Time-lapse VSP monitoring of CO2 sequestration at the CaMI Field Research Station
The Containment and Monitoring Institute Field Research Station (CaMI.FRS) is a carbon sequestration field experiment near Brooks, Alberta. CO2 is injected into a saline aquifer in the 6 m thick, 10% porosity Basal Belly River Sandstone at 300 m depth. This CO2 plume simulates a shallow leak scenario from an industrial scale CO2 storage reservoir. Vertical Seismic Profiles (VSP) were collected between 2017 and 2021 to determine a detection threshold and delineate the plume. These data had high repeatability, with permanent borehole sensors and identical source coordinates. Seasonal variations in surface and near-surface conditions were the main source of dissimilarity between baseline and monitor data. A time-lapse compliant processing workflow was developed for the 10 Hz ?? 150 Hz shallow data. This workflow produced directly comparable amplitudes between baseline and monitor data, except at higher frequency bands affected by variable near-surface filtering. Spectral differences were eliminated with high-cut filters designed for each source location. Avoiding the use of shaping filters preserved the subtle amplitude response of the CO2 plume in the remaining bandwidth. After 33 t of injection, the CO2 plume produced an observable time-lapse amplitude anomaly on multiple geophone datasets on two monitoring lines. The interpreted CO2 plume matched forward modeling expectations of approximately 50 m lateral extent, but with an asymmetric distribution around the injection well. Equivalent DAS datasets possessed lower signal strength than geophone data, with instrument noise interfering with the CO2 amplitude response. A weak seismic anomaly was observed on only one DAS monitoring line. The high-confidence geophone results establish a leak detection threshold at 33 t of CO2 for this geological setting. The ability to detect a 33 t CO2 plume with VSP field data lends confidence to Measurement, Monitoring, and Verification capabilities at industrial operations with injection rates of kilotonnes or megatonnes per year. The time-lapse compliant VSP workflow will help inform shallow monitoring procedures, while the CO2 delineation will contribute to future multi-disciplinary studies at CaMI.FRS.