Monitoring methane gas migration in a near surface confined aquifer using electrical resistivity tomography
Timothy A. Cary, Rachel Lauer, Kristopher A. H. Innanen
An 85% Methane composite gas was injected into a near surface confined aquifer at a rate of 1.5 m3 per day, for 66 days from June 12th to August 16th, 2018. The field site, located in north-eastern British Columbia, is characterized as a fluvioglacial depositional environment which is consistent with the setting of the majority of energy wells in Alberta and British Columbia, Canada. 12 m of diamitic clay seal the injection target; a 14 m thick aquifer consisting of interbedded fine-grained sands and silts. Injection was focused at the base of the aquifer at 26 m depth. Electrical resistivity tomography (ERT), combined with distributed temperature sensing (DTS), was employed to monitor the migration and fate of the gas plume during and after the injection period. Three ERT lines were permanently installed for time lapse monitoring, two parallel and one orthogonal to groundwater flow (NW-SE), centered on or close to the injection location. Dipole-dipole and gradient arrays were employed on five occasions during injection and the data combined and inverted using RES2DINV to produce time lapse difference images. Results show resistivity increases of 15-25% near the injection zone. The gas plume is interpreted as spreading laterally until buoyancy driven preferential pathways are encountered to shallower depths. Resistivity increases of 15-25% are also seen at 10-12 m depth that coincide with gas flow observed at a monitoring well at 12m depth. DTS data were incorporated to correct the inversions for temperature effects. The general structure of the resistivity changes remains the same after temperature corrections are applied.