This comment letter response contains two parts:   

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was assigned a Letter ID that corresponds to the last four digits of the docket number of the

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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

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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 

NOV " 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, 99352 
p. 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 follow the 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. 
p. 3 

Docket ID No. EPA-HQ-OW-2008-0390 

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 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. 

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. 4 

Docket ID No. EPA-HQ-OW-2008-0390 

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 problem with 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 

3 
p. 5 

Docket ID No. EPA-HQ-OW-2008-0390 

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. 6 

Docket ID No. EPA-HQ-OW-2008-0390 

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 low compressibility, all of which suggest that the impact (i .e ., pressure change) of the injection activity could be 

detected at great distances from the 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 from the 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). 
p. 7 

Docket ID No. EPA-HQ-OW-2008-0390 

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 Review Reevaluation 

Proposed Regulation 

p. 43507, c. 3 

When monitoring data differ significantly from modeled predictions or when there 

are 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. 
p. 8 

Docket ID No. EPA-HQ-OW-2008-0390 

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) . 
p. 9 

Docket ID No. EPA-HQ-OW-2008-0390 

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 

8 
p. 10 

Docket ID No. EPA-HQ-OW-2008-0390 

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 allow for 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 . 
p. 11 

Docket ID No. EPA-HQ-OW-2008-0390 

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 . 

10 
p. 12 

Docket ID No. EPA-HQ-OW-2008-0390 

Model results that should be used to determine if a code is valid for solving a specified problem 

include plume area and maximum plume 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. 
p. 13 

Docket ID No. EPA-HQ-OW-2008-0390 

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 below the 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 minimum injection 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. 

12 
p. 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 

13 
p. 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.html 
p. 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 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 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 

15 
p. 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 Sequestration Partnerships 

Comment: This response provides recommended changes to the EPA proposed rule on 

underground injection control on CO2 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 of fer constructive comments for the 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 seven RCSPs funded by the U.S. Department of Energy 

and coordinated through the National Energy Technology Laboratory (NETL). In addition to the 

information cited in the preamble to the proposed 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 DOE partnership sites: the Escatawpa, Mississippi 

project; the Aneth Field project in Paradox Basin, Southeast Utah; 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 require the presence of multiple confining layers, does 

not promote sound policy and should be removed from the proposed regulation. 

Individual Response: EPA believes that the Class VI requirements, 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, the potential presence of impurities in captured 

CO2, and its mobility within subsurface geologic formations. 

EPA retains the requirement to identify additional containment/confining zones, if required by 

the Director, as proposed (§146.83(b)). EPA acknowledges 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 containment 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, and also provide additional sites for monitoring, 

mitigation, and remediation. The Agency's decision 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 characterizing 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 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 

Individual Response: EPA believes that, while the commenter 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 must 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 phases of the injected CO2 stream, 

based on available site characterization, monitoring, and operational data. 

Furthermore, any changes to the AoR would necessitate a review of and potential change in the 

testing and monitoring plan, consistent with the commenter's assertion 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 requires 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 reevaluation (as well as following significant changes 

to the facility, or when required by the Director). The relationship between the AoR reevaluation 

and the testing and monitoring plan should allow for considerations suggested by the commenter. 

2
EPA believes that by linking development and review of the testing 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 frequency (seeking comment) 

Comment: 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. 

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 protection 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 believes 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 discretion to require more 

frequent AoR reevaluation if he or she has reason to 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 delineation, as specified by EPA in the 

proposed regulations. 

Individual Response: EPA acknowledges the commenter’s support and agrees that predicting 

the complex multiphase, buoyant flow of the CO2, 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 transport, as well as changes in formation 

pressure, in three dimensions, as a function of time 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 faults, 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/Modeling 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 probable circumstances are considered. 

Individual Response: EPA acknowledges that there will be some uncertainty involved in AoR 

modeling. An EPA-prepared technical background document, "Computational Modeling of 

Underground Injection of Carbon Dioxide for Determination 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 requirements 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 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." 

