Methane Emissions from the Marcellus Shale in Southwestern Pennsylvania and Northern West Virginia Based on Airborne Measurements

Natural gas production in the United States has increased rapidly over the past decade, along with concerns about methane (CH ₄ ) fugitive emissions and its climate impacts. Quantification of CH ₄ emissions from oil and natural gas (O&NG) operations is important for establishing scientifically sound policies for mitigating greenhouse gases. We use the aircraft mass balance approach for three flight experiments in August and September 2015 to estimate CH ₄ emissions from O&NG operations over the southwestern Marcellus Shale. We estimate a mean CH ₄ emission rate as 21.2 kg/s with 28% coming from O&NG operations. The mean CH ₄ emission rate from O&NG operations was estimated to be 1.1% of total NG production. The individual best-estimate emission rates from the three flight experiments ranged from 0.78 to 1.5%, with overall limits of 0% and 3.5%. These emission rates are at the low end of other top-down studies, but consistent with the few observational studies in the Marcellus Shale region as well as the U.S. Environmental Protection Agency CH ₄ inventory. A substantial source of CH ₄ (~70% of observed CH ₄ emissions) was found to contain little ethane, possibly due to coalbed CH ₄ emitted either directly from coal mines or from wells drilled through coalbed layers in O&NG operations. Recent regulations requiring capture of gas from the completion-venting step of hydraulic fracturing appear to have reduced the atmospheric release of CH ₄ . Our study suggests that for a 20-year time scale, energy derived from the combustion of natural gas extracted from this region likely exerts a net climate benefit compared to coal.