Methane source attribution in a U.S. dry gas basin using spatial patterns of ground and airborne ethane and methane measurements

An intensive coordinated airborne and ground-based measurement study was conducted in the Fayetteville Shale in northwestern Arkansas during September and October 2015 to compare and explain potential discrepancies between top-down and bottom-up estimates of regional natural gas (NG) methane (CH4) emissions. In situ mobile downwind measurements are used to document the ethane to methane enhancement ratios (ERs) in emission plumes from NG operations in the region. Enhancement ratios are low (<2% for 87% of NG sources sampled) in this dry gas-producing region and normally distributed around 1.3% in the western half of the study area. A few sampled landfills emitted CH4 but no ethane (C2H6). Sampling drives around large chicken farms, prevalent in the region, did not detect significant downwind CH4 enhancements. In situ airborne measurements of C2H6 and CH4 from area-scale surveys over and downwind of the region documented the resulting ERs from a mix of CH4 sources. Based on these measurements, we show that on average during the measurement windows 85–95% of total CH4 emissions in the western half of the Fayetteville Shale originated from NG sources, which agrees well with bottom-up estimates from the same field study. Lower mixing ratios measured over the eastern half of the region did not support the ER analysis due to the low signal-to-noise on C2H6 measurements.