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Venting and leaking of methane from shale gas development: response to Cathles et al. Robert W. Howarth & Renee Santoro & Anthony Ingraffea Received: 10 December 2011 / Accepted: 10 January 2012 / Published online: 1 February 2012 # The Author(s) 2012. This article is published with open access at Springerlink.com Abstract In April 2011, we published the first comprehensive analysis of greenhouse gas (GHG) emissions from shale gas obtained by hydraulic fracturing, with a focus on methane emis
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  Venting and leaking of methane from shale gasdevelopment: response to Cathles et al. Robert W. Howarth  &  Renee Santoro  & Anthony Ingraffea Received: 10 December 2011 /Accepted: 10 January 2012 /Published online: 1 February 2012 # The Author(s) 2012. This article is published with open access at Springerlink.com Abstract  In April 2011, we published the first comprehensive analysis of greenhouse gas(GHG) emissions from shale gas obtained by hydraulic fracturing, with a focus on methaneemissions. Our analysis was challenged by Cathles et al. (2012). Here, we respond to thosecriticisms. We stand by our approach and findings. The latest EPA estimate for methaneemissions from shale gas falls within the range of our estimates but not those of Cathles et al.which are substantially lower. Cathles et al. believe the focus should be just on electricitygeneration, and the global warming potential of methane should be considered only on a 100-year time scale. Our analysis covered both electricity (30% of US usage) and heat generation (the largest usage), and we evaluated both 20- and 100-year integrated timeframes for methane. Both time frames are important, but the decadal scale is critical, giventhe urgent need to avoid climate-system tipping points. Using all available information andthe latest climate science, we conclude that for most uses, the GHG footprint of shale gas isgreater than that of other fossil fuels on time scales of up to 100 years. When used togenerate electricity, the shale-gas footprint is still significantly greater than that of coal at decadal time scales but is less at the century scale. We reiterate our conclusion from our April 2011 paper that shale gas is not a suitable bridge fuel for the 21st Century. 1 Introduction Promoters view shale gas as a bridge fuel that allows continued reliance on fossil fuels whilereducing greenhouse gas (GHG) emissions. Our April 2011 paper in  Climatic Change challenged this view (Howarth et al. 2011). In the first comprehensive analysis of theGHG emissions from shale gas, we concluded that methane emissions lead to a large Climatic Change (2012) 113:537  –  549DOI 10.1007/s10584-012-0401-0 Electronic supplementary material  The online version of this article (doi:10.1007/s10584-012-0401-0)contains supplementary material, which is available to authorized users.R. W. Howarth ( * ) :  R. SantoroDepartment of Ecology & Evolutionary Biology, Cornell University, Ithaca, NY 14853, USAe-mail: rwh2@cornell.eduA. Ingraffea School of Civil and Environmental Engineering, Cornell University, Ithaca, NY 14853, USA  GHG footprint, particularly at decadal time scales. Cathles et al. (2012) challenged our work. Here, we respond to the criticisms of Cathles et al. (2012), and show that most havelittle merit. Further, we compare and contrast our assumptions and approach with other studies and with new information made available since our paper was published. After carefully considering all of these, we stand by the analysis and conclusions we publishedin Howarth et al. (2011). 2 Methane emissions during entire life cycle for shale gas and conventional gas Cathles et al. (2012) state our methane emissions are too high and are  “ at odds with previousstudies. ”  We strongly disagree. Table 1 compares our estimates for both conventional gasand shale gas (Howarth et al. 2011) with 9 other studies, including 7 that have only becomeavailable since our paper was published in April 2011, listed chronologically by time of  publication. See Electronic Supplementary Materials for details on conversions and calcu-lations. Prior to our study, published estimates existed only for conventional gas. As wediscussed in Howarth et al. (2011), the estimate of Hayhoe et al. (2002) is very close to our  mean value for conventional gas, while the estimate from Jamarillo et al. (2007) is lower andshould probably be considered too low because of their reliance on emission factors from a 1996 EPA report (Harrison et al. 1996). Increasing evidence over the past 15 years hassuggested the 1996 factors were low (Howarth et al. 2011). In November 2010, EPA (2010) released parts of their first re-assessment of the 1996 