Methane Emissions from Conventional and Unconventional Natural Gas Production Sites in the Marcellus Shale Basin

Abstract

There is a need for continued assessment of methane (CH4) emissions associated with natural gas (NG) production, especially as recent advancements in horizontal drilling combined with staged hydraulic fracturing technologies have dramatically increased NG production (we refer to these wells as "unconventional" NG wells). In this study, we measured facility-level CH4 emissions rates from the NG production sector in the Marcellus region, and compared CH4 emissions between unconventional NG (UNG) well pad sites and the relatively smaller and older "conventional" NG (CvNG) sites that consist of wells drilled vertically into permeable geologic formations. A top-down tracer-flux CH4 measurement approach utilizing mobile downwind intercepts of CH4, ethane, and tracer (nitrous oxide and acetylene) plumes was performed at 18 CvNG sites (19 individual wells) and 17 UNG sites (88 individual wells). The 17 UNG sites included four sites undergoing completion flowback (FB). The mean facility-level CH4 emission rate among UNG well pad sites in routine production (18.8 kg/h (95% confidence interval (CI) on the mean of 12.0-26.8 kg/h)) was 23 times greater than the mean CH4 emissions from CvNG sites. These differences were attributed, in part, to the large size (based on number of wells and ancillary NG production equipment) and the significantly higher production rate of UNG sites. However, CvNG sites generally had much higher production-normalized CH4 emission rates (median: 11%; range: 0.35-91%) compared to UNG sites (median: 0.13%, range: 0.01-1.2%), likely resulting from a greater prevalence of avoidable process operating conditions (e.g., unresolved equipment maintenance issues). At the regional scale, we estimate that total annual CH4 emissions from 88 500 combined CvNG well pads in Pennsylvania and West Virginia (660 Gg (95% CI: 500 to 800 Gg)) exceeded that from 3390 UNG well pads by 170 Gg, reflecting the large number of CvNG wells and the comparably large fraction of CH4 lost per unit production. The new emissions data suggest that the recently instituted Pennsylvania CH4 emissions inventory substantially underestimates measured facility-level CH4 emissions by >10-40 times for five UNG sites in this study.

Mobile measurement of methane emissions from natural gas developments in northeastern British Columbia, Canada

“…As shown in Fig. 8, compressor stations emitted most frequently (70 % emission frequency), which we expected based on the results of Omara et al (2016). However, due to our low sample size relative to well pads, we would need to sample more compressor stations to arrive at a statistically significant estimate.…”

“…Our minimum detection limit of 0.59 g s −1 was used as the emission factor for wells. Facility emission volumes are from Omara et al (2016) amounts of infrastructure allows for a comparison between our CH 4 volume estimate and those of Peischl et al (2016) and Karion et al (2013).…”

Quantifying methane emissions from natural gas production in north-eastern Pennsylvania

“…To convert the production rate into an emission rate, we need to assume a first guess as to the expected leakage from wells in the area. A first-guess natural gas emission rate of 0.13 % was applied to the production value of each of the 7000+ producing unconventional wells based on the median rate from Omara et al (2016). The natural gas emission rate was then converted to a CH 4 emission rate by assuming a CH 4 composition in the natural gas of 95 % (Peischl et al, 2015).…”

“…Emissions from sources in the natural gas industry can be temporally variable and have a wide range of values depending on a number of factors, such as the quality and age of the device and the gas pressure moving through the component. Furthermore, recent studies have shown that a majority of emissions come from a small percentage of devices, often referred to as "super-emitters", creating a long-tail distribution of emission sources (Brandt et al, 2014;Omara et al, 2016;Zavala-Araiza et al, 2015Frankenberg et al, 2016). These factors make it difficult to sample enough devices and adequately describe the mean emission rate, thus allowing for significant representation errors in the emission factors.…”

“…Reaching peak production by the end of 2015, the Marcellus is the largest producer of shale in the USA, producing 17 000 million standard cubic feet per day (MMSCFD) of natural gas (EIA, 2016a). A bottom-up study measuring emissions from 17 unconventional well sites in the Marcellus found a median emission rate from the wells of 0.13 % of production but estimated a mean emission rate between 0.38 and 0.86 % of production due to the potential presence of super-emitters, which would skew the mean emission rate towards values higher than the median (Omara et al, 2016). An aircraft mass balance study over north-eastern Pennsylvania calculated an emission rate between 0.18 and 0.41 %, a number that accounted for emissions from the production, processing, and transmission of the gas (Peischl et al, 2015).…”