Orphaned Oil & Gas Well Methane Emission Rates Quantified with Gaussian Plume Inversions of Ambient Observations

Abstract. Annually, ~ 3.6 million abandoned oil and gas wells in the U.S. emit a combined ~ 2.6 Tg methane (CH₄) adversely affecting climate and regional air quality. However, these estimates depend on emission factors derived from measuring sub-populations of wells that vary by orders of magnitude due to very limited field sampling and poorly characterized distributions. Currently, U.S. protocols to remediate orphaned wells lacks standardized quantification methods needed to both prioritize plugging and account for emission reductions. Therefore, sensitive, reliable, affordable, and scalable CH₄ flux quantification methods are needed. We report the use of a simple Gaussian plume method where the dispersion parameters are constrained by in situ ground measurements of CH₄ concentration at four locations 7.5 – 49 m downwind of the orphan well as well as local winds to estimate the leak rate from an orphan well. We derive a flux of 10.53 ± 1.16 kg CH₄ h^-1 during a venting procedure in April 2023 that agrees with the directly measured volumetric flow rate of 9.00 ± 0.25 kg CH₄ h^-1. This is 71 % greater than the 5.3 kg CH₄ h^-1 flux measured 7-months prior. Additionally, we discovered a secondary leak through the surface-casing inferred as 0.43–0.67 kg CH₄ h^-1 by both our ground Gaussian analysis and by transecting the plume with an uncrewed aerial system (UAS). We show that in situ determination of the dispersion parameters used in our Gaussian inversions allows us to measure methane emissions to 15 % accuracy significantly reducing errors when compared to standard practice of assuming stability class. Our results help develop simpler methods and protocols for robust orphan well emission quantification that can be used for reporting.