methane emission factors, increasingsome emissions factors by orders of magnitude. EPA (2011a ), released just after our paper was published in April, used these new factors to re-assess and update the U.S. nationalGHG inventory, leading to a 2-fold increase in total methane emissions from the natural gasindustry. Table 1  Comparison of published estimates for full life-cycle methane emissions from conventional gas andshale gas, expressed per unit of Lower Heating Value (gC MJ − 1 ). Studies are listed by chronology of  publication dateConventional gas Shale gasHayhoe et al. (2002) 0.57 *Jamarillo et al. (2007) 0.15 *Howarth et al. (2011) 0.26  –  0.96 0.55  –  1.2EPA (2011a ) 0.38 0.60 + Jiang et al. (2011) * 0.30Fulton et al.(2011) 0.38 ++ *Hultman et al. (2011) 0.35 0.57Skone et al. (2011) 0.27 0.37Burnham et al. (2011) 0.39 0.29Cathles et al. (2012) 0.14  –  0.36 0.14  –  0.36See Electronic Supplemental Materials for details on conversions * Estimates not provided in these reports + Includes emissions from coal-bed methane, and therefore may under-estimate shale gas emissions ++ Based on average for all gas production in the US, not just conventional gas, and so somewhat over-estimates conventional gas emissions538 Climatic Change (2012) 113:537  –  549  The new estimate for methane emissions from conventional gas in the EPA (2011a )inventory, 0.38 g C MJ − 1 , is within the range of our estimates: 0.26 to 0.96 g C MJ − 1 (Table 1). As discussed below, we believe the new EPA estimate may still be too low, due toa low estimate for emissions during gas transmission, storage, and distribution. Several of the other recent estimates for conventional gas are very close to the new EPA estimate(Fulton et al. 2011; Hultman et al. 2011; Burnham et al. 2011). The Skone et al. (2011) value is 29% lower than the EPA estimate and is very similar to our lower-end number. Cathles et al. (2012) present a range of values, with their high end estimate of 0.36 g C MJ − 1  beingsimilar to the EPA estimate but their low end estimate (0.14 g C MJ − 1 ) far lower than anyother estimate, except for the Jamarillo et al. (2007) estimate based on the old 1996 EPAemission factors.For shale gas, the estimate derived from EPA (2011a ) of 0.60 g C MJ − 1 is within our estimated range of 0.55 to 1.2 g C MJ − 1 (Table 1); as with conventional gas, we feel the EPAestimate may not adequately reflect methane emissions from transmission, storage, anddistribution. Hultman et al. (2011) provide an estimate only slightly less than the EPAnumber. In contrast, several other studies present shale gas emission estimates that are 38%(Skone et al. 2011) to 50% lower (Jiang et al. 2011; Burnham et al. 2011) than the EPA estimate. The Cathles et al. (2012) emission estimates are 40% to 77% lower than the EPAvalues, and represent the lowest estimates given in any study.In an analysis of a PowerPoint presentation by Skone that provided the basis for Skone et al. (2011), Hughes (2011a ) concludes that a major difference between our work and that of  Skone and colleagues was the estimated lifetime gas production from a well, an important factor since emissions are normalized to production. Hughes (2011a ) suggests that Skonesignificantly overestimated this lifetime production, and thereby underestimated the emis-sions per unit of energy available from gas production (see Electronic SupplementalMaterials). We agree, and believe this criticism also applies to Jiang et al. (2011). The lifetime production of shale-gas wells remains uncertain, since the shale-gas technology is sonew (Howarth and Ingraffea  2011). Some industry sources estimate a 30-year lifetime, but the oldest shale-gas wells from high-volume hydraulic fracturing are only a decade old, and production of shale-gas wells falls off much more rapidly than for conventional gas wells.Further, increasing evidence suggests that shale-gas production often has been exaggerated(Berman 2010; Hughes 2011a , 2011b; Urbina  2011a , 2011b). Our high-end methane estimates for both conventional gas and shale gas are substantiallyhigher than EPA (2011a ) (Table 1), due to higher emission estimates for gas storage, transmission, and distribution ( “ downstream ”  emissions). Note that our estimated rangefor emissions at the shale-gas wells ( “ upstream ”  emissions of 0.34 to 0.58 g C MJ − 1 ) agreevery well with the EPA estimate (0.43 g C MJ − 1 ; see Electronic Supplementary Materials).While EPA has updated many emission factors for natural gas systems since 2010 (EPA2010, 2011a , 2011b), they continue to rely on the 1996 EPA study for downstream emissions. Updates to this assumption currently are under consideration (EPA 2011a ). Inthe meanwhile, we believe the EPA estimates are too low (Howarth et al. 2011). Note that the