Abandoned oil and gas wells in Western Canada: methane measurements and emission estimates

Emissions of greenhouse gases such as carbon dioxide, methane and nitrous oxide from human activities contribute to climate change and global mean surface temperature warming. Reductions in near-term climate forcers such as methane, which has a global warming potential 25 times more potent than carbon dioxide on a 100-year time frame, are crucial to limit further global temperature rises in the near term and allow time for long term and large-scale strategies to come into effect. In Canada and the U.S., the oil and gas sector contribute to 41% and 31% respectively of methane emissions annually but has the highest potential for technologically feasible reduction in the short term compared to other sectors such as waste or agriculture. Accurate quantification of methane emissions across all sectors, including oil and gas, are needed to inform national inventories, regulations and reduction strategies that are key to mitigating climate change. Methane leakage from abandoned oil and gas wells not only contributes to methane emissions from the oil and gas sector, but also poses a risk to groundwater through subsurface leakage caused by well integrity issues. Despite this, the number of methane emission measurements from abandoned oil and gas wells is small compared to the total population across U.S. and Canada. Additionally, many provinces and states with current and previous history of oil and gas development still have no available direct point-source based measurements, including Canada’s largest oil and gas producing provinces of Alberta and Saskatchewan. Furthermore, existing measurements do not differentiate between emissions from aboveground well infrastructure leaks and emissions from surface casing vent flows, an indicator of subsurface leakage. We conducted chamber-based methane emission measurements of 238 abandoned oil and gas wells across Alberta and Saskatchewan, Canada. We separately quantified emissions from surface casing vents and other emissions from the wellhead infrastructure (including near well gas migration) to develop component-specific emission factors. By combining our measurement-based emission factors with publicly available datasets on abandoned oil and gas wells, we estimated Canada-wide emissions from abandoned wells including the contribution of emissions from surface casing vent flows associated with subsurface leakage. From our measurements we estimated methane emissions from abandoned wells in Canada to be 85-95 kilotonnes of methane per year, of which surface casing vent emissions represent 75-82% (70 kilotonnes of methane per year). Within our sample set we also measured a super high emitter with a methane emission rate of (5.2x106 mg CH4/h), two to three times higher than the largest previously published measurement from an abandoned oil and gas well. By comparing the occurrence of surface casing vent flows within our sample set to two previous studies based on provincial datasets we found that subsurface leaks are three to five higher than previously estimated. We conclude that subsurface leakage is a major contributor to methane emissions from abandoned oil and gas wells and that additional point-source and component-based measurements are needed to accurately quantify emissions and determine the prevalence of well integrity issues in abandoned and active well populations. Moreover, the impact of well attributes on methane leakage and temporal variability of emissions from abandoned oil and gas wells also need further investigation. Comprehensive studies at oil and gas wells that combine methane emissions measurements with investigations of other environmental impacts such as groundwater contamination are needed to create mitigation strategies that address emissions and broader environmental impacts

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