Princeton University

REVIEW COMMENTS

Michael A. CeliaDepartment of Civil and Environmental Engineering

Princeton University

Princeton University

SUMMARY OF COMMENTS BY

� David Yoxtheimer, Penn State University

� Avner Vengosh, Duke University

� Michael Celia, Princeton University

COMMENTS BY D. YOXTHEIMER

� The most likely cause of groundwater degradation is from surface

spills of drilling related wastes that could seep downward into an

aquifer.

� Improperly constructed gas or oil wells could also pose a risk to

groundwater quality if not adequately sealed.

� Contamination of shallow aquifers by deep fluids is more likely to be

caused by inadequate well completions than by induced fractures.

� Regarding accidents and leaks above ground, lined well pads with

secondary containment is a common preventive measure.

Princeton University

COMMENTS BY A. VENGOSH

� Chemical and isotope data may be used to identify the amount of flow

between the deep and shallow zones. This can provide more convincing

verification of models.

� One of the key missing points in this report is the simulation of gas

migration due to loss of well integrity.

� The recent work of Osborn et al. (2011) should be cited in the report.

That work shows geochemical and isotopic evidence for stray gas

migration in northeastern Pennsylvania, with leakage along well casing

the likely pathway.

� The notion that “migration paths and sources (of gas) cannot be

determined adequately” as stated on page 22 is not correct.

� A framework for a viable monitoring program is essential and should be

addressed in the report.

Princeton University

COMMENTS BY M. CELIA

� The approach based on “worst-case scenarios” should be reconsidered.

An alternate approach that includes elements of probabilistic models

might be considered.

� Definitions of key parameters need to be stated explicitly, especially with

respect to averaging length scales.

� Leaky wells should be included in the analysis.

� Are there scenarios where the fracking procedure is repeated, so that

more than one two-hour period of imposed high pressure occurs?

� The specific domains, and especially the initial and boundry conditions,

need to be stated more clearly.

� The range of parameter values used in the simulations should be

consistent across the different scenarios.

Princeton University

COMMENTS BY M. CELIA (PAGE 2)

� Several opportunities for collaboration appear to exist, including (1)

inclusion of geochemistry/isotope analysis in the models, and (2)

interactions with the Above-Ground Processes group to define scenarios

with sources of GW contamination being above the potable aquifer

(instead of below).

� Some consideration of the (large) volumes of source water needed for

fracking may be considered, especially if these are drawn from the

subsurface.

� Some consideration of competing uses of the subsurface might be

considered. For example, fracking might preclude CCS due to fracturing

of caprock.

Princeton University

COMMENTS BY M. CELIA (PAGE 3)

� The overall approach used in this study addresses the important

environmental questions associated with shale gas production.

� The quality of the research team is excellent.

� All three external reviewers appreciate the opportunity to participate in

this important activity.

Princeton University