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Responses on each topic area are compiled and are also available in the rulemaking docket.

Letter ID: 0288 

Commenter: Anonymous7 

 

Letter ID: 0288 Comment ID: 25136 Letter Lines: 17 to 53 

Comment Code: 3100 2.1 Proposed Geologic Siting Requirements (40 CFR 146.83) 

Comment: COMMENT ON CARBON SEQUESTRATION 

Geology, like hydrology is a science of averaged parameter values. When 

determining lithologic properties (e.g.. transmissivity, thickness, threedimensional position, etc.) of a geologic stratum, scientists estimate these 

properties based on collection of data at a small number points selected from an 

infinite number of possible points. The more deeply buried the stratum, the 

greater the cost of data collection and, so, the number of data points tends to 

diminish with depth. There is no way to know how representative the collected 

data are of a geologic stratum. 

As an example, imagine driving down a road at 60 mph and hitting a hole in the 

highway. In 20 million years from now, that highway likely will be buried under 

hundreds of feet of sediment. If a dozen test holes are drilled from the land 

surface, though the highway roadbed, the chance that that hole would be 

intersected is minuscule, yet that hole in the roadbed may be the most important 

feature of the highway. 

Not all data are collected from drilling cores, there also are temperature data, 

geophysical data, etc. An uncertainty analysis is performed to provide a sense 

of how reliable the modeling results are, but the bottom line" is that the "weak 

links" in the protectiveness of a confining unit are still likely to be unknown. 

While this lack of knowledge is not a lethal blow to modeling for, say, watersupply, these weak links can lead to serious consequences where the purpose is 

modeling the injection of supercritical CO2 fluids. 

When we think of very deep strata that are, say, thousands of feet below land 

surface, we take some "comfort" in their depth. We take this comfort, because 

it's difficult to imagine that, absent a deep mappable fault, a fluid rupturing a 

very deep confining layer is likely to reach the land surface. However, where 

injection strata and confining units are shallower, our level of comfort 

decreases, and rightly so. It is because of the lack of sufficient data to 

accurately define the nature of the subsurface, that EPA needs to take a 

protective approach to options for: 1) injecting into formations in or above the 

lowermost USDW, 2) injecting below the lowermost USDW when that USDW is 

"relatively" shallow, and 3) injecting into a formation overlain by a brittle 

confining layer, unless that confining layer is very deep. 

1
2

Individual Response: EPA agrees that permitting decisions regarding GS sites must account for 

a large amount of site-specific information. For this reason, the GS regulation requires the owner 

or operator to submit data about the site, including information on the porosity, permeability, 

mineralogy, and geochemistry of the targeted injection and confining zones. Operators must also 

present information on the compatibility of the CO2 stream with fluids in the injection zone and 

minerals in both the injection and the confining zone(s), as determined through the formation 

testing program and on the compatibility of the CO2 with the materials used to construct the 

well, as required at §146.82(c)(3).