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 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. 

Individual Response: EPA notes that the Agency will not perform independent "tests" or 

validation of individual modeling products. EPA believes 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 clarifies that an EPA-prepared technical background document, "Computational Modeling 

of Underground Injection of Carbon Dioxide for Determination 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 requirements 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 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 based this background document in part on guidance drafted by the National Research 

Council and on EPA's Council on Regulatory Environmental Modeling (CREM) document. EPA 

refers interested commenters to the document "Guidance on the Development, Evaluation and 

Application of Environmental Models," (74 FR 14551, March 31, 2009). 

EPA is developing guidance on AoR and corrective action at GS Class VI sites to assist owners 

or operators and Directors in evaluating requirements and proposed AoR and corrective action 

plans under §146.84. The guidance describes EPA's recommended approaches for the data to be 

considered in an initial AoR delineation, how to apply computational 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/groundwater/uic/wells_sequestration.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 below the lowermost USDW. 

Individual Response: EPA acknowledges that there are limited circumstances where injection 

above the lowermost 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 process by which owners or operators can seek a waiver of the Class VI injection 

depth requirements for injection 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 owners or operators of Class II EOR/EGR wells 

transitioning to Class VI injection for GS. Additionally, the final GS rule does not categorically 

preclude or prohibit injection into any type of formation. 

EPA believes that these requirements will ensure USDW protection while providing flexibility to 

UIC program Directors and owners or operators who undertake CO2 injection for GS. The 

Agency believes this approach: 1) responds to concerns about local and regional geologic storage 

5
capacity limitations imposed by the proposed injection depth requirements; 2) allows for a more 

site-specific assessment of injection depth for GS projects; 3) accommodates injection into 

different formation types; 4) allows 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 aquifer 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 drinking water needs. 

For additional information on the waiver approach and 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: 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 geophysical wire-line logging surveys. 

Individual Response: EPA disagrees that a five-year external MIT is appropriate. The final GS 

regulation retains the requirement that owners or operators of Class VI wells must demonstrate 

injection well external mechanical integrity at least once annually under §146.89(c). EPA 

believes this is justifiable for the protection of USDWs from endangerment given the unique 

nature of CO2 GS relative to other injection operations, including the potential 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 changes, the temperature and pressure effects 

associated with injecting supercritical CO2, and the buoyant nature of the injected CO2. Because 

of the unknown long-term effects of CO2 GS, EPA chose a more conservative initial approach in 

the early stages of deployment for GS. 

In addition, a comprehensive review conducted by EPA of MIT results from 1993 to 1998 found 

that Class I wells injecting corrosive fluids fail MITs at rates two to three times higher than those 

that inject non-corrosive fluids (USEPA 1999). Given the potential for CO2 to be corrosive (e.g., 

in the presence of water or aqueous formation fluids), there is a potential risk factor for MIT 

failure and, for this reason, EPA believes that the tailored MIT requirements in the final GS rule 

are appropriate. 

EPA believes that the MITs described in §146.89(c) of the rule an approved tracer log or 

temperature or noise logs are effective methods to demonstrate external MI, provide an important 

early warning of potential problems 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 adaptive 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 identified by EPA include 

a requirement for a confining zone with suitable properties to contain the 

injected CO2 (§ 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 wellcharacterized 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 CO2 migration out of the storage 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). In addition, the technical support 

document provided by EPA for the proposed rule 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 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 problem with 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 

CO2 in the proposed regulations for Class VI wells could lead to the incorrect 

perception by the public that injection and storage of CO2 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 CO2 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, NewYork. 

Individual Response: EPA believes that the Class VI requirements, 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, the 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 fluid movement, allow for pressure dissipation, and 

provide additional sites for monitoring, mitigation, 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 this additional discretion is an 

important element of assuring that GS activities will not endanger USDWs. 

