Time-lapse VSP results from the CaMI Field Research Station
Brendan Kolkman-Quinn, Donald C. Lawton
The Containment and Monitoring Institute operates a carbon sequestration Field Research Station (CaMI.FRS) where 15 tonnes of CO2 have been injected between 2017 and 2019. Walk-away vertical seismic profiles (VSP) were collected in 2017, 2018, and 2019, for the purpose of time-lapse monitoring of the CO2 plume. Two geophone and three straight-fiber DAS walk-away VSP datasets were processed. The repeatability of the time-lapse data was measured by comparing the normalized-root-mean-square and predictability values of the various datasets. High amplitude, near-offset shots resulted in large residual amplitudes and were excluded from the time-lapse results. The geophone data was found to have significantly better repeatability than the DAS data. NRMS values as low as 8%-12% were obtained for the geophone data, compared to lowest values of 21% to 29% for the DAS data. The 18m offset of the 2019-2017 geophone datasets showed a negative amplitude anomaly with positive side-lobes. This anomaly was broadly consistent with expectations of the CO2 plume's effect on reflection amplitudes. Confidence in this interpretation was low. The anomaly was confined to one 3m binned trace in one dataset, and not far above the background residual amplitudes. No anomaly was evident in the 2019-2018 time-lapse, between which 7t of CO2 had been injected. Applying a time-variant inverse-Q filter to the data improved vertical resolution but exacerbated amplitude residuals, worsening the repeatability metrics. Confident interpretation and characterization of the CO2 plume will require more repeatable, consistent time-lapse results. The cause of low repeatability was attributed mainly to scaling issues between time-lapse surveys, as opposed to specific processing steps in the standard VSP workflow. Better matching of amplitudes between baseline and monitor shot gathers is critical to reducing background residual amplitudes in the time-lapse results.