Assessing the potential to use repeated ambient noise seismic tomography to detect CO2 leaks: Application to the Aquistore storage site

• Published in: International journal of greenhouse gas control 2018-04, Vol.71, p.20-35
• Main Authors: Stork, Anna L., Allmark, Claire, Curtis, Andrew, Kendall, J.-Michael, White, Don J.
• Format: Article
• Language: English
• Subjects: Ambient noise tomography; Aquistore; Carbon capture and storage; CCS; CO2 leakage detection; Passive seismic monitoring
• ISSN: 1750-5836; 1878-0148
• DOI: 10.1016/j.ijggc.2018.02.007

•Ambient noise interferometry (ANI) study to monitor the Aquistore CO2 storage site.•First pre- and post-injection noise tomography survey of a major CO2 storage site.•In specific circumstances, Rayleigh wave tomography would detect CO2 leakage. The Aquistore project in Saskatchewan, Canada provides carbon dioxide (CO2) storage for the world's first combined commercial power plant and carbon capture and storage (CCS) project. CO2 has been injected at a depth of 3.2 km since April 2015 and a permanent near surface geophone array provides passive seismic monitoring. The ability to identify any containment breach is a vital part of risk management and reduction for CO2 storage sites. We therefore investigate the potential to monitor seismic velocity changes following a hypothetical leak of CO2 from the reservoir using passive monitoring methods. We estimate the expected shear-wave velocity change with CO2 saturation, and using data from the geophone array we investigate whether ambient noise interferometry (ANI) and a tomographic inversion for Rayleigh wave group-velocity maps could provide a suitable CO2 leakage detection tool. To assess the repeatability of the method, we conduct, for the first time, a time-lapse ambient noise tomography survey of a CO2 storage site to cover time periods preceding and following injection start-up. Sensitivity analysis results indicate that usable surface wave data derived from the current array configuration are sensitive to depths of ∼400 m and shallower. We do not expect to observe any changes due to CO2 migration at such shallow depths and the estimated seismic velocities pre- and post-injection agree to within 60 m s−1, which is on the order of double the predicted velocity change with CO2 saturation. Therefore, due to uncertainties in travel-time picks (5–15%) and variations in the obtained velocity structure between consecutive days (up to 20%), we would be unable to resolve the expected seismic velocity change with an influx of CO2 at 400 m (∼3–4%). Additionally, the noise source variability does not allow stable velocity estimates to be made in the time-frame of currently-available data. Consequently, in the event of a CO2 leak at the Aquistore site, using the standard ambient noise analysis methods applied herein, Rayleigh wave tomography could be deployed to detect velocity changes due to CO2 saturation only if (a) a wider aperture surface array was in place to allow longer period surface waves to be used, providi