The Agency believes that providing the Director discretion 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 characterization by the owner or operator (under §146.82) and 

providing an appropriate degree of flexibility to the Director to evaluate site-specific geologic 

data will afford the greatest protection 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 regarding characterizing the regional geology and hydrogeology of a 

proposed GS site as well as performing the necessary geochemical, geomechanical, and 

geophysical analyses of the injection and confining zone. EPA expects that this guidance will 

assist permit writers in making an objective decision about requiring additional confining layers. 

The guidance can be found on EPA’s website at 

http://water.epa.gov/type/groundwater/uic/wells_sequestration.cfm. 

To the comment that the geologic siting requirements could create a negative public perception, 

EPA believes that a demonstration 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/Modeling CO2 movement 

Comment: 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. 

9
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. 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 CO2) has low compressibility, 

the displaced fluid (i.e., brine) has low 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 injection activity could 

be detected at great distances from the injection well(s). 

Individual Response: EPA believes that, while the commenter 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 must 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 phases of the injected CO2 stream, 

based on available site characterization, monitoring, and operational data. 

Furthermore, any changes to the AoR would necessitate a review of and potential change in the 

testing and monitoring plan, consistent with the commenter's assertion 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 requires 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 reevaluation (as well as following significant changes 

to the facility, or when required by the Director). The relationship between the AoR reevaluation 

and the testing and monitoring plan should allow for considerations suggested by the commenter. 

10
EPA believes that by linking development and review of the testing 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-specific 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 modeling that accounts for available site 

characterization, monitoring, and operational data. 

EPA anticipates that the delineated AoR will serve as the foundation for identifying areas in 

which corrective action and monitoring associated with the site would occur, and that the AoR 

and corrective action plan and the testing and monitoring plan would be based on the delineated 

AoR. EPA is developing guidance for owners or operators on developing project-specific plans; 

the guidance will be available on EPA's web site, at 

http://water.epa.gov/type/groundwater/uic/wells_sequestration.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. Within 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/Modeling 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 formation and caprock, well injection 

pressure will be limited to some fraction below the fracturing pressure. As CO2 

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 CO2 results in elevated pressures both inside and beyond the 

CO2 plume. Elevated pressures within the CO2 plume have the potential to force 

CO2 and formation fluid through permeable wells, fractures, or faults that 

11
penetrate the formation. Elevated pressures 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 greater 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 the 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 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 CO2 into the injection formation is 

correlated with the area of elevated formation pressure, which is usually larger 

than the area that contains the injected CO2 mass. For confined formations, this 

area is defined by the lateral pressure distribution around the injection 

well(s). 

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 The area of monitoring is based 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 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 AoMhas a mass-based definition. - The AoR has a pressure-differential-based 

12
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. 

Individual Response: EPA believes that, while the commenter 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 must 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 phases of the injected CO2 stream, 

based on available site characterization, monitoring, and operational data. 

Furthermore, any changes to the AoR would necessitate a review of and potential change in the 

testing and monitoring plan, consistent with the commenter's assertion 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 requires 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 reevaluation (as well as following significant changes 

to the facility, or when required by the Director). The relationship between the AoR reevaluation 

and the testing and monitoring plan should allow for considerations suggested by the commenter. 

EPA believes that by linking development and review of the testing 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 frequency (seeking comment) 

Comment: AREA OF REVIEW REEVALUATION 

PROPOSED REGULATION 

p. 43507, c. 3 When monitoring data differ significantly from modeled 

redictions or when there are 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 

13
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. 

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 CO2 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 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 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. 

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 protection 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 believes 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 discretion to require more 

frequent AoR reevaluation if he or she has reason to 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 conditions that warrant reevaluation, 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/groundwater/uic/wells_sequestration.cfm. 

14
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 (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 CO2 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 CO2 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). 

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 conclude that these 

simplified approaches are clearly not sufficient to predict the extent of an 

injected CO2 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 CO2. 

