter ID: 0207 Comment ID: 22443 Letter Lines: 213 to 279

Comment Code: 3300 2.2.1 Delineating the AoR/M odeling CO2 movement Comment Code: 4600 2.6 Proposed Plume and Pressure Front Monitoring Requirements (40 CFR 146.90)

Comment: Injection of CO2 into deep saline formations is driven by differences between the well injection pressure and formation pressure. To avoid inducing fractures or opening existing fractures in the fo rmation and caprock, well injection pressure will be limited to some fracti on below the fracturing pressure. As CO2 migrates into the formation the form ation pressure increases around the injection well. The pressure distribution in the formation ar ound the injection well depends on the formation properties, structure, and injection rate. Injection formations will be required to be confined ve rtically and/or horizontally to prevent the vertical migration of inj ected fluids and saline reservoir fluids into USDWs.
Confined formations (e.g., a caprock overlying a horizontally extensive formation) allow the formation fluid to be displaced laterally by the injected fluid. Injecting CO2 results in elevatedpressures both inside and beyond the CO2 plume. Elevated pressures within th e CO2 plume have the potential to force CO2 and formation fluid through permeable wells, fractures, or faults that

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### penetrate the formation. Elevated pre ssures outside the CO2 plume have the potential to force formation fluid through permeable wells, fractures or faults that penetrate the formation. The higher the formation pressure is above the ambient formation pressure, the greate r the potential for CO2 and formation fluid to leak through permeable wells, fractures, and faults. The lateral pressure distribution is a function of the injection rate and the lateral distribution of formation permeability. When CO2 injection is halted, elevated pressures decay over time as th e injected and formation fluids migrate away from the injection well(s).
The proposed regulations define an AoR and Corrective Action as being an area within which artificial pene trations may act as conduits for fluid movement into USDWs (e.g., active and abandoned wells) and, as needed, require the applicant to perform corrective action to seal th ese open wells. The potential for fluid movement into USDWs from the injection of CO2 into the injection formation is correlated with the area of elevated form ation pressure, which is usually larger than the area that contains the injected CO2 mass. For confined formations, this area is defined by the lateral pressu re distribution around the injection well(s).

PNNL RECOMMENDATION
EPA should define two regulatory regions surrounding the geol ogic sequestration project and provide speci fic definitions for thos e regions; 1) Area of Monitoring (AoM) and 2) Area of Review; wh ere the AoM is used to define the spatial distribution of monitoring wells and the AoR is used to define the limit of corrective actions:
Area of Monitoring (AoM): The region surrounding the geologic sequestration project that The area of monitoring is ba sed on monitoring data contains 95% of the carbon dioxide mass. and computational modeling that accounts for the physical and chemical properties of all phases of the injected carbon dioxide stream. In the absence of monitoring data , the area of monitoring is based on computational modeling.
Area of Review (AoR): The region surrounding the geologic sequestration reservoir that has or is indicated to de velop pressures that are greater than 10% different from the original hydrost atic formation fluid pressure, but no less than the Area of Monitoring. The area of review is based on monitoring data and computational modeling that account s for the physical and chemical properties of all phases of the injected carbon dioxide stream. In the absence of monitoring data, the area of reviewis based on computational modeling.
The proposed definitions of AoM and AoR have several important components:

- The AoMhas a mass-based definition. - Th e AoR has a pressure-differential-based

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### definition. - The AoR is never smaller thanthe AoM. - The AoR and AoM may be asymmetrical around the injection well(s). - The AoR and AoM may vary over time. - The carbon dioxide mass should be determined using a combination of monitoring data and computational modeling. - Pressure diffe rentials are based on equilibrium hydrostatic conditions.
Individual Response: EPA believes that, while the commente r offers different terminology for the AoR and the area to be monitored, the GS rule addresses the commenter's suggestions. Notably, the owner or operator is required ( under 146.90) to develop and implement a testing and monitoring plan for each GS project that mu st consider the delineated AoR that includes both the CO2 plume and associated pressure front and incorporates many of the attributes suggested by the commenter. EPA clarifies that delineation of the AoR, as required at 146.84(c), must account for the physical and chemical properties of all phase s of the injected CO2 stream, based on available site characterization, monitoring, and operational data.
Furthermore, any changes to the AoR would necess itate a review of and po tential change in the testing and monitoring plan, consistent with the commenter's a ssertion that the AoR may vary over time. The final GS regulation includes a requirement at 146.84(e) that the AoR be reevaluated at least every 5 years, and also re quires that the owner or operator periodically review the AoR and corrective action, testing and monitoring, and emergency and remedial response plans within one year of an AoR reeval uation (as well as follo wing significant changes to the facility, or when required by the Directo r). The relationship between the AoR reevaluation and the testing and monitoring plan should allow fo r considerations suggested by the commenter.
EPA believes that by linking develo pment and review of the testi ng and monitoring plan to the AoR evaluation and reevaluation process, the GS rule, in effect, incorporates many of the commenter's considerations for an area of monitoring.

Letter ID: 0207 Comment ID: 22444 Letter Lines: 281 to 316

Comment Code: 3530 2.2.3.1 AoR reevaluation fre quency (seeking comment)

Comment: AREA OF REVIEW REEVALUATION

PROPOSED REGULATION
p. 43507, c. 3 When monitoring data di ffer significantly from modeled redictions or when there are appreci able operational changes (e.g., rates) reevaluation may be mandated prior to the minimum fixed frequency. At no time would area of review reeval uations occur less frequentlythan every 10 years.

DISCUSSION
The conclusion reached by EPA that the Ao R should be reevaluated over time based on comparisons of incoming monitoring and si te characterization data with model

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### predictions is very important. However, the definition of "significantly" needs to be clarified in the re gulation, otherwise interpretation will be a constant point of confusion.
Based on PNNL's experience in modeling GS projects, comparison of monitoring data with model prediction, especially in the initial years of operation, would be extremely valuable in assessing our understanding of the re servoir behavior.
Models of CO2 injection into deep form ations are sensitive to estimates of permeability in the formation. Permeability can vary over orders of magnitude in a formation, and this can have a large im pact on estimates of pressure response, and hence the AoR. As the size of the injected CO2 plume grows with time, it will encounter areas of the formation that may have different permeability than was measured at the injection well, and the AoR may vary over time.

