Quantifying alkane emissions in the Eagle Ford Shale using boundary layer enhancement

Abstract

Abstract. The Eagle Ford Shale in southern Texas is home to a booming unconventional oil and gas industry, the climate and air quality impacts of which remain poorly quantified due to uncertain emission estimates. We used the atmospheric enhancement of alkanes from Texas Commission on Environmental Quality volatile organic compound monitors across the shale, in combination with back trajectory and dispersion modeling, to quantify C 2 -C 4 alkane emissions for a region in southern Texas, including the core of the Eagle Ford, for a set of 68 days from July 2013 to December 2015. Emissions were partitioned into raw natural gas and liquid storage tank sources using gas and headspace composition data, respectively, and observed enhancement ratios. We also estimate methane emissions based on typical ethane-to-methane ratios in gaseous emissions. The median emission rate from raw natural gas sources in the shale, calculated as a percentage of the total produced natural gas in the upwind region, was 0.7 % with an interquartile range (IQR) of 0.5-1.3 %, below the US Environmental Protection Agency's (EPA) current estimates. However, storage tanks contributed 17 % of methane emissions, 55 % of ethane, 82 % percent of propane, 90 % of n-butane, and 83 % of isobutane emissions. The inclusion of liquid storage tank emissions results in a median emission rate of 1.0 % (IQR of 0.7-1.6 %) relative to produced natural gas, overlapping the current EPA estimate of roughly 1.6 %. We conclude that emissions from liquid storage tanks are likely a major source for the observed non-methane hydrocarbon enhancements in the Northern Hemisphere.

Quantifying Methane and Ethane Emissions to the Atmosphere From Central and Western U.S. Oil and Natural Gas Production Regions

“…For the Eagle Ford region, there was a general lack of information on natural gas composition until the study by Zhang et al (). A previous study of Eagle Ford emissions (Roest & Schade, ) relied on reports to the Texas Commission on Environmental Quality (Pring, ) to estimate the natural gas composition from this region. We use the results from Zhang et al () to represent eastern Eagle Ford natural gas composition, and the multiple studies cited in section 5.2, which includes the study by Pring (), to represent western Eagle Ford natural gas composition.…”

“…If they had instead used our estimated abundance, 68%, their combined average percentage of produced natural gas emitted to the atmosphere would have been 1.6 ± 0.6%, which is still lower than, but within the combined uncertainties of, our estimate. Roest and Schade () used light alkane data from various sources to estimate the composition of raw natural gas and condensate tank gas from the region and then used the relative enhancements of light alkanes to attribute CH 4 emissions to raw natural gas and condensate tank emissions. It is therefore difficult to estimate how the inclusion of the data from Zhang et al () would affect their analysis.…”

Source apportionment of non-methane hydrocarbons, NOx and H2S data from a central monitoring station in the Eagle Ford shale, Texas

“…Pentane, and in extension all alkane VMRs, minimized for southeasterly air trajectories (Figure 4a), while isopentane-to-pentane ratios maximized for those directions (Figure 4b). Urban isopentaneto-pentane ratios, dominated by car traffic emissions, are typically ≥2 (Parrish et al, 1998;Warneke et al, 2007;Roest and Schade, 2017), while the same ratio is closer to 1 in areas affected dominantly by oil and gas exploration sources (Gilman et al, 2013;Petron et al, 2014;Thompson et al, 2014). For air mass trajectories from the SE sector (approx.…”

“…For air mass trajectories from the SE sector (approx. 120-170 degrees), air mass origins typically lie in the Gulf of Mexico (Roest and Schade, 2017), spend comparatively short periods over land and, traveling perpendicular to the shale axis, short distances over the main shale exploration areas (Figure 1). At the same time, these air masses traverse the city of Kenedy and the US-181 traffic corridor en-route to Karnes City.…”

Atmospheric Implications of Large C2‐C5 Alkane Emissions From the U.S. Oil and Gas Industry

“…Contribution of the Oil and Gas Sector to Emissions of C 2 -C 5 Alkanes 3.1.1. Ethane and Propane The oil and gas sector emits C 2 H 6 and C 3 H 8 primarily due to leakage during the production, processing, and transportation of natural gas (Gilman et al, 2013;Kort et al, 2016;Pétron et al, 2012;Roest & Schade, 2017). Trace amounts of C 2 H 6 and C 3 H 8 can also be produced during hydrocarbon combustion processes (Basevich et al, 2012;Gomer & Kistiakowsky, 1951;Sangwan et al, 2015;Thynne, 1962).…”