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 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 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 AoRdelineation, 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 CO2 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.). CambridgeUniversity Press, NewYork. 

Individual Response: EPA acknowledges the commenter’s support and agrees that predicting 

the complex multiphase, buoyant flow of the CO2, 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 transport, as well as changes in formation 

pressure, in three dimensions, as a function of time 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 faults, 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 CO2 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 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 

16
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 

ncertainty in model 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 CO2 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 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 circumstances are considered. This would 

allow for a better assessment of the AoR that addresses the inherent uncertainty 

in computational fluid flow modeling of the injection process. 

REFERENCES 

17
IPCC. 2005. IPCC Special Report: Carbon Dioxide Capture and Storage. Summary for 

PolicyMakers and Technical Summary, B. Metz, O. Davidson, H. de Coninck, M. 

Loos, and L. Meyer (eds.). Cambridge University Press, NewYork. 

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. 

NationalEnergy Technology Laboratory. 

Individual Response: EPA acknowledges that there will be some uncertainty involved in AoR 

modeling. An EPA-prepared technical background document, "Computational Modeling of 

Underground Injection of Carbon Dioxide for Determination 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 requirements 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 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 based this background document in part on guidance drafted by the National Research 

Council and on EPA's Council on Regulatory Environmental Modeling (CREM) document. EPA 

refers interested commenters to the document "Guidance on the Development, Evaluation and 

Application of Environmental Models," (74 FR 14551, March 31, 2009). 

EPA agrees that in some cases, results from test-cases may be appropriate. In the technical 

background document (USEPA, 2008d), EPA provided several 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-problems and comparisons to widely used codes 

presented?" Another was "Was relevant site characterization data gathered at a sufficiently high 

spatial resolution to inform model development? Was this data used in the model in a way that 

considers uncertainty?" 

Regarding the suggestion that EPA establish a suite of test problems to demonstrate the 

suitability of the computational code, EPA notes that the Agency will not perform independent 

"tests" or validation of individual modeling products. EPA believes 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 Environmental Modeling (CREM) document. EPA 

18
refers interested commenters to the document "Guidance on the Development, Evaluation and 

Application of Environmental Models," (74 FR 14551, March 31, 2009). 

EPA is developing guidance on AoR and corrective action at GS Class VI sites to assist owners 

or operators and Directors in evaluating requirements and proposed AoR and corrective action 

plans under §146.84. The guidance describes EPA's recommended approaches for the data to be 

considered in an initial AoR delineation, how to apply computational 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/groundwater/uic/wells_sequestration.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 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 CO2 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 CO2 sequestration under the 

conditions present at the specific 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 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 CO2 sequestration at a dissimilar site, it may not be 

19
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 surface 

climatic extremes, injectate composition, single versus multiple wells, 

conditions (i.e., pressure reduction wells, and horizontal versus vertical 

injection wells), and physical/ 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 experts in the field of CO2 

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 green house 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. 

Model results that should be used to determine 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 planes, injected mass over the time period of 

simulation, pressure distribution, 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 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 

20
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. vander Meer andW.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, TKovscek, C Oldenburg, 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-prepared technical background document, 

"Computational Modeling of Underground Injection of Carbon Dioxide for Determination 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 

requirements 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 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 test-cases may be appropriate. In the technical 

background document (USEPA, 2008d), EPA provided several 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-problems and comparisons to widely used codes 

presented?" Another was "Was relevant site characterization data gathered at a sufficiently high 

spatial resolution to inform model development? Was this data used in the model in a way that 

considers uncertainty?" 

21
Regarding the suggestion that EPA establish a suite of test problems to demonstrate the 

suitability of the computational code, EPA notes that the Agency will not perform independent 

"tests" or validation of individual modeling products. EPA believes 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 Environmental Modeling (CREM) document. EPA 

refers interested commenters to the document "Guidance on the Development, Evaluation and 

Application of Environmental Models," (74 FR 14551, March 31, 2009). 