PNNL RECOMMENDATION
We recommend that comparisons of monitoringdata with model results be made and reported annually, to ensure that the site is behaving as expected or predicted. If at any time the monitoring data or comparisons of observed and predicted pressure response suggest anomalous behavi or, this could suggest a need for a reevaluation of the AoR (and AoM; see prev ious comments), at th e discretion of the Director.
Individual Response: EPA agrees with commenters that reevaluations of the AoR every ten years may not be sufficient to appropriately model plume and pressure front movement, establish conditions for corrective action, and ensure prot ection of USDWs from endangerment. For these reasons, the Agency changed the minimum frequency for the AoR reevaluation at §146.84(e) from 10 years to at least every 5 years in the final GS rule. EPA belie ves that this revised frequency addresses commenters' concerns about the inherent uncertainties in modeling CO2 movement, the emerging nature of GS technology, risk-based targeting of monitoring activities, and the unique nature of CO2 GS. In addition, the Director has discre tion to require more frequent AoR reevaluation if he or she has reasonto believe, based on site-specific information, that a five-year reevaluation schedule is not sufficiently protective of USDWs.
EPA also agrees with the commenter that inconsistencies between monitoring data and predictions about CO2 plume and pressure front movement would warrant a reevaluation of the AoR. For this reason, the AoR and corrective action plan must specify the site-specific monitoring and operational c onditions that warrant reev aluation, as specified in §146.84(b)(2)(ii).
EPA is developing guidance on preparing and updating AoR and corrective action plans that will clarify the requirements for reevaluations. The guidance will be available on EPA's web site, at http://water.epa.gov/type/groundwat er/uic/wells_se questration.cfm.

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### Letter ID: 0207 Comment ID: 22445 Letter Lines: 318 to 376

Comment Code: 3310 2.2.1.1 Comments in favor of requiring computational models to delineate the AoR (seeking comment)

Comment: USING MULTIPHASE COMPUTATIONAL MODELS TO DELINEATE THE AoR

PROPOSED REGULATION
p. 43506, c. 1 Current UIC regulations require that the owner or operator define the Area of Review (AoR), within which the owner or operator must identify all penetrations (regardle ss of property ownership) in the confining zone and the injection zone and determine whether th ey have been properly completed or plugged.
p. 43506, c. 2 EPA believes that predicting the complex multi-phase buoyant flow of the CO2, coinjectates, and compounds th at may be mobilized due to injection requires the sophistication of multiphase co mputational models. EPA proposes that the owners or operators of GS wells de lineate the AoR for CO2 GS sites using computational fluid flow models desi gned for the specific site conditions and injection regime.

DISCUSSION
The use of multiphase computational models for prediction of plume areas has been supported by several groups, including th e Intergovernmental Panel on Climate Change (IPCC, (2005), which suggested the us e of modeling for evaluation of the long-term distribution of CO2 in the subsur face. This modeling could be used as the basis for identification of the AoR for a given site. The IPCC report makes clear that the models used for these e fforts should be multiphase models that incorporate the important physical, chemical, and geomechanical processes present at a given site (IPCC 2005).
Whereas a prescribed radius or simple mathematical computation, as included in the current UIC regulations, may be adequate for some UIC sites, research and experience in modeling CO2 sequestration leads us to conclu de that these simplified approaches are clearly not suffi cient to predict the extent of an injected CO2 plume. EPA should be commende d for requiring the use of multiphase computational models for delineating the Ao R. We know that initial models of the Sleipner site suggested relatively simple and predic table migration of CO2. However, time lapse seismic monitoring of the injected plume revealed the importance of an internal shale formati on within the injectio n reservoir that has significantly altered th e direction and extent of CO2 migration from those early predictions (Chadwick et al. 2004). This finding emphasizes the importance of computational fluid flow modeling capable of handling complex

15

### internal heterogeneity for prediction of CO2 plume geometries, which are unlikely to be cylindrical or closely pred icted using the appro aches that have been previously used in UIC applications.

PNNL RECOMMENDATION
We support the use of sophisticated, multiphase, computational models designed for site-specific conditions for AoRdelin eation, as specified by EPA in the proposed regulations.

REFERENCES
R.A. Chadwick, P. Zweigel, U. Greger sen, G.A. Kirby, S. Holloway and P.N. Johannessen. 2004. Geological reservoir charac terization of a CO2 storage site : the Utsira Sand, Sleipner, Northe rn North Sea, Energy 29:1371-1381.
IPCC. 2005. IPCC Special Report: Carbon Dioxi de Capture and Stor age. Summary for Policy Makers and Technical Summary, B. Metz, O. Davidson, H. de Coninck, M. Loos, and L. Meyer (eds.). Cambri dgeUniversity Press, NewYork.
Individual Response: EPA acknowledges the commenter’s s upport and agrees that predicting the complex multiphase, buoyant flow of the CO 2, co-injectates, and compounds that may be mobilized due to injection requires the sophistication of computational models, as required at §146.84(c)(1). Models can predict CO2 fate and tr ansport, as well as changes in formation pressure, in three dimensions, as a function of ti me in order to delineate the subsurface storage site and the AoR. Models can account for site-specific and predicted operating conditions as well as geologic heterogeneities and potential migration through fau lts, fractures, and artificial penetrations.

Letter ID: 0207 Comment ID: 22446 Letter Lines: 378 to 446

Comment Code: 3330 2.2.1.3 Applicability /adequacy of computational models (seeking comment)

Comment: Uncertainty Associated with Modeling CO 2 Movement and Reservoir Pressure

PROPOSED REGULATION
p. 43506, c. 3 In suggesting the use of computational flow models for delineating the AoR of GS sites, EPA notes their potential "to predict CO2 movement in response to varying conditions or scenarios, such as changing injection rates, or the presence or absen ce of fractures or faults in confining layers." However, the Proposed Rule does not explicitly disc uss uncertainty assessment in the use of modeling for AoR delineation.

