Estimating Methane Emissions from Orphaned Oil and Gas Wells in Kentucky

From February 2023 through August 2024, Kentucky Geological Survey (KGS) scientists measured methane emissions on 119 orphaned oil and gas wells distributed among 26 counties in eastern, south-central, and western Kentucky. Early measurements were done in partnership with the Well Done Foundation using a Ventbuster flowmeter (n = 41), whereas later measurements (n = 78) were done by KGS scientists using a SEMTECH High-Flow 2 sampler or a static chamber designed and fabricated at the University of Kentucky. The measurements were done in support of the Kentucky Division of Oil and Gas plugging program funded by the Bipartisan Infrastructure Law’s State Orphaned Well Program (Initial Grant Program, IGP). In addition, KGS scientists conducted 244 screening and post-plugging measurements on 32 wells. Methane emissions measurements—termed ‘main protocol’—were conducted on oil (n = 82), gas (n = 32), and injection (n = 5) wells. The average methane emission rate for all wells is 30.7 g/hr; however, the data are right-skewed because a small number of wells (n = 9) account for 90% of emissions. Emissions from gas wells (66%) are more likely to exceed 1 g/hr as compared to oil wells (24%), and average emissions for gas wells (77.1 g/hr) are greater than oil (14.5 g/hr) or injection wells (4.1 g/hr), although the standard deviations overlap. The magnitude of methane emissions is not correlated with completion age (r = 0.07) or total depth (r = 0.11). It appears, however, that wells with complete or nearly complete infrastructure are more likely to be high methane emitters as compared to wells where all or part of the well is open to atmosphere. We believe the infrastructure acts to maintain at least a partial lid on the reservoir, thereby maintaining reservoir pressure to drive methane emissions. Maintenance of reservoir pressure also precludes intrusion of atmospheric gas into the reservoir. Using the measured methane emissions (i.e., statistical sample) to estimate emissions for wells plugged in the IGP was done using statistical bootstrapping. This procedure, which uses measured data to produce a probability distribution for the mean, is useful for defining confidence intervals for non-normally distributed data. The results provide lower and upper bounds of 11 and 55 g/hr for the 95% confidence interval for the average emission rate. Applied to wells in the IGP, this suggests a range of avoided methane emissions equal to 61.2 to 305.9 tonnes per year.