TRANSPORT OF FUGITIVE GAS TO THE VADOSE ZONE AND ATMOSPHERE DURING A CONTROLLED RELEASE OF NATURAL GAS INTO AN AQUIFER IN NORTHEASTERN BRITISH COLUMBIA, CANADA

The expansion of unconventional oil and gas development has led to growing concern for the environmental impacts of gas migration (GM), which occurs at some petroleum wells. GM is the transport of natural gas outside of the well casing and leads to mobile fugitive gas (FG) that penetrates the neighbouring geological formations and can impact aquifers. FG that migrates to the surface contributes greenhouse gas emissions that are difficult to quantify. Despite these concerns, few studies have directly investigated the migration of natural gas in the subsurface.

This study aims to increase knowledge of GM and FG through a controlled natural gas release experiment using a range of monitoring techniques. The experiment was conducted in northeastern British Columbia (BC), Canada, a region of active unconventional natural gas development. The experimental site consisted of heterogeneous quaternary glacial deposits with a relatively continuous confining clay layer overlaying a sand aquifer. Roughly 100 m³of a synthetic natural gas mixture was injected at the base of the aquifer at a constant rate for 70 days.

This presentation will focus on monitoring of FG transport in the unsaturated zone and surface effluxes. Twelve long-term chambers were used to measure CO₂and CH₄effluxes, providing high resolution time-series data. Survey chamber measurements at 105 locations allowed for spatially distributed measurements at lower frequency. In additions, soil gas samples were collected from 22 soil gas sampling ports. The results illustrate that the injected gas moved upgradient against groundwater flow and broke through at the surface a month after the injection. Once the gas was detected, elevated CH₄fluxes were continuously detected at the surface and began to decrease exponentially once the injection was stopped. Soil gas composition and isotopic data further support that the injected gas moved through the soil and that CH₄was microbially oxidised to CO₂.