DISCUSSION

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### Although we agree with the EPA that comput ational multiphase fluid flow models are appropriate for delineating the AoR of GS sites, the Proposed Rule does not adequately address how uncertainty should be managed in regards to modeling GS sites, and we consider this to be animportant aspect. By not explicitly addressing uncertainty, the Proposed Rule infers that a single deterministic model result would be appropriate for determ ining the AoR and/or for site review and reevaluation. This ignores the fact thatthere is always a certain level of ncertainty in model results due to the cumulative variability in input data and uncertainty in model parameters.
The carbon sequestration roadmap produ ced by the National Energy Technology Laboratory (DOE 2007) notes th at one of the goals of fi eld tests conducted during the Validation Phase is to "Validate and re fine current CO2 reservoir models for various geologic sequestration sites." Va lidation does not prove that a given model is correct, but it does provide additional constraints on the model parameters. However, in the case of salin e formation storage, history matching is generally not feasible for constraining uncertainties, due to a lack of underground data for comparison (IP CC 2005). Therefore the IPCC (2005) recommended that systematic parameter variation Routines and statistical functions should be included in future c oupled simulators to allow uncertainty estimates for numerical reservoir simulation results.
Uncertainty analysis is a valuable analyti cal tool and is critical for providing confidence in model results. Several approaches can be taken to capture the uncertainty present in any site modeli ng exercise (e.g., Neuman and Wierenga 2003). Three commonly used alternatives are Monte Carlo simulation, the bounding approach, and sensitivity anal ysis. The type of uncertainty analysis appropriate to a particular problem depends on the complexity of the problem and the available data.
Probabilistic approaches like those sugge sted by the IPCC (2005) and Neuman and Wierenga(2003) would provide useful assess ments of the uncertainty associated with the predicted CO2 plume footprint, and could be used to evaluate the risk associated with various choices of theAoR.

PNNL RECOMMENDATION
A formal process for uncertainty assessment of the CO2 plume footprint should be incorporated in the Proposed Rule. This uncertainty assessment could be based on a bounding approach, a sensitivity analysis , Monte Carlo simulation or other method, provided the range of probable circ umstances are considered. This would allow for a better assessment of the AoR th at addresses the inhe rent uncertainty in computational fluid flow mode ling of the injection process.

REFERENCES

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### IPCC. 2005. IPCC Special Report: Carbon Dioxi de Capture and Stor age. Summary for PolicyMakers and Technical Summary, B. Me tz, O. Davidson, H. de Coninck, M. Loos, and L. Meyer (eds.). Cambridge University Press, NewYork.
Neuman, SP, and PJ Wierenga. 2003, A Co mprehensive Strategy of Hydrogeologic Modeling and Uncertainty Analysis for Nucl ear Facilities and Sites. Technical Rpt. NUREG/CR-6805, U. S. Nuclear Re gulatory Commission, Washington DC.
U.S . DOE. 2007. Carbon Sequestration T echnology and Program Plan Roadmap. NationalEnergy Technology Laboratory.
Individual Response: EPA acknowledges that there will be some uncertainty involved in AoR modeling. An EPA-prepared technical backgr ound document, "Computational Modeling of Underground Injection of Carbon Dioxide for De termination of Area-of-Review and Potential Risk to Underground Sources of Drinking Water" (cited as USEPA, 2008d) was provided in the regulatory docket for the proposed GS rule. This document discusses, for example, uncertainty analysis in detail. This document was provided as a background to clarify what information EPA analyzed in order to draft the modeling require ments at 146.84(c)(1), as well as what actions EPA expects owners or operators and Directors to take in this area. For example, the document states: "The requirement for an a-priori AoR delineation based on modelin g predictions stresses the need for uncertainty and sensitivity analyses for the initial prediction. Sensitivity analysis is recommended as the principal evaluation tool fo r characterizing the most and least important sources of uncertainty in environmental models."
EPA based this background document in part on guidance drafted by the National Research Council and on EPA's Council on Regulatory Envi ronmental Modeling (CREM) document. EPA refers interested commenters to the document "Guidance on the Development, Evaluation and Application of Environmental M odels," (74 FR 14551, March 31, 2009).
EPA agrees that in some cases, results from te st-cases may be appropriate. In the technical background document (USEPA, 2008d), EPA provided se veral questions that should be raised during evaluation of models. One of these questions was "If using an independently developed or unverified code, are results from test-probl ems and comparisons to widely used codes presented?" Another was "Was relevant site char acterization data gathered at a sufficiently high spatial resolution to inform model development? Wa s this data used in the model in a way that considers uncertainty?"
Regarding the suggestion that EPA establish a suite of test problem s to demonstrate the suitability of the computational code, EPA notes that the Agency will not perform independent "tests" or validation of indivi dual modeling products. EPA believe s that operators should have the flexibility to select models with which they are familiar and that are most appropriate to site conditions and available data types and amounts.
EPA based this background document in part on guidance drafted by the National Research Council and on EPA's Council on Regulatory Envi ronmental Modeling (CREM) document. EPA

18

### refers interested commenters to the document "Guidance on the Development, Evaluation and Application of Environmental M odels," (74 FR 14551, March 31, 2009).
EPA is developing guidance on AoR and corrective action at GS Class VI s ites to assist owners or operators and Directors in evaluating requi rements and proposed AoR and corrective action plans under §146.84. The guidance describes EPA's reco mmended approaches for the data to be considered in an initial AoR delineation, how to apply computat ional modeling to delineate the AoR, performing corrective action at GS sites, and, when appropriate, completing AoR reevaluations. The guidance will be available on EPA's web site, at http://water.epa.gov/type/groundwat er/uic/wells_se questration.cfm.

Letter ID: 0207 Comment ID: 22447 Letter Lines: 448 to 545

Comment Code: 3330 2.2.1.3 Applicability /adequacy of computational models (seeking comment) Comment Code: 3350 2.2.1.5 Use of proprietary models for GS sites (seeking comment)

Comment: VALIDATION OF MULTIPHASE COMPUTATIONAL MODELS

PROPOSED REGULATION
p. 43507, c. 1 Models used for GS sites s hould be based on accepted science and should be validated. In some cases, owne rs or operators may choose to use proprietary models (i.e., not available for free to th e general public). EPA is aware that the use of proprietary code s may prevent full evaluation of model results (e.g ., NRC, 2007). Several popul ar codes in the petroleum-reservoir engineering discipline are pr oprietary and owners or ope rators of particular sites may prefer to use these codes as th ey have previous experience with them, and they have been used in peer-review ed studies to model CO2 sequestration. When using a proprietary model, owners or operators should cl early disclose the code assumptions, relevant equations, and scientific basis.

