Little evidence for significant increases of methane emissions from oil and gas operations in the U.S.

Recent studies on whether methane (CH4) emissions from oil and natural gas (ONG) operations in the U.S. have increased are still inconclusive. To provide observational evidence we carefully analyzed the in-situ CH4 measurements from the NOAA/ESRL Global Greenhouse Gas Reference Network (GGGRN) for the best estimates of CH4 trends for 2006-2016. Methane data from more than 20 surface and aircraft sites across the U.S. were included in this study. Variations of sampling frequencies in different seasons were taken into account for accurate trend detection. Correlations among measurements within short sampling intervals were also considered for uncertainty estimates. We found that most of our sites had similar CH4 trends of 6 ppb/yr, which was comparable with the recent global background CH4 trend. Substantially higher growth rates were found at the Southern Great Plain site in Oklahoma (SGP, downwind of the Eagle Ford, Barnett Shale and Woodford ONG fields) and the Dahlen sites in North Dakota (DND, downwind of the Bakken ONG field), which indicated influences from regional ONG activities. Ethane (C2H6) measurements from SGP (C2H6 measurements were not available from DND) and propane (C3H8) measurements from both SPG and DND exhibited significant increasing trends, while trends at other sites were either non-significant (trend < 2*S.D.) or only marginally significant. Linear correlations were well identified for surface C3H8 and CH4 enhancements at these two sites, relative to observations at higher altitudes. However, by applying the observed enhancement ratios of surface C3H8 /CH4 and the C3H8 trends (as indicator for ONG emissions) on CH4 trend estimates, we would infer much larger surface CH4 trends than what we actually observed at these two sites. This discrepancy suggests that using enhancement ratios of C3H8 /CH4 is not likely a reliable approach to compute CH4 emission trends.