EPA is developing guidance on AoR and corrective action at GS Class VI sites to assist owners 

or operators and Directors in evaluating requirements and proposed AoR and corrective action 

plans under §146.84. The guidance describes EPA's recommended approaches for the data to be 

considered in an initial AoR delineation, how to apply computational 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/groundwater/uic/wells_sequestration.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 depth (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 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 below the lowermost USDWs, minimizing the potential for 

endangerment. 

p. 43502-43503, section Hon 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. 

22
The EPA is soliciting comments on these issues: 

1. Whether CO2 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 CO2 injection receiving bed. 

4. Whether the EPA should set a minimum injection 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 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 CO2 storage. In addition, 

laboratory experiments suggest that regional, multi-gigaton capacity for CO2 

storage. mineralization of large volumes of CO2 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 CO2 into 

basalts, deep saline reservoirs, and other potential reservoirs only when they 

are beneath the lowermost USDW. 

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 

23
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. 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. 

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 does not categorically preclude injection 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 receive and confine injected CO2, as required at 

§146.83 of the GS rule. The Class VI siting, well 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, basalts, 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 associated 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 anticipates 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 the Class V experimental 

technology well classification to clarify permitting for Class V wells. This addendum will 

address shifting of Class V experimental projects to full-scale GS projects and the future use of 

the Class V experimental technology well subclass for research and testing of new GS 

technologies. 

EPA acknowledges that there are limited circumstances where injection above the lowermost 

USDW can be performed in a manner that protects USDWs. The final GS rule at §146.95 

24
establishes a process by which owners or operators can seek a waiver of the Class VI injection 

depth requirements. The waiver approach requires the submission of extensive, site-specific 

information (in conjunction with the permit application submittal) to demonstrate that injection 

of CO2 into a formation that is above or between USDWs can be done in a manner that is 

protective of USDWs. The requirements are designed 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 focuses on ensuring protection of USDWs, as required 

under the SDWA, by establishing regulations to ensure the safe injection 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 considered when injection depth waivers are sought. Therefore, the 

final GS rule at §146.95(b) requires the Director to consider these issues in determining whether 

an injection depth waiver is appropriate. Specifically, the Director must consider whether 

alternative injection sites 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, "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 II 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. 

Individual Response: The final rule allows for the expansion 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 transitioning from Class II EOR/EGR wells to Class VI. 

This approach is designed to ensure that the differences between traditional Class II EOR/EGR 

operations and Class VI operations are considered during the aquifer exemption application 

process and the Class VI permitting process. These differences include the anticipated large CO2 

injection volumes associated with GS, the buoyant and mobile nature of the injectate, and its 

corrosivity in the presence of water. The Agency 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 wells transitioning to Class VI GS operations. 

EPA clarifies that all decisions regarding whether 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 permit 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 zone 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 alternatives that would allow injection 

between USDWsor above the lowest USDW: 

1 . Injection approval for GS between USDWs 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, 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 

USDWsby the state. 

26
We do not support the proposed restriction of GS to stratigraphic zones below 

the lowermost USDW. 

- Prohibition of CO2 injection into saline formations above USDWs is overly 

restrictive and would severely limit GS in states such as Wyoming, where deep 

sedimentary formations in much of the 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 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 CO2, 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 CO2 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 USDWis not based 

on real technical risk of a GS project. 

NETL. 2007. National Carbon Sequestration Atlas. 

http://www.netl.doe.gov/technologies/carbon seq/refshelf/atlas/index.html 

Individual Response: As described above, EPA acknowledges that there are limited 

circumstances where injection above the lowermost 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 process by which owners or operators can seek a waiver of the 

Class VI injection depth requirements for injection 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 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 aquifers should be clarified. Aquifer 

27
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 CO2 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. 

Individual Response: EPA acknowledges that there are limited circumstances where injection 

above the lowermost USDW can be performed in a manner that protects USDWs from 

endangerment, and the final GS rule does not categorically preclude or prohibit injection into any 

type of formation. 