DISCUSSION
We agree that computational models used for GS sites should be based on accepted science and should be validated. However, the proposed rule does not define validation. We believe that these models must be validated in terms of their appropriateness for simulating the complex ities of CO2 sequestration under the conditions present at the sp ecific site being evaluated. The fact that a code has been validated and previously used to model CO2 sequestration (even if peerreviewed) does not ensure that it is a ppropriate for use at all sites and under all conditions. For example, it is possible that, for a specific site, certain physical and/or geochemical pr ocesses must be considered that are not available in a given computational code. Whereas that code may have reasonably and successfully modeled CO2 sequestration at a dissimilar site, it may not be

19

### appropriate for the site being considered.
It may be appropriate for EPA to establis h a suite of test pr oblems that span the breadth of GS reservoir types (i.e., deep saline formations, depleted oil and gas reservoirs, faulted or fractured reservoirs, unmineable coal seams, basalts, salt domes, reactive carbonate s, and shales), injection surface climatic extremes, injectate composition, single versus multiple wells, conditions (i.e., pressure reduction wells , and horizontal ve rsus vertical injection wells), and physi cal/ geochemical processes deemed important in modeling CO2 sequestration (i.e. ., changes of state and chemical reactions). We suggest that this suite of problems be defined by expe rts in the field of CO2 sequestration modeling and code development, and that a variety of computational codes used in modeling GS sites be benchm arked using this suite. A subset of the suite of problems may be sufficient to va lidate a code for use at a particular site, however this subset mu st be rigorously defined (by EPA or the State, if it has established primacy for UIC regulation) based on the conditions specific to the site in question to ensure that the model being validated is appropriate for use at the site. Efforts are already underway to evaluate and compare computational codes being used to model GS sites. For example, several code intercomparison studies have been conduc ted to evaluate th e capabilities and accuracy of numerical simulators for ge ological storage of green house gases (Pruess et al., 2004 and Law et al., 2003). In addition, the Inter-Partnership Simulation and Risk Working Group withinthe DOE Regional Partnerships is embarking on an effort to compare overa ll modeling methodologies on a jointly accessible data set. This will be more inclusive than a code intercomparison effort as the methodologies and conceptu al models used may vary widely.

Model results that should be used to dete rmine if a code is valid for solving a specified problem include plume area and maximum plume radius (as defined by 95% of the CO2 mass) at particular time plan es, injected mass over the time period of simulation, pressure distribut ion, and CO2 phase). Based on the code benchmarks and expert knowledge, a tolerance must be defined that determines what constitutes acceptable results. This suite of test problems could be modified or augmented as the state-of -the-science progresses in modeling, ensuring that any computational code being used at a partic ular site is current with regard to accepted scientific principles and rigor.
In addition to validating part icular computational codes using a set of standard benchmarks, the implementation of the con ceptual model should also be validated by comparing model results to observed data, and by using m odel calibration and history matching to improve the model resu lts. EPA understands the importance of history-matching of model simulations w ith monitoring as stated on p. 43507, c. 1.

PNNL RECOMMENDATION

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### EPA should establish a suite of test probl ems with specified parameters that could be used by the applicant to de monstrate the suitability of the computational code to be used for analysis of the proposed site. As computational approaches and methods impr ove, this suite of problems should be modified and/or updated to incorporate the state-of -the-knowledge in GS modeling.
We agree that proprietary m odels can be used, provided th ey can demonstrate they produce results within the defined acceptable tolerance, in addition to disclosing the code assumptions, relevant equations, and scientific basis. These same requirements must, of course, be me t by all computational codes being used at GS sites.

REFERENCES:
Law, D.H.-S., L.G.H. vander Meer andW.D . Gunter, 2003: Compar ison of numerical simulators for greenhouse gas storage in co al beds, Part II: Flue gas injection. Proceedings of the 6th Internationa l Conference on Greenhouse Gas Control Technologies (GHGT-6), J. Gale and Y. Kaya (eds.), 1-4 October 2002, Kyoto, Japan, Pergamon, v.I, 563-568. Geologi schen Gesellschaft,149,3,449-464.
Pruess K, J Garcia, TKovscek, C Oldenbur g, J Rutqvist, C Steefel, and TXu. 2002. Intercomparison of Numerical Simulation Codes for Geologic Disposal of CO2. LBNL-51813, Lawrence Berkeley National Laboratory, Berkeley, California.
Individual Response: EPA clarifies that an EPA-prepar ed technical background document, "Computational Modeling of Unde rground Injection of Carbon Dioxide for Determination of Area-of-Review and Potential Risk to Undergro und Sources of Drinki ng Water" (cited as USEPA, 2008d) was provided in the regulatory docket for the pr oposed GS rule. This document discusses, for example, uncertainty analysis in detail. This document was provided as a background to clarify what information EPA analyzed in order to draft the modeling requirements at §146.84(c)(1), as well as what ac tions EPA expects owners or operators and Directors to take in this area. For example, the document states: "The requirement for an a-priori AoR delineation based on modeling predictions stresses the need for uncertainty and sensitivity analyses for the initial prediction. Sensitivity analysis is recommended as the principal evaluation tool for characterizing the most and least important sources of uncertainty in environmental models."
EPA agrees that in some cases, results from te st-cases may be appropriate. In the technical background document (USEPA, 2008d), EPA provided se veral questions that should be raised during evaluation of models. One of these questions was "If using an independently developed or unverified code, are results from test-probl ems and comparisons to widely used codes presented?" Another was "Was relevant site char acterization data gathered at a sufficiently high spatial resolution to inform model development? Wa s this data used in the model in a way that considers uncertainty?"

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### Regarding the suggestion that EPA establish a suite of test problem s to demonstrate the suitability of the computational code, EPA notes that the Agency will not perform independent "tests" or validation of indivi dual modeling products. EPA believe s that operators should have the flexibility to select models with which they are familiar and that are most appropriate to site conditions and available data types and amounts.
EPA based this background document in part on guidance drafted by the National Research Council and on EPA's Council on Regulatory Envi ronmental Modeling (CREM) document. EPA refers interested commenters to the document "Guidance on the Development, Evaluation and Application of Environmental M odels," (74 FR 14551, March 31, 2009).
EPA is developing guidance on AoR and corrective action at GS Class VI s ites to assist owners or operators and Directors in evaluating requi rements and proposed AoR and corrective action plans under §146.84. The guidance describes EPA's reco mmended approaches for the data to be considered in an initial AoR delineation, how to apply computat ional modeling to delineate the AoR, performing corrective action at GS sites, and, when appropriate, completing AoR reevaluations. The guidance will be available on EPA's web site, at http://water.epa.gov/type/groundwat er/uic/wells_se questration.cfm.

