Geophysical response to simulated methane migration in groundwater based on a controlled injection experiment in a sandy unconfined aquifer

Geophysical methods have the capacity to detect and characterize gas-phase dynamics in groundwater. Suitable methods can be deployed at surface or within boreholes depending on the required depth of investigation, spatial/temporal resolution, and geologic conditions. While the application of geophysical methods to monitor immiscible phase contaminants in the subsurface has been extensively documented, the effects of hydraulic properties and flow system conditions on the nature of the geophysical responses used to elucidate multi-phase fluid flow remains underdeveloped. A series of numerical 2-dimensional multi-phase flow and geophysical model simulations based on a controlled methane release experiment in the Borden unconfined sand aquifer was carried out to assess the influence of porous media hydraulic properties and flow system conditions on geophysical signatures associated with transient gas-phase saturation and gas migration behaviour. Specifically, the utility of electrical resistivity tomography (ERT) and ground-penetrating radar (GPR) to monitor gas-phase plume dynamics in shallow groundwater flow systems is examined. ERT and GPR responses to gas-phase distribution and migration during a 72-day methane gas injection and subsequent recovery period was calculated using a numerical multi-phase flow model (CFbio) simulating four distinct parameterizations of the sandy aquifer system. Geophysical models showed that ERT was effective at imaging the central position of the plume but was less effective at detecting thinner lateral migration pathways extending beyond the primary high gas saturation bulb. Conversely, GPR was able to detect thin gas pools emanating from the primary gas bulb and small-scale vertical preferential pathways arising from capillary boundaries with contrasting saturations; however, gradational boundaries proved to be more difficult to resolve using GPR. This study demonstrates that ERT and GPR can be very useful tools in combination for longer-term monitoring of stray gas leakage from decommissioned hydrocarbon wells in shallow granular media freshwater aquifers, especially given the likelihood of strong lateral migration.