Estimate of Methane Emissions from the Marcellus Shale in Southwest Pennsylvania and Northern West Virginia Using Aircraft Observations

Natural gas production in the United States has increased rapidly over the past decade, along with concerns about methane (CH₄) leakage into the atmosphere and its climate impact. Quantification of methane emissions from oil and natural gas operations is important for establishing scientifically sound and cost-effective policies for mitigating greenhouse gases (GHGs). We present an estimate of methane emissions from oil and natural gas operations in the Marcellus Shale in Southwest Pennsylvania and Northern West Virginia based on aircraft measurements. The mass balance approach was used for three flights on three different days in August and September 2015 and methane emissions from a 77 km × 55 km area are estimated to be 39.2±2.3 kg CH₄ s^-1 (or 1.24±0.07 Tg CH₄ yr^-1). Among the total CH₄ emissions, 22.9±2.4 kg CH₄ hr⁻¹ (or 0.72 ± 0.07 Tg CH₄ yr^-1) was emitted by oil and natural gas operations during their production and processing stages. Based on the total natural gas production and CH₄ emissions from sectors other than oil and natural gas operations in this area, we estimate an averaged CH₄ leakage rate from oil and natural gas operations to be 4.1±0.5% (3.6% - 4.6%). This leakage rate is broadly consistent with the results from several recent top-down studies, but is higher than the results from other observational studies and the leakage rate estimated based on the US EPA methane emission inventory for methane. This work demonstrates the mass balance approach as a valuable tool for estimating CH₄ emissions from oil and natural gas operations. Further atmospheric measurements are needed to better assess inventories of CH₄ emissions.