Letter ID: 0207 Comment ID: 22448 Letter Lines: 549 to 624 ; 705 to 709

Comment Code: 4324 2.4.4.1.2 Comments opposed to requiring injection below the lowermost USDW Comment Code: 4340 2.4.4.2 Minimum injection de pth (seeking comment) Comment Code: 4430 2.4.4.3.3 Injection into coal seams above USDWs (seeking comment)

Comment: Stratigraphic Restriction of Saline and Other Reservoirs to Below Lowermost USDW

PROPOSED REGULATION FOR SALINE FORMATIONS
p. 43494, c. 2 Saline formations: Deep and geographically extensive sedimentary rock layers saturated with waters or br ines that have a high total dissolved solids (TDS) content (i.e., >10,000 mg/L TDS).
p. 43505, c. 1 1. Proposed Geologic S iting Requirements Existing UIC requirements for siting injection wells include identification of geologic formations suitable to receive the injected fluids and confine them such that they are isolated below the lowermos t USDWs, minimizing the potential for endangerment.
p. 43502-43503, section Hon target geologic form ations As with unmineable coal seams, EPA seeks comment on prohibiting inje ction into such formations [basalts, salt domes, and shales] if they are above the lowermost USDW.

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### The EPA is soliciting comments on these issues:
1. Whether CO2 injection should be allowe d into an injection zone (saline sedimentary formations, unmi nable coal seam, basalt, salt caverns, shales, or other reservoirs) above the lowermost USDW , when the Director determines that geologic conditions exist that will prevent fluid movement into adjacent USDWs.
2. Whether a designated USDW should be ex empted to allow for GS, and under what conditions should these exemptions be approved.
3. Whether all USDWs should be exempted below a CO2 injection receiving bed.

4. Whether the EPA should set a minimum injection depth requirement for GS, rather than require that such injectio n take place below the lowermost USDW.

DISCUSSION
Three of the most important issues here are (a) the physical location of saline formations or other types of geologic reservoirs for GS, relative to USDWs; (b) under what circumstances the EPA should remove one or more water-bearing zones from the list of USDWs to allow injectio n; and (c) under what other conditions should the EPA allow injection above a USDW.
In addition to injection into saline fo rmations, EPA recognizes that basalt, unmineable coal seams, salt domes, abandone d mines, black shales and other types of reservoirs may be promising emerging ta rgets. Basalts, in particular appear to have considerable potential for GS Res earch by PNNL and othe rs indicates that extruded basalt flow tops and flow bases may have considerable potential for GS. Research by PNNL and others indicates that extruded basa lt flow tops and flow bases may have regional, multi-gigaton capacity for CO2 storage. In addition, laboratory experiments suggest that regi onal, multi-gigaton capacity for CO2 storage. mineralization of large volumes of CO2 may proceed within a few years of injection (McGra il et al, 2006).
In the proposed rule, the new Class VI wells would be allowed to inject CO2 into basalts, deep saline reservoirs, and othe r potential reservoirs only when they are beneath the lowermost USDW.
A USDW is defined by the EPA as "An a quifer or portion of an aquifer that supplies any public water system or thatcontains a sufficient quantity of ground water to supply a public water syst em, and currently supplies drinking water for human consumption, or that c ontains fewer than 10,000 mg/1 total dissolved solids and is not an exempted aquifer." This defini tion would include many non-potable aquifers that would be candidates for GS.

The EPA's reliance on the standard defi nition of a USDW as being solely

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### mg/L TDS is far too restrictive. In many states, this standard is not applied - all groundwaters are protected regardless of water quality. In at least one state GS operations would be permitted if removing anthropogenic CO2 is considered a greater public benefit than the loss of a marginal aquifer.
The EPA is aware that restricting GS to a position below all EPA defined USDWs would severely limit GS in many areas. Unde r the current proposed rule, directors may grant injection into deep, marginal US DWs that directly overlie crystalline basement rock and/or unmineable coal s eams. In addition to these exceptions, the EPA would consider allowing operation of GS wells where receiving formations are above the lowermost USDW only where ther e are thousands of feet between the injection zone and the over lying and underlying USDWs.

REFERENCES
McGrail, B. P., H. T. Schaef, A. M. Ho, Y. Chien, J. J. Dooley, and C. L. Davidson. 2006. Potential for Carbon Dioxide Sequestration in Flood Basalts, J. Geophys. Res. 111, B12201.
Individual Response: Regarding injection into basalts, coal seams, and salt domes, EPA clarifies that the GS rule doe s not categorically preclude inje ction of CO2 into any non-USDW formation types. The purpose of the GS rule is to ensure protection of USDWs from endangerment, and the rule requires that owners or operators demonstrate that wells are sited in suitable systems with geologic formations to r eceive and confine injected CO2, as required at §146.83 of the GS rule. The Class VI siting, we ll construction and operation, monitoring and testing, and post-injection site care and site closure requirements are designed to ensure that injected CO2 remains within the target formation. If an owner or operator can demonstrate to the Director's satisfaction that GS in coal seams, ba salts, salt domes, or shales can be accomplished in a manner that protects USDWs and meets all of the requirements of the GS rule, the project may be permitted as a Class VI well.
EPA acknowledges that there may be unknown risks a ssociated with GS, particularly regarding GS into formations such as coal seams, basalts , and salt domes, which have been the subject of limited experimental projects in the past. EPA anti cipates that in the short-term, GS into these types of formations will continue to be experimental (i.e., if they are to test GS technologies and collect data), and will likely be permitted as Class V experimental technology wells and be subject to UIC Program Guidance #83.
EPA is preparing an addendum to UICPG #83 that describes the use of th e Class V experimental technology well classification to clarify perm itting for Class V wells. This addendum will address shifting of Class V experi mental projects to full-scale GS projects and the future use of the Class V experimental technology well s ubclass for research and testing of new GS technologies.
EPA acknowledges that there are limited circum stances where injectio n above the lowermost USDW can be performed in a manner that pr otects USDWs. The final GS rule at §146.95

