This comment letter response contains two parts:   

(1) The original submittal from the commenter.

(2) A data base printout of EPA’s response to each point raised by the commenter.

Notes about the response letters:

• The response letters were generated by a data base used to track the content of all public

comment submittals and ensure that the Agency responds to each. Each comment letter

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

original comment (as posted on www.regulations.gov).

• Each response letter includes: comment text; an EPA‐assigned comment code; and an

individual response. These are described and explained below.  

• The data base contains Comment text from the letters submitted by commenters.  

Many hard copy letters were scanned into electronic format for inclusion in the data

base. As a result, there may be some typographical errors attributable to optical

character recognition issues.  However, all original submittals were reviewed in the

process of drafting responses to ensure that the comment content was understood and

appropriately addressed.

• The content of each comment letter was divided into topic areas and assigned a unique

Comment ID.  Additionally, one or more Comment Codes (i.e., unique topic area ID) was

assigned to each discrete point raised by a commenter. This information was added to

the data base. To inform final rulemaking decisions, all comments and information

received on a given topic were reviewed by technical experts in each rule area (e.g., on

construction requirements).  

• Following decisions on the final rule requirements, Individual responses were drafted

by topic area to ensure completeness and consistency of the responses with the final

rule and other responses to similar comments.  Each discrete point made by a

commenter was given a unique response.

• Upon completion of comment responses, all information was entered into the data base.

Each letter response is a compilation of each comment and its individual response,

compiled in the order of the original comment letter.  As a result of this process, there is

repetition in the response text.  The Agency believes this approach ensures completeness

and attention to each comment letter that was submitted.   

• All comments in a topic area were reviewed, evaluated, and responded to concurrently.  

Responses on each topic area are compiled and are also available in the rulemaking docket.
Comment on Need for Basin-Scale Evaluation of CO2 Storage 

Jean-Philippe Nicot 

Bureau of Economic Geology, The University of Texas at Austin 

It is obvious that early projects will not have basin-scale consequences. However the addition of 

successive projects and the increase in cumulative mass injected will, on the long run, have a 

regional impact. Although CO2 plumes might not, elevated pressure footprints will overlap, 

suggesting that a somehow coordinated approach is needed from the start (even if it is 

understood that it is not an issue initially). Pressure zone of influence overlap, at what could be 

considered a semi-regional level, could pressurize potential leakage pathways (faults, boreholes) 

in a way that would not have been a concern for a single project. Some mechanisms must be put 

in place that will formally inform second and following entrants of projects initiated earlier. 

Impact of CO2 injection at a more regional level consists in displacing saline and brackish water 

up dip into fresh water aquifers. This cannot be a direct concern of individual operators but must 

be the responsibility of the regulatory entity. One of the limitations of the references cited in the 

NODA is the lack of data, in particular of information on shale/clayey layers acting as a cushion 

to absorb and slowly diffuse back the increase in pressure. It follows that the responsible 

regulatory agency must collect this type of information from all operators in order to be able to 

address the regional capacity. And this, largely before pressure overlap becomes an issue. As an 

example of practical approach, in an ideal world, each operator would provide information about 

formations overlying its injection site(s) either through direct feedback on the parameters of 

interest as measured in the field or by providing core samples. 
Letter ID: 0381  

Commenter: Jean‐Philippe Nicot  

Organization: University of Texas at Austin, Bureau of Economic Geology (BEG)  

Letter ID: 0381 Comment ID: 28099 Letter Lines: 20 to 26  

Comment Code: 2220 2.2.2. Basin‐Scale Hydrologic Impacts of CO2 Storage (seeking comment)  

Comment: Comment related to the need of considering basin‐scale evaluation early  

on. Impact of numerous large CCS projects has to be considered at the basin  

scale. Often lack of data makes modeling of such large scale injection  

problematic. Regulators need to be proactive and develop a mechanism for all  

operators, especially early entrants (at which time there is no such issue) to  

provide the information needed later for assessment and modeling of basin‐scale  

impacts.  

