Measurements of Atmospheric Methane Emissions from Stray GasMigration: A Case Study from the Marcellus Shale

Understanding emissions of methane from legacyand ongoing shale gas development requires both regional studiesthat assess the frequency of emissions and case studies that assesscausation. We present thefirst direct measurements of emissions ina case study of a putatively leaking gas well in the largest shale gasplay in the United States. We quantify atmospheric methaneemissions in farmland >2 km from the nearest shale gas well citedfor casing and cementing issues. Wefind that emissions are highlyheterogeneous as they travel long distances in the subsurface.Emissions were measured near observed patches of dead vegetationand methane bubbling from a stream. An eddy covariancefluxtower, chamberflux measurements, and a survey of enhancementsof the near-surface methane mole fraction were used to quantifyemissions and evaluate the spatial and temporal variability. We combined eddy covariance measurements with the survey of themethane mole fraction to estimate total emissions over the study area (2,800 m2). Estimated at similar to 6kgCH4day-1, emissions werespatially heterogeneous but showed no temporal trends over 6 months. The isotopic signature of the atmospheric CH4source(delta 13CH4) was equal to-29 parts per thousand, consistent with methane of thermogenic origin and similar to the isotopic signature of the gasreported from the nearest shale gas well. While the magnitude of emissions from the potential leak is modest compared to largeemitters identified among shale gas production sites, it is large compared to estimates of emissions from single abandoned wells.Since other areas of emissions have been identified close to this putatively leaking well, our estimate of emissions likely representsonly a portion of total emissions from this event. More comprehensive quantification will require more extensive spatial andtemporal sampling of the locations of gas migration to the surface as well as an investigation into the mechanisms of subsurface gasmigration. This work highlights an example of atmospheric methane emissions from potential stray gas migration at a location farfrom a well pad, and further research should explore the frequency and mechanisms behind these types of events to inform carefuland strategic natural gas development.