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### establishes a process by which owne rs or operators can seek a waiv er of the Class VI injection depth requirements. The waiver approach requir es the submission of extensive, site-specific information (in conjunction with the permit appli cation submittal) to dem onstrate that injection of CO2 into a formation that is above or be tween USDWs can be done in a manner that is protective of USDWs. The requirements are desi gned to ensure the owner or operator, the Director, and the Regional Administrator consider, on a site specific basis, the implications, benefits, and challenges associated with GS and ensuring water availability and USDW protection.
EPA is not changing the USDW definition; changing the definition of a USDW is outside the scope of the GS rulemaking. The GS rule focuse s on ensuring protection of USDWs, as required under the SDWA, by establishing regul ations to ensure the safe in jection of CO2 for long-term storage.
EPA believes that there is increasing demand for water resources and that technology improvements may make high TDS aquifers viable drinking water sources. EPA believes these potential resources should be c onsidered when injection depth wa ivers are sought. Therefore, the final GS rule at §146.95(b) requires the Director to consider thes e issues in determining whether an injection depth waiver is appropriate. Spec ifically, the Director must consider whether alternative injection si tes are available; community needs, demands, and supply from drinking water resources; planned needs, potential and/or future use of USDWs and non-USDWs in the area; and a plan for securing alternative resources or treating USDW formation waters in the event of contamination related to the Class VI injection activity.

Letter ID: 0207 Comment ID: 22449 Letter Lines: 626 to 635

Comment Code: 4450 2.4.4.4 Use of aquifer exemptions (seeking comment)

Comment: As currently defined by the EPA, "aquife r exemptions" permanently remove a specified aquifer from protection as a US DW. Aquifer exemptions are currently granted for mineral or hydrocarbon expl oration by Class III solution mining wells, or by Class II oil and gas related wells, when there is no reasonable expectation that the formation will be us ed as a drinking water supply. It is important to note that the EPA has not previously considered the purpose of creating additional capacity for the emplacement of fluids as sufficient cause for granting an exemption. Under the ne w rule, it would be possible for operators to apply to change the designation of some aquifers being used for Class II injection. These aquifers would n eed a new, separate exemption for GS.
Individual Response: The final rule allows for the expans ion of the areal extent of existing aquifer exemptions for Class II EOR/EGR wells transitioning to Class VI injection for GS pursuant to requirements at §§146.4 and 144.7(d). Class VI owners or operators granted expansions of existing Class II EOR/EGR aquifer exemptions for GS projects must meet all of the tailored requirements for GS wells in the final GS rule. However, the final rule precludes the

25

### issuance of new aquifer exemptions for Class VI GS wells; that is, aquifer exemptions will only be granted for projects that are transitioni ng from Class II EOR/EGR wells to Class VI.
This approach is designed to ensure that th e differences between traditional Class II EOR/EGR operations and Class VI operati ons are considered during the aquifer exemption application process and the Class VI permitting process. Thes e differences include the anticipated large CO2 injection volumes associated withGS, the buoyant and mobile nature of the injectate, and its corrosivity in the presence of water. The Agen cy believes that this process will encourage owners or operators and Directors to consider the use of alternative formations for GS, including non-USDW formations through the waiver process, prior to applying for or approving aquifer exemption expansions for Class II EOR/EGR we lls transitioning to Cl ass VI GS operations.
EPA clarifies that all decisions regarding whethe r expansion of existing aquifer exemptions will be at the Director's discretion and based on site -specific information. If an owner or operator applies for a Class VI permit to inject CO2 into a previously exempted aquifer (non-USDW) that is located above and/or between USDWs, the perm it applicant must also apply for a waiver of the injection depth requirements pursuant to §146 .95 to ensure that if a waiver is granted, USDWs above and below the injection z one are protected from endangerment.
For additional information on aquifer exemptions for Class VI wells, please see the final rule and preamble, as well as EPA's responses to comments on the Notice of Data Availability and Request for Comment.

Letter ID: 0207 Comment ID: 22450 Letter Lines: 637 to 681 ; 711 to 712

Comment Code: 4324 2.4.4.1.2 Comments opposed to requiring injection below the lowermost USDW

Comment: The EPA suggests the following possible alte rnatives that would allow injection between USDWsor above the lowest USDW:
1 . Injection approval for GS between USDW s formations is at the Director's discretion in circumstances in which it maybe demonstrated that USDWs wouldnot be endangered.
2. Directors may be allowed to exempt, attheir discretion, all USDWs below a GS injection zone. (This would permanently rem ove these aquifers from protection as USDWs).

PNNL RECOMMENDATION
GS should be permitted in the following:
1) reservoirs containing >10,000 mg/1L TDS, 2) where the formation water is
USDWsby the state.

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### We do not support the proposed restriction of GS to stratigraphic zones below the lowermost USDW.
- Prohibition of CO2 injection into salin e formations above USDWs is overly restrictive and would severe ly limit GS in states such as Wyoming, where deep sedimentary formations in much of th e state have TDS lower than 10,000 mg/1 (NETL). Thus almost any formation considered for GS would likely overlie a low TDS formation.
- GS should be allowed into an injection zone above the lowermost USDW (whether saline or other formation) when the Direct or determines that geologic conditions exist that will prevent fluid movement into nearby USDWs. Conditions that will prevent downward movement of fluids include the presence of intact low permeability beds between the receivi ng bed and the underlying USDW. In consideration of the buoyant behavior of injected CO2, the requirement of thousands of feet of intervening formations may be less important than the demonstration of the sealing capacity of th e intervening layers. We believe that "thousands of feet" is a vague requirement, when the more rigorous test is the demonstration of the sealing capac ity of the intervening layers.

