Investigating the role of Canadian non-producing oil and gas wells in subsurface-atmosphere methane fluxes through geochemical signatures and methane origins

The presence of approximately 400,000 non-producing oil and gas wells (OGWs) in Canada and millions more globally poses significant environmental and safety issues. These wells leak methane (CH₄) and other pollutants, which exacerbate climate change, pose explosion hazards, contaminate drinking water, and damage plants and animals. Plugging all existing non-producing OGWs would cost several hundred billion dollars1, making this approach virtually impossible. However, since only 10% of these wells are responsible for >90% of the emissions2, a better strategy may be to identify and target high-emitting wells for more effective and economical mitigation efforts. It is therefore important to fully understand the processes governing methane leakage and their subsequent emissions through non-producing OGWs. Another important aspect of these efforts is the identification of well integrity failures, which may not necessarily cause high emissions to the atmosphere but can cause subsurface fluid migration, even for low-emitting wells. A modern OGW typically consists of a system of casings and cement, providing multiple barriers designed to prevent contamination. The surface casing vent (SCV), installed at the wellhead, is designed to vent gas from the annular space between the surface casing and the next casing string. Generally, methane emissions at the SCV are viewed as a sign of well integrity failure but could be unrelated if the casing intersects natural fluid migration pathways.In this study, we compiled the geochemical data of 365 OGWs from Canada, with measurements made at the component level (wellhead, SCV and surrounding soil) wherever possible. By analyzing δ13C and δ2H isotopic signatures and gas compositions, we identified the origins of our samples as primary microbial, secondary microbial, thermogenic, or abiotic. These origins were only attributed to a third of the studied wells for at least one of the three components (wellhead, SCV and surrounding soil), due to the sensitivity of this approach. We found that the presence of thermogenic methane at the SCV is a good indicator of high-emitting wells, with magnitudes of emissions 100 times higher than microbial emissions. Furthermore, our analysis revealed that a considerable number of emitting wells (~23%) produce methane of microbial origin, which is higher than previously thought (8% in the only existing meta-analysis), and with emission magnitudes that exceed previous estimates by a factor of 1,000. These results suggest that non-producing OGWs could act as bridges facilitating the diffusion of subsurface microbial methane emissions into the atmosphere. Finally, we generally found similar geochemical signatures of methane in corresponding wellhead and SCVs, suggesting that the structural integrity of these wells has been compromised and they can act as one single entity.1. Raimi, D., Krupnick, A. J., Shah, J.-S. & Thompson, A. Decommissioning Orphaned and Abandoned Oil and Gas Wells: New Estimates and Cost Drivers. Environ. Sci. Technol. 55, 10224–10230 (2021). 2. Williams, J. P., Regehr, A. & Kang, M. Methane Emissions from Abandoned Oil and Gas Wells in Canada and the United States. Environ. Sci. Technol. 55, 563–570 (2021).

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