Fate Of Fugitive Natural Gas In Heterogeneous Near-Surface Sediments In A Region Of Extensive Petroleum Resource Development

Fugitive natural gas released in the subsurface from leaking oil and gas wells can affect groundwater quality and generate significant greenhouse gas emissions to the atmosphere. We released natural gas into a Western Canada Sedimentary Basin (WCSB) groundwater system located in an area of petroleum resource development. Through 55 depth-discrete monitoring points installed up to 26 m deep, we tracked spatiotemporal evolution of dissolved gases over 760 days. Fugitive gas was diverted and mostly retained in the subsurface by capillary barriers, resulting in highly irregular distribution and dissolution of multicomponent gas constituents. Gas wetness changed significantly during migration, although stable-carbon isotope ratios did not. We expect that where a surface diamict is present, typical of the WCSB, a significant portion of fugitive gas released from leaky wells will be retained in the subsurface, mitigating greenhouse gas emissions to the atmosphere but inferring greater risk on groundwater.Plain Language Summary The impacts of fugitive gas from leaky oil and gas wells remain poorly understood but include greenhouse gas emissions to the atmosphere and degradation of potable groundwater resources. To assess migration patterns, environmental impacts, and fate of fugitive gas, we released natural gas in a shallow groundwater system, representative of much of the Western Canada Sedimentary Basin (WCSB) where extensive petroleum resource development takes place and similar to 0.6% of wells have been confirmed as releasing natural gas into the subsurface. Our results indicate that for clayrich surficial sediments typical of the WCSB, fugitive gas is likely to remain trapped in the subsurface and to slowly dissolve in groundwater in an irregular manner over a long period. The different constituents that form natural gas dissolved at different rates, changing their molecular ratios; stable-carbon isotope ratios remained stable. Our findings suggest that where glacial till forms the surficial geology, subsurface fugitive gas released from leaky oil and gas wells remains trapped in the ground, limiting emissions of greenhouse gases to the atmosphere but potentially impacting groundwater.