Origins of Subsurface Methane Leaking from Nonproducing Oil and Gas Wells in Canada

Canada is home to hundreds of thousands of oil and gas wells that are no longer producing, each potentially providing a pathway for subsurface methane to be emitted into the atmosphere and contribute to global warming. The origins and drivers of high methane emissions from these wells remain uncertain. In this study, we applied a bespoke geochemical framework to samples collected from 401 Canadian nonproducing wells and determined methane origins for 112 samples originating from either surface casing vent (SCV) or wellhead emissions at 100 of these wells. We found that the presence of thermogenic methane at the SCV is a good indicator of high-emission wells, with average emission magnitudes 500 times higher than those of microbial sources. Our results also show that half of the studied wells emit a mix of thermogenic and microbial methane. Compared to previous estimates, our percentage of wells emitting microbial methane (∼23%) is three times higher, and the associated median emission rate is 1000 times greater. Finally, we highlight the potential of geochemical data and our analysis framework to inform future measurements and well integrity assessments, thereby providing critical information to guide mitigation and repair strategies.