Fate of leaking natural gas in soil near oil and gas wells in Western Canada

Unwanted leakage of natural gas (>90 % v/v methane) occurs at ca. one third of the oil and gas wells in Western Canada. The delta 13C and concentrations of C1-C4 and CO 2 gases in soil gas samples collected from the heavy oil district of Alberta and Saskatchewan demonstrate that although aerobic bacterial oxidation of leaking gas in soil occurs at a number of well sites, there is little evidence for oxidation at many others. Long term monitoring at three leaking well sites demonstrated that methanotrophic bacteria metabolize >99 % v/v of the leaking gas in summer. Soil freezing, however, reduces oxidation by 60 % in winter. Oxidation of CH4 is also inhibited in soil contaminated with heavy oil.;Authigenic calcites of bacteriogenic and abiotic origin, containing up to 100 % CH4-derived carbon, precipitate in soils near the wells. The delta13C and delta18O of bacteriogenic calcite suggest that it forms in early spring, when soil temperature is too low for methanogenesis. This, along with the discovery of associated authigenic pyrite, suggests that the precipitation of bacteriogenic calcite may be related to the anaerobic oxidation of leaking gas.;This study should provide a strong impetus for the regulatory agencies in Western Canada to review their guidelines regarding leaking well remediation. Results also imply that a wider use of delta13C analyses by the oil and gas industry may help to significantly reduce unwanted CH 4 emissions in Western Canada.;Estimated kinetic isotope fractionation factors epsilonCH4-CO2 exhibit seasonal variability and correlate negatively with soil temperature. Results suggest that larger epsilonCH4-CO2 values in winter are related to the lower activity of the Methane Mono Oxygenaze (MMO) enzyme at low temperature. Elevated soil H2 contents and very low epsilon CH4-CO2 values in the summer suggest that methanogenic microorganisms also inhabit the soils at well sites not contaminated with oil. The O 2 consuming methanotrophs provide habitats for the methanogens by rendering parts of the soil near the wells anaerobic. The low amount and comparatively high delta13C of soil organic matter at those sites also suggest that fermentative bacteria from the methanogenic consortium metabolize biomass generated by the methanotrophs.