- The draft rule is far too restrictiv e and is not based on technical risk factors. Risk of CO2 contamination to a US DW is lower for aquifers that underlie an injection reservoir due to the inherent buoyancy of th e CO2. Vertical migration can be interrupted completely by the pres ence of one or more low permeability layers. Hence, a prohibition on injecting a bove the shallowest USDWis not based on real technical risk of a GS project.
NETL. 2007. National Carbon Sequestration Atlas. http://www.netl.doe.gov/technologies/car bon seq/refshelf/atlas/index.html
Individual Response: As described above, EPA acknowledges that there are limited circumstances where injection above the lowerm ost USDW can be performed in a manner that protects USDWs from endangerment, as long as appropriate protections are in place. The final GS rule at §146.95 establishes a pr ocess by which owners or operato rs can seek a waiver of the Class VI injection depth requirements for inje ction into non-USDW form ations above and/or between USDWs. The final rule also establishes limited circum stances under which expansions of the areal extent of aquifer exemptions may be granted fo r owners or operators of Class II EOR/EGR wells transitioning to Class VI injection for GS.

Letter ID: 0207 Comment ID: 22451 Letter Lines: 683 to 692

Comment Code: 4460 2.4.4.4.1 Comments in favor of allowing aquifer exemptions in certain circumstances

Comment: - The conditions for exemptions of aqui fers should be clarified. Aquifer

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### exemptions should be allowed for GS ("a r eceiving bed for Class VI wells") when it can be demonstrated that there is no reasonable expectation that the formation will be considered for a drinking water source, and when it can be demonstrated that CO2 is highly unlikelyto migrate updip into a portion of the formation that may be considered for use as a USDW.
- Blanket aquifer exemption of all USDWs be low a GS zone is not needed if there is demonstration of a competent seal be tween the injection zone and the first underlying USDW.
Individual Response: EPA acknowledges that there are lim ited circumstances where injection above the lowermost USDW can be performe d in a manner that protects USDWs from endangerment, and the final GS rule does not cate gorically preclude or prohi bit injection into any type of formation.
The final GS rule, at §146.95, establishes a proc ess by which owners or operators can seek a waiver of the Class VI injection depth requir ements for injection into non-USDW formations above and/or between USDWs. The final rule al so establishes limited ci rcumstances under which expansions of the areal extent of aquifer exemptions may be granted for owners or operators of Class II EOR/EGR wells transitioni ng to Class VI injection for GS pursuant to requirements at §§146.4 and 144.7(d).
The Agency clarifies that the aquifer exempti on requirements and the injection depth waiver requirements serve different purposes. Class VI owners or operators granted expansions of existing Class II EOR/EGR aquifer exemptions fo r GS projects must meet all of the tailored requirements for GS wells in the final GS ru le, including the requirements at §146.83 for site criteria. EPA believes that, in most cases, the inj ection depth waiver is a more appropriate option than aquifer exemptions for Class VI injection, and believes that aquifer exemption expansions for GS should be granted in limited circumstances.
If an owner or operator applies for a Class VI permit to inject CO2 into a previously exempted aquifer (non-USDW) that is lo cated above and/or between USDW s, the permit applicant must also apply for a waiver of the injection depth re quirements pursuant to §146.95 to ensure that if a waiver is granted, USDWs above and below the inje ction zone are protected from endangerment.
EPA clarifies that all decisions regarding whethe r expansion of existing aquifer exemptions will be at the Director's discretion and based on site-specific information. EPA is developing guidance on the injection depth waiver proces s. The guidance describes the recommended approach an owner or operator may follow wh en submitting a waiver application, processing waiver applications by the UIC and PWSS Program Directors, and additional processes that may apply to projects that have been issued waivers. This guidance will be available on EPA’s website at http://water.epa.gov/type/groundwater/uic/wells _sequestration.cfm
For additional information on aquifer exemptions for Class VI wells, please see the final rule and preamble, as well as EPA's responses to comments on the Notice of Data Availability and Request for Comment.

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### Letter ID: 0207 Comment ID: 22452 Letter Lines: 694 to 696

Comment Code: 2163 1.6.3.1 Comments in favor of injection into unmineable coal seams

Comment: - No blanket prohibition should be introduced for any potential non-saline reservoir; promising GS targets of basalt, unmineable coal seams, etc., should be evaluated with methodologies similar to those for saline formations.
Individual Response: EPA clarifies that the GS rule does not preclude injection into any nonUSDWs formations, provided the site-suitability requirements in the GS rule are met. EPA acknowledges that there may be ri sks associated with GS into certain formation types. For this reason, the Agency is using an adaptive approach to regulate GS injection. EPA will collect and review project data on early GS pr ojects, including those that injectinto various formation types, to determine whether the appropriate amount and types of information and documentation are being collected. EPA plans, every six years, to collect and review data on GS projects to determine whether the appropriate amount and types of information and appropriate documentation are being collected, and to determ ine if modifications to the requirements are appropriate or necessary.

Letter ID: 0207 Comment ID: 22453 Letter Lines: 698 to 703

Comment Code: 4344 2.4.4.2.2 Comments opposed to setting a minimum injection depth

Comment: - GS is better served by neither mandati ng a set minimum depth for injection nor by requiring injection take place below the lowermost USDW. Minimum depth requirements would be unnecessarily proscrip tive . Shallow injections could be done for very small commercial producers or in cases where reservoir engineering practices (such as pre-diss olution of the CO2into formation water) are done so that no free CO2 phase exists.
Individual Response: EPA agrees that it is notappropriate to set a mi nimum injection depth for GS projects and the Agency believes that a determ ination of site-suitabili ty for GS (i.e., that subsurface formations will receiv e and confine injected CO2) s hould be made on a site-specific basis. The GS rule, at §146.82, requires the owne r or operator to provide extensive site- and project-specific information to the Director to demonstrate that the site meets the minimum site criteria at §146.83, that the CO2 will be contai ned within the injection formation, and that USDWs will be protected from endangerment.
EPA acknowledges that there are limited circum stances where injectio n above the lowermost USDW can be performed in a manner that protec ts USDWs, as long as appropriate protections are in place. The final GS rule at §146.95 establis hes a process by which owners or operators can seek a waiver of the Class VI injection de pth requirements for in jection into non-USDW formations above and/or between USDWs. The waiver approach requires the submission of extensive, site-specific information (in conjunc tion with the permit appl ication submission) to demonstrate that injection of CO2 into a non-US DW that is above or between USDWs can be

29

### done in a manner that is protective of USDWs. Th e requirements are designed to ensure that the owner or operator, the Director, and the Regional Administrator cons ider, on a site specific basis, the implications, benefits, and challenges associat ed with GS and ensuring water availability and USDW protection. For additional information on the waiver approach, please see the final rule and preamble, as well as EPA's responses to comments on the Notice of Data Availability and Request for Comment.