The final GS rule, at §146.95, establishes a process by which owners or operators can seek a 

waiver of the Class VI injection depth requirements for injection 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 owners or operators of 

Class II EOR/EGR wells transitioning to Class VI injection for GS pursuant to requirements at 

§§146.4 and 144.7(d). 

The Agency clarifies that the aquifer exemption 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 for GS projects must meet all of the tailored 

requirements for GS wells in the final GS rule, including the requirements at §146.83 for site 

criteria. EPA believes that, in most cases, the injection 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 located above and/or between USDWs, the permit 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 zone are protected from endangerment. 

EPA clarifies that all decisions regarding whether 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 process. The guidance describes the recommended 

approach an owner or operator may follow when 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. 

28
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 risks 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 projects, including those that inject into 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 determine 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 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 CO2into formation water) are done so 

that no free CO2 phase exists. 

Individual Response: EPA agrees that it is not appropriate to set a minimum injection depth for 

GS projects and the Agency believes that a determination of site-suitability for GS (i.e., that 

subsurface formations will receive and confine injected CO2) should be made on a site-specific 

basis. The GS rule, at §146.82, requires the owner 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 contained within the injection formation, and that 

USDWs will be protected from endangerment. 

EPA acknowledges that there are limited circumstances where injection above the lowermost 

USDW can be performed in a manner that protects USDWs, as long as appropriate protections 

are in place. The final GS rule at §146.95 establishes a process by which owners or operators can 

seek a waiver of the Class VI injection depth requirements for injection into non-USDW 

formations above and/or between USDWs. The waiver approach requires the submission of 

extensive, site-specific information (in conjunction with the permit application submission) to 

demonstrate that injection of CO2 into a non-USDW that is above or between USDWs can be 

29
done in a manner that is protective of USDWs. The requirements are designed to ensure that 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. 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 their injection wells by continuously 

monitoring injection pressure, flow rate, and injected volumes... 

p. 43514, c. 1 ...require owners and operators of CO2 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 USDWsgiven 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 CO2 injection operation) 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 interrupt daily plant operations. 

In general, we believe that annual external MIT evaluations would be excessive 

30
and overly disruptive for normal plant operations. This is because quantitative 

external MIT evaluation tests would require: 

- Removal of the surface wellhead and internal CO2 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 

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 CO2 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 

31
32

detection limits. Environmental Geology, 54(6), 1207-1215, 

Individual Response: EPA disagrees that a five-year external MIT is appropriate. The final GS 

regulation retains the requirement that owners or operators of Class VI wells must demonstrate 

injection well external mechanical integrity at least once annually under §146.89(c). EPA 

believes this is justifiable for the protection of USDWs from endangerment given the unique 

nature of CO2 GS relative to other injection operations, including the potential 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 changes, the temperature and pressure effects 

associated with injecting supercritical CO2, and the buoyant nature of the injected CO2. Because 

of the unknown long-term effects of CO2 GS, EPA chose a more conservative initial approach in 

the early stages of deployment for GS. 

In addition, a comprehensive review conducted by EPA of MIT results from 1993 to 1998 found 

that Class I wells injecting corrosive fluids fail MITs at rates two to three times higher than those 

that inject non-corrosive fluids (USEPA 1999). Given the potential for CO2 to be corrosive (e.g., 

in the presence of water or aqueous formation fluids), there is a potential risk factor for MIT 

failure and, for this reason, EPA believes that the tailored MIT requirements in the final GS rule 

are appropriate. Regarding concerns 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 well can be timed to avoid complete interruption of the project. 

EPA believes that the external MITs described in §146.89(c) of the rule (e.g., an approved tracer 

log or temperature or noise logs) are effective methods to demonstrate external MI, provide an 

important early warning of potential problems 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 clarifies that the regulations 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 this requirement at §146.89(b). 

EPA adds that it is using an adaptive 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.