Individual Response: The final GS rule focuses on permitting of individual GS wells and therefore does

not include requirements for basin‐scale evaluations. However, EPA agrees that proactive management

of basin scale concerns to maintain pressures within acceptable ranges is important, and encourages

owners or operators and permitting authorities to communicate planned GS activities to maximize

coordination and minimize pressure interference. If a permit applicant (or the permitting authority) is

aware of pending GS projects near a proposed well, the AoR delineation performed pursuant to §146.84

should account for the impacts of multiple owners and operators injecting within a single basin. EPA

anticipates that, where multiple operators are involved, the Director would not approve modeling that

does not account for changes that may result from all injection operations. The proximity of injection

sites to each other and the relative timing of each injection operation might indicate a possible increase

in risk for the containment of the total CO2 and/or the effects of hydraulic perturbations and brine

displacement. For additional information regarding EPA’s requirements for AoR delineation for GS

projects, see the responses to comments on the proposed rule in the rulemaking docket.  

Letter ID: 0381 Comment ID: 28100 Letter Lines: 32 to 53  

Comment Code: 2220 2.2.2. Basin‐Scale Hydrologic Impacts of CO2 Storage (seeking comment)  

Comment: It is obvious that early projects will not have basin‐scale consequences.  

However the addition of successive projects and the increase in cumulative mass  

injected will, on the long run, have a regional impact. Although CO2 plumes  

might not, elevated pressure footprints will overlap, suggesting that a somehow  
coordinated approach is needed from the start (even if it is understood that it  

is not an issue initially). Pressure zone of influence overlap, at what could be  

considered a semi‐regional level, could pressurize potential leakage pathways  

(faults, boreholes) in a way that would not have been a concern for a single  

project. Some mechanisms must be put in place that will formally inform second  

and following entrants of projects initiated earlier. Impact of CO2 injection at  

a more regional level consists in displacing saline and brackish water up dip  

into fresh water aquifers. This cannot be a direct concern of individual  

operators but must be the responsibility of the regulatory entity. One of the  

limitations of the references cited in the NODA is the lack of data, in  

particular of information on shale/clayey layers acting as a cushion to absorb  

and slowly diffuse back the increase in pressure. It follows that the  

responsible regulatory agency must collect this type of information from all  

operators in order to be able to address the regional capacity. And this,  

largely before pressure overlap becomes an issue. As an example of practical  

approach, in an ideal world, each operator would provide information about  

formations overlying its injection site(s) either through direct feedback on the  

parameters of interest as measured in the field or by providing core samples.  

Individual Response: EPA agrees with the commenter about the significance of collecting necessary site

characterization data and consideration of the cumulative effects of multiple injection sites in a given

formation. Therefore, the final GS rule requires a detailed site‐specific evaluation of local geologic

conditions (§146.82) and modeling to delineate the AoR (§146.84) investigations for actual reservoirs.

EPA clarifies that the final GS rule does not include requirements for basin‐scale evaluations. However,

EPA agrees that proactive management of basin scale concerns to maintain pressures within acceptable

ranges is important, and encourages owners or operators and permitting authorities to communicate

planned GS activities to maximize coordination and minimize pressure interference. If a permit applicant

(or the permitting authority) is aware of pending GS projects near a proposed well, the AoR delineation

performed pursuant to §146.84 should account for the impacts of multiple owners and operators

injecting within a single basin. EPA anticipates that, where multiple operators are involved, the Director

would not approve modeling that does not account for changes that may result from all injection

operations. The proximity of injection sites to each other and the relative timing of each injection

operation might indicate a possible increase in risk for the containment of the total CO2 and/or the

effects of hydraulic perturbations and brine displacement. For additional information regarding EPA’s

requirements for AoR delineation for GS projects, see the responses to comments on the proposed rule

in the rulemaking docket.

EPA recognizes that these overlap issues will become more prevalent as deployment of GS increases,

and will collect and review data on early GS projects and how they may interact. As part of its adaptive

rulemaking approach, 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.