Methane Emissions from Abandoned Oil and Gas Wells in Alberta and Saskatchewan, Canada: The Role of Surface Casing Vent Flows.

Abandoned oil and gas wells can act as leakage pathways for methane, a potent greenhouse gas, and other fluids to migrate through the subsurface and to the atmosphere. National estimates of methane emissions remain highly uncertain, and available measurements do not provide details on whether the emissions are associated with well integrity failure (indicating subsurface leaks) or aboveground well infrastructure leaks. Therefore, we directly measured methane emission rates from 238 unplugged and plugged abandoned wells across Alberta and Saskatchewan, Canada, separately quantified emissions from surface casing vents and other emissions from the wellhead (non-surface casing vent), and developed emission factors to estimate Canada-wide emissions from abandoned wells. Our highest measured emission rate (5.2 × 10 mg CH/hr) from an unplugged gas well was two to three times higher than the largest previously published emission rate from an abandoned well. We estimated methane emissions from abandoned wells in Canada to be 85-93 kilotonnes of methane per year, of which surface casing vent emissions represented 75-82% (70 kilotonnes of methane per year). We found that subsurface leaks, as evidenced by surface casing vent flows, occurred at 32% of abandoned wells in Alberta, substantially higher than previously estimated using provincial data alone (6 and 11%). Therefore, well integrity failures and groundwater contamination are likely to be more common than previous studies suggest.