Harmonization of initial estimates of shale gas life cycle greenhouse gas emissions for electric power generation

Footnotes

• ↵¹To whom correspondence should be addressed. Email: garvin.heath{at}nrel.gov.

• Author contributions: G.A.H. designed research; G.A.H. and P.O. performed research; G.A.H. and P.O. analyzed data; and G.A.H., D.J.A., and M.B. wrote the paper.

• The authors declare no conflict of interest.

• ⁱWeber and Clavin (25) is a Monte Carlo-based synthesis of results from six unconventional gas LCAs and thus included for perspective but not harmonization.

• ⁱⁱFor rhetorical simplicity, these eight LCAs will sometimes be referred to by the first author’s last name.

• ⁱⁱⁱRestimulation is a synonymous term to recompletion; workover can refer to well maintenance without hydraulic fracturing (29, 30), or mean recompletion (31).

• ^ivHeath et al. (15) didn’t create their own comparison case, but rather compared Barnett shale results to harmonization of 42 references (26) that collectively focus on a diverse set of conventional or domestic gas types. Laurenzi and Jersey (16) compared their results to coal only; we use the results of O’Donoughue et al. (26) for comparison with Laurenzi and Jersey where necessary.

• ^vNote that each reference could report more than one estimate.

• ^viThe findings reported here are robust to use of the probability distribution of EUR for wells in all shale gas plays in the United States. Both distributions are described in SI Text and Table S4.

• ^viiThere is also the possibility of differences in definition of methane leakage, where some might only include methane contained in natural gas that is unintentionally released to the atmosphere (often referred to as fugitive emissions), others might additionally include methane in natural gas intentionally leaked to the atmosphere (often referred to as vented emissions), and still others might additionally include methane not just contained in leakage of natural gas but also emissions of methane from combustion or even from tanks storing coproducts (condensate or oil). For instance, Heath et al. (15) included fugitive, vented, and combustion-emitted methane in their leakage estimate, whereas Skone et al. (11) and Burnham et al. (13) only included fugitive and vented methane emissions. Harmonization to a common definition of methane leakage was beyond the scope of this study because the data were not reported at this resolution.

• ^viiiA leakage rate reported in these units enables rapid estimation of methane emissions based on a known amount of produced natural gas.

• ^ixFor gas produced from oil wells, only GHG emissions starting with gas processing are assigned to the natural gas industry; the EPA assigns oil production GHG emissions, including those related to associated gas, to the oil industry (48). This approach is consistent with what is known as the product-purpose coproduct allocation philosophy (47).

• ^xSee www.nrel.gov/harmonization for a complete list.

• ^xiThe unconventional gas well EUR used here is the median for active shale plays in the United States (2.2 bcf) as determined through our analysis of EIA data (41), and well lifetime is assumed to be 30 y. Conventional gas well EUR is assumed to be 1 bcf (a central estimate from LCAs considered in this study), and well lifetime is 30 y.

• ↵*This Direct Submission article had a prearranged editor.

• This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1309334111/-/DCSupplemental.