Monitoring the evolution and migration of a methane gas plume in an unconfined sandy aquifer using time-lapse GPR and ERT

Abstract

Fugitive methane (CH₄) leakage associated with conventional and unconventional petroleum development (e.g., shale gas) may pose significant risks to shallow groundwater. While the potential threat of stray (CH₄) gas in aquifers has been acknowledged, few studies have examined the nature of its migration and fate in a shallow groundwater flow system. This study examines the geophysical responses observed from surface during a 72day field-scale simulated CH₄ leak in an unconfined sandy aquifer at Canadian Forces Base Borden, Canada, to better understand the transient behaviour of fugitive CH₄ gas in the subsurface. Time-lapse ground-penetrating radar (GPR) and electrical resistivity tomography (ERT) were used to monitor the distribution and migration of the gas-phase and assess any impacts to groundwater hydrochemistry. Geophysical measurements captured the transient formation of a CH₄ gas plume emanating from the injector, which was accompanied by an increase in total dissolved gas pressure (P_TDG). Subsequent reductions in P_TDG were accompanied by reduced bulk resistivity around the injector along with an increase in the GPR reflectivity along horizontal bedding reflectors farther downgradient. Repeat temporal GPR reflection profiling identified three events with major peaks in reflectivity, interpreted to represent episodic lateral CH₄ gas release events into the aquifer. Here, a gradual increase in P_TDG near the injector caused a sudden lateral breakthrough of gas in the direction of groundwater flow, causing free-phase CH₄ to migrate much farther than anticipated based on groundwater advection. CH₄ accumulated along subtle permeability boundaries demarcated by grain-scale bedding within the aquifer characteristic of numerous Borden-aquifer multi-phase flow experiments. Diminishing reflectivity over a period of days to weeks suggests buoyancy-driven migration to the vadose zone and/or CH₄ dissolution into groundwater. Lateral and vertical CH₄ migration was primarily governed by subtle, yet measurable heterogeneity and anisotropy in the aquifer.