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Measurements have shown that abandoned oil and gas wells emit methane to the atmosphere, but the estimates of methane emissions at the national scales remain highly uncertain. Here, we provide an overview of available measurement data and studies investigating factors linked to high methane-emitting abandoned wells. We then analyze abandoned oil and gas well data in Canada and the United States to estimate methane emissions for both countries and evaluate uncertainties in the national estimates. Available measurement data indicate that average methane emission rates used as emission factors vary by 3 orders of magnitude or more, even after accounting for plugging status. Plugging status has been shown to be an important predictor of high methane emitting wells; however, there may be other important factors such as age, depth, fluid type and geographical region. Such well attribute data are …