CO2 Sequestration Site Characterization and Time-lapse Monitoring Using Reflection Seismic Methods
Abdullah A. Alshuhail
Of the various disciplines involved in carbon capture and storage, seismic methods are critically important. In this dissertation, two case studies in west-central Alberta were investigated by employing reflection seismic methods, rock physics and numerical modelling. In the first case study, regional and local seismic site characterization was undertaken as part of the Wabamun Area CO2 Sequestration Project. The results show that P-wave reflection seismology can be an effective tool in regional and local mapping of the continuity of the carbonate Nisku aquifer as well as indelineating geologic discontinuities, such as karsting, that may compromise storage integrity. Furthermore, the information provided by the seismic data was valuable when integrated with petrophysical data in order to reduce the ambiguity in identifying CO2 injection 'sweet spots'. Results from the fluid substitution and numerical forward seismic modelling suggest that CO2 anomalies in stiff carbonate aquifers like the Nisku Formation are small and so is the change in seismic response. For instance, the maximum change in reflection time and NRMS amplitude in time-lapse P-wave reflection surface seismic data was found to be ~ 1.5 ms and ~ 24%, respectively. Detection of these small changes depends on a number of factors, including data repeatability, frequency bandwidth and CO2 saturation scheme. The change in the S-wave properties is muchsmaller than in the acoustic properties suggesting that it is unlikely that PS-wave would be successful in identifying CO2 anomaly.
The second case study pertains to 4-D seismic monitoring at the Pembina- Cardium CO2 Pilot Project site where multi-component surface seismic and walk-away vertical seismic profile methods were implemented as part of the monitoring program. The quality of these data, in particular, was compromised by interference caused by infrastructure development which resulted in the loss of ~ 20% of the seismic shot locations. The 4-D information contributed by the PS-wave surface seismic data was also limited due to the small change in the S-wave properties. Although the magnitude of the predicted change in the acoustic properties was within the detection range, unambiguous identification and mapping of the injected CO2 in the Cardium sandstone reservoir could not be achieved due, primarily, to the CO2 confinement to a thin layer within the reservoir. However, the lack of CO2 anomaly above the reservoir indicates that no upward migration of the CO2 plume was taking place during the injection program. This observation was supported by the results from fluid substitution and forward seismic modelling which show that the P-wave seismic response would be quite sensitive to upward migration of the plume. The dissertation concludes by outlining some of the recommendations, considerations and challenges involved in the implementation of seismic and rock physics methods in CO2 sequestration.