Groundwater methane in relation to oil and gas development and shallow coal seams in the Denver-Julesburg Basin of Colorado

Owen A. Sherwood, Jessica D. Rogers, Greg Lackey, Troy L. Burke, Stephen G. Osborn, and Joseph N. Ryan
PNAS first published July 11, 2016 https://doi.org/10.1073/pnas.1523267113

  1. Edited by Peter H. Gleick, Pacific Institute for Studies in Development, Environment, and Security, Oakland, CA, and approved June 7, 2016 (received for review November 24, 2015)

Significance

The impact of unconventional oil and gas development on groundwater quality remains controversial. We use an archive of public domain data to examine factors influencing the distribution and sources of groundwater methane in the oil- and gas-producing Denver-Julesburg Basin of Colorado. Thermogenic stray gas sourced from deep oil and gas reservoirs impacted 42 water wells in 32 separate cases at a rate of about two cases per year from 2001 to 2014. The rate did not change after the introduction of horizontal drilling combined with high-volume hydraulic fracturing in 2010. The risk of stray gas contamination ranged from 0.12% of 35,000 water wells in the basin (lower estimate) to 4.5% of the 924 water wells that were tested (upper estimate).

Abstract

Unconventional oil and gas development has generated intense public concerns about potential impacts to groundwater quality. Specific pathways of contamination have been identified; however, overall rates of contamination remain ambiguous. We used an archive of geochemical data collected from 1988 to 2014 to determine the sources and occurrence of groundwater methane in the Denver-Julesburg Basin of northeastern Colorado. This 60,000-km2 region has a 60-y-long history of hydraulic fracturing, with horizontal drilling and high-volume hydraulic fracturing beginning in 2010. Of 924 sampled water wells in the basin, dissolved methane was detected in 593 wells at depths of 20–190 m. Based on carbon and hydrogen stable isotopes and gas molecular ratios, most of this methane was microbially generated, likely within shallow coal seams. A total of 42 water wells contained thermogenic stray gas originating from underlying oil and gas producing formations. Inadequate surface casing and leaks in production casing and wellhead seals in older, vertical oil and gas wells were identified as stray gas migration pathways. The rate of oil and gas wellbore failure was estimated as 0.06% of the 54,000 oil and gas wells in the basin (lower estimate) to 0.15% of the 20,700 wells in the area where stray gas contamination occurred (upper estimate) and has remained steady at about two cases per year since 2001. These results show that wellbore barrier failure, not high-volume hydraulic fracturing in horizontal wells, is the main cause of thermogenic stray gas migration in this oil- and gas-producing basin.

Footnotes

  • ↵1To whom correspondence should be addressed. Email: owen.sherwood{at}colorado.edu.

  • Author contributions: O.A.S. and J.N.R. designed research; O.A.S., J.D.R., G.L., T.L.B., and S.G.O. performed research; O.A.S., J.D.R., G.L., T.L.B., and S.G.O. analyzed data; and O.A.S. and J.N.R. wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission.

  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1523267113/-/DCSupplemental.

Freely available online through the PNAS open access option.

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Groundwater impacts of unconventional oil and gas
Owen A. Sherwood, Jessica D. Rogers, Greg Lackey, Troy L. Burke, Stephen G. Osborn, Joseph N. Ryan
Proceedings of the National Academy of Sciences Jul 2016, 201523267; DOI: 10.1073/pnas.1523267113
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