downstream emission estimates of Hultman et al. (2011) are similar to EPA (2011a ), while those of Jiang et al. (2011) are 43% less, Skone et al. (2011) 38% less, and Burnham et  al. (2011) 31% less (Electronic Supplemental Materials). One problem with the 1996 emission factors is that they were not based on random sampling or a comprehensiveassessment of actual industry practices, but rather only analyzed emissions from modelfacilities run by companies that voluntarily participated (Kirchgessner et al. 1997). Theaverage long-distance gas transmission pipeline in the U.S. is more than 50 years old, andmany cities rely on gas distribution systems that are 80 to 100 years old, but these older  Climatic Change (2012) 113:537  –  549 539  systems were not part of the 1996 EPA assessment. Our range of estimates for methaneemissions during gas storage, transmission, and distribution falls well within the range given by Hayhoe et al. (2002), and our mean estimate is virtually identical to their   “  best estimate ” (Howarth et al. 2011). Nonetheless, we readily admit that these estimates are highlyuncertain. There is an urgent need for better measurement of methane fluxes from all partsof the natural gas industry, but particularly during completion of unconventional wells andfrom storage, transmission, and distribution sectors (Howarth et al. 2011).EPA proposed new regulations in October 2009 that would require regular reporting onGHG emissions, including methane, from natural gas systems (EPA 2011c). ChesapeakeEnergy Corporation, the American Gas Association, and others filed legal challenges tothese regulations (Nelson 2011). Nonetheless, final implementation of the regulations seemslikely. As of November 2011, EPA has extended the deadline for the first reporting toSeptember 2012 (EPA 2011c). These regulations should help evaluate methane pollution,although actual measurements of venting and leakage rates will not be required, and thereporting requirement as proposed could be met using EPA emission factors. Field measure-ments across a range of well types, pipeline and storage systems, and geographic locationsare important for better characterizing methane emissions. 3 How much methane is vented during completion of shale-gas wells? During the weeks following hydraulic fracturing, frac-return liquids flow back to the surface,accompanied by large volumes of natural gas. We estimated substantial methane venting tothe atmosphere at this time, leading to a higher GHG footprint for shale gas than for conventional gas (Howarth et al. 2011). Cathles et al. (2012) claim we are wrong and assert  that methane emissions from shale-gas and conventional gas wells should be equivalent.They provide four arguments: 1) a physical argument that large flows of gas are not possiblewhile frac fluids fill the well; 2) an assertion that venting of methane to the atmospherewould be unsafe; 3) a statement that we incorrectly used data on methane capture duringflowback to estimate venting; and 4) an assertion that venting of methane is not in theeconomic interests of industry. We disagree with each point, and note our methane emissionestimates during well completion and flowback are quite consistent with both those of EPA(2010, 2011a , b) and Hultman et al. (2011). Cathles et al. state that gas venting during flowback is low, since the liquids in the wellinterfere with the free flow of gas, and imply that this condition continues until the well goesinto production. While it is true that liquids can restrict gas flow early in the flow-back  period, gas is freely vented in the latter stages. According to EPA (2011d), during wellcleanup following hydraulic fracturing  “  backflow emissions are a result of free gas being produced by the well during well cleanup event, when the well also happens to be producingliquids (mostly water) and sand. The high rate backflow, with intermittent slugs of water andsand along with free gas, is typically directed to an impoundment or vessels until the well isfully cleaned up, where the free gas vents to the atmosphere while the water and sand remainin the impoundment or vessels. ”  The methane emissions are  “ vented as the backflow entersthe impoundment or vessels ”  (EPA 2011d). Initial flowback is 100% liquid, but this quickly becomes a two-phase flow of liquid and gas as backpressure within the fractures declines(Soliman & Hunt  1985; Willberg et al. 1998; Yang et al. 2010; EPA 2011a , d). The gas  produced is not in solution, but rather is free-flowing with the liquid in this frothy mix. Thegas cannot be put into production and sent to sales until flowback rates are sufficientlydecreased to impose pipeline pressure. 540 Climatic Change (2012) 113:537  –  549

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