Methane emissions from Canada’s inactive oil and gas wells are up to seven times higher than government estimates, a new McGill study finds—with a relatively small number of high emitters responsible for most of the pollution.

The study analyzes the methane footprints of sites with non-producing wells across Canada, focusing on emissions from two sources: above-ground wellhead equipment, and surface casing vents (SCVs)—pipes designed to prevent the buildup of pressure within a wellbore. These sites emit an average of 230 kilotonnes of emissions each year, estimate the researchers, far more than the 34 kilotonnes reported in the federal government’s 2024 National Inventory Report (NIR). However, the study’s statistical uncertainty indicates that the actual number could be anywhere between 51 to 560 kilotonnes annually.

A few high-emitting wells dominate the emissions. “For example, one well can emit as much as 100 wells combined,” study co-author Jade Boutot, a PhD student in civil engineering, told CBC News, adding that those wells should be prioritized for remediation.

The McGill study’s wellhead emission estimate is about half the federal estimate, but estimates for SCV emissions are 16 times greater than NIR figures. It showed lower uncertainty than the NIR for wellhead emissions estimates, but comparatively higher uncertainty for SCV emissions.

Environment and Climate Change Canada said it is reviewing the research and may include it in a review of how it estimates methane emissions, reported CBC News.

Methane emissions from non-producing wells are difficult to estimate accurately in Canada and elsewhere due to limited direct measurements and uncertainty about the exact number of wells. In Canada, non-producing wells make up more than 70% of the total number of oil and gas wells.

But Canada’s 2024 NIR—which covered data as recent as 2022—reports 409,319 inactive wells, 15% fewer than the 471,276 wells the researchers counted. “Our well count is unlikely to be an overestimate, as it corresponds to the wells in government databases with unique identifiers,” write the researchers.

The most recent NIR released in 2025 using data from 2023, placed the total number of abandoned oil and gas wells at approximately 423,000.

The McGill study reassesses the scale of emissions from these non-producing wells based on a dataset of methane flow-rate measurements for 494 of them. The dataset covers five provinces—Alberta, Saskatchewan, Ontario, British Columbia, and Quebec—using publicly available provincial and territorial data that includes 105 previously unmeasured wells. The authors say the dataset is “the largest measurement database using a consistent methodology.”

Researchers collected data about well attributes that include geographic location, well status, and other specifications. They also took 678 measurements across the sites in the dataset to directly quantify flow rate with a static-chamber methodology, which measures gas accumulation in an enclosed area over time.

They acknowledge that, given the study’s small sample size, “emission estimates remain highly uncertain.”

Alberta had the highest methane flow rates for both wellheads and SCVs. Saskatchewan’s wellheads showed a comparable rate, but measurements for SCVs were markedly lower there. The distribution of both wellhead and SCV methane flow rates was skewed by large emitters, with 98% of emissions coming from only the top 12% of wellheads and the top 2.1% of SCVs.