Time-lapse analysis of CaMI.FRS CO₂ VSP data

Brendan Kolkman-Quinn, Donald C. Lawton, Marie Macquet

The Containment and Monitoring Institute Field Research Station (CaMI.FRS) operates a carbon sequestration experiment near Brooks, Alberta. By March 2021, 33 tonnes of CO₂ had been injected into a Basal Belly River Fm sandstone at 300m depth, as a simulation of a shallow leak from a CO₂ storage reservoir. Vertical Seismic Profiles (VSP) were collected between 2017 and 2021 to determine the detection threshold and monitor the plume of injected CO₂. These field data had high repeatability, with permanent borehole geophones and identical shot coordinates, with only surface conditions adding variability. A finite-difference VSP forward model was created to predict the 2D time-lapse response of the CO₂ plume and allow for testing of processing parameters with noiseless data. The 10Hz-150Hz field data required careful processing. A time-lapse compliant processing workflow was developed by paring down a standard VSP workflow, with the goal of avoiding unnecessary alterations to amplitude and phase. This simplified and minimized the cross-equalization between baseline and monitor amplitude spectra: Applying a pre-stack shaping-filter preserved most of the frequency content, at the cost of diminishing the amplitude of the CO₂ time-lapse anomaly. Alternatively, applying custom high-cut filters to the baseline and monitor shot gather pairs reduced the overall frequency content but preserved amplitude contrasts in the reservoir, yielding a higher-confidence result. The 2019-2017 time-lapse difference for a 15t of injection did not unequivocally detect the plume, showing a faint CO₂ anomaly amid background residuals of similar amplitude. The 2021-2017 results showed a higher-amplitude time-lapse anomaly similar to model’s predictions. The normalized-root-mean-square (NRMS) difference between reflection amplitudes ranged from 10%-20% in the field data, above the noiseless model’s NRMS of 8%-13%. The interpreted CO₂ plume had a 45m-51m lateral extent with an asymmetric distribution around the injection well, indicating greater permeation towards the south-west than to the north-east. A detection threshold between 15t and 33t of CO₂ in a 10% porosity reservoir has been confidently established. These findings will help inform Monitoring, Measurement and Verification (MMV) procedures at future CO₂ sequestration operations.