Methane Emissions from Abandoned Oil and Gas Wells: New Measurements and Analysis of Temporal Variations and Land Cover

Abstract

Abandoned oil and gas (AOG) wells can provide pathways for the migration of subsurface fluids such as methane to the atmosphere. Recent estimates from direct measurements of methane fluxes show that methane emissions from AOG wells are in the range of 4-7% of Pennsylvania’s anthropogenic emissions. These emissions are not included in any emissions inventory. Field measurement campaigns have been conducted from July 2013 through October 2014 at a total of forty-seven well sites. These campaigns involved measurements of gas fluxes from wells and the immediate surrounding area as well as at control sites near the wells. Eighty-one percent of the wells have positive flow rates (they emit methane), 6.4% have flow rates indistinguishable from zero, and 13% have small negative flow rates. The presence of other alkanes (ethane, propane, and n-butane) in the wells with positive flow rates suggests that the methane emissions are not produced from microbial processes and are thus of thermogenic origin. The isotopic composition of the methane also indicates that the methane is of thermogenic origin. A total of 23 wells have each been measured between 2 and 9 times. These multiple measurements allow the variability in methane emission rates at these wells to be assessed, including the degree to which the variations are associated with seasonal changes. Overall the high-emitting wells (wells with flow rates greater than 104 mg/hr of CH4) displayed only a small degree of temporal variability compared to low-emitting wells. There appears to be no relationship between the number of times a well is measured and the variability in flow rate measurements. Preliminary results indicate that season does not affect the emission rates for wells or for control locations near wells. Land cover (forest, grassland, or wetland) is also explored as a potential cause for variation among different well and control measurements. Preliminary results indicate that land cover does not impact the emission rates for well locations, but does affect the emission rates for control locations. These results could impact the way future measurements are conducted and also serve to provide a more accurate emission rate estimate.