Letter ID: 0207 Comment ID: 22455 Letter Lines: 714 to 795

Comment Code: 4546 2.5.2.2 MIT frequency (seeking comment)

Comment: PROPOSED MECHANICAL INTEGRITY TESTING REQUIREMENT

PROPOSED REGULATION
p. 43513, c. 3 ...require owners and operators of Class VI GS project to monitor internal mechanical integrity of th eir injection wells by continuously monitoring injection pressure, flowrate, and injected volumes...
p. 43514, c. 1 ...require owners and operat ors of CO2 wells to demonstrate injection well external m echanical integrity at l east once annually. The EPA seeks comment on the appropriate frequency of internal and external MITs for GS injection wells.

DISCUSSION
During the injection period the proposed rule states that internal MIT should be performed continuously by monitoring pr essures (e.g., injection and annular zones), injection rates and volumes, etc., and external MIT should be performed annually. Continuous monitoring of injection rates, volumes , and pressures during the injection phase seems reasona ble and appropriate. The semiannual reporting frequency of such resu lts also seems reasonable.
The proposed rule also states that extern al MIT be performed annually during the injection phase. The rationale for this is to protect USDWsgiven the potential corrosive effects on injection well compone nts (steel casing and cement) that are exposed to the GS stream and the buoyant nature of the injected fluid.
To execute an annual external MIT, the injection well would be required to be shut down (i.e., cease CO2 injection operati on) and the internal CO2 injection delivery tubing system to be physically removed from the injection well. At a minimum, this would require that all GS projects, no matter how large, to have a backup injection well or multiple injection wells to be available so as to not adversely impact or interruptdaily plant operations.
In general, we believe that annual extern al MIT evaluations w ould be excessive

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### and overly disruptive for normal plant operations. This is because quantitative external MIT evaluationtests would require:
- Removal of the surface wellhead and internal CO2 injection delivery tubing system
- Performing detailed wireline geophysic al surveys for cement-seal and well casing integrity (e.g., ce ment-bond log), and
- If warranted by the results of the wi reline geophysical surveys, conducting direct leakage-detection tests with in stalled internal te st equipment (e.g., downhole internal straddle-packer system) of suspect ed areas of concern
The performance of the various aforementi oned elements of an external MIT and re-installation of components of the inj ection well system are likely to take between 2 to 5 days, depending on the ch aracteristics of the injection well (e.g., injection reservoir depth), complexity of the internal CO2 injection well system, and whether quantitative leakage detection tests are warranted. Because of extended time required to conduct an exte rnal MIT program, the performance of fixed-annual external MIT evaluations ( outside the schedule of planned plant operation maintenance shut-downs) are belie ved to be excessively disruptive.

PNNL RECOMMENDATION

We recommend that an extern al MIT be performed on a recurrence interval of every 5 years, as is now typically done for other classes of UIC wells. The external MIT should rely primarily on results obtained from geophysical wire-line logging surveys, and (if warranted by wireline geophysical survey results), conducting direct leakage-detection tests of suspect ed areas of concern. If the external MIT indicates the potential for a compromi se to the cement seal characteristics or well casing integrity, then addition al, more quantitative tests (e.g. Gasda et al. 2008) and subsequent correc tive actions may be warranted.

The recommended external MIT recurrenc e assessment of every 5 years is predicated on no indications of injecti on system compromise that would be observed by changes in monitored intern al MIT parameters during injection (e.g., injection or annular zone pressure), or pressure fall-off test response following normal, temporary operational shut-down periods. If at any time the monitoring parameters or pressure data responses indicate an omalous injection test system behavior, then this could indicate the need for an external MIT evaluation to be performed, at the discretion of the Director.

REFERENCE
Gasda, S. E., J. M. Nordbotten, and M. A. Celia. 2008. Determining effective wellbore permeability from a field pressure test: a numerical analysis of

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### 32 detection limits. Environmental Geology, 54(6), 1207-1215,
Individual Response: EPA disagrees that a five-year extern al MIT is appropriate. The final GS regulation retains the requirement that owners or operators of Class VI wells must demonstrate injection well external mech anical integrity at least once annually under §146.89(c). EPA believes this is justifiable for the protecti on of USDWs from endangerment given the unique nature of CO2 GS relative to other injection operations, including the pote ntial corrosive impact of CO2 in the presence of water and impurities on well materials, the potential geomechanical impacts to well integrity due to CO2 phase ch anges, the temperature and pressure effects associated with injecting supercritical CO2, and the buoyant nature of th e injected CO2. Because of the unknown long-term effects of CO2 GS, EPA chose a more cons ervative initial approach in the early stages of deployment for GS.
In addition, a comprehensive review conducte d by EPA of MIT results from 1993 to 1998 found that Class I wells injecting corrosi ve fluids fail MITs at rates two to three times higher than those that inject non-corrosive fluids (USEPA 1999). Given the potentia l for CO2 to be corrosive (e.g., in the presence of water or aqueous formation flui ds), there is a potential risk factor for MIT failure and, for this reason, EPA be lieves that the tailored MIT requi rements in the final GS rule are appropriate. Regarding concer ns about disruptions of plant operations, EPA believes that, due to the expected size of GS fields, many GS operations will have more than one injection well, and that testing of each we ll can be timed to avoid complete interruption of the project.

EPA believes that the external MI Ts described in §146.89(c) of the ru le (e.g., an approved tracer log or temperature or noise logs ) are effective methods to demons trate external MI, provide an important early warning of pot ential problems in the injectio n well system, and represent standard techniques that have been used succes sfully by the drilling industry and for the UIC Program for decades. EPA clarifies that the re gulations provides the Director discretion to require additional methods or, with the Administrator's approval, to approve the use of alternative MIT methods based on specific information about the site or the well. See §146.89(e).

EPA acknowledges the commenter’s support of continuous monitoring to demonstrate internal MI; the final rule retains th is requirement at §146.89(b).
EPA adds that it is using an ad aptive approach to regulating GS. The Agency plans to collect and review data every six years on GS projects, including MIT data, to determine whether the appropriate amount and types of information and appropriate documentation are being collected, and to determine if modifications to the requirements are appropriate or necessary.