Active surface and borehole seismic monitoring of a small supercritical CO2 injection into the subsurface: experience from the CO2CRC Otway Project

Abstract Time-lapse (TL) (4D) seismic monitoring of injected CO2 in geological formationsis being increasingly employed as the principal method for ensuring containment of CO2 and testing conformance of predicted plume behavior. However, to bring further confidence in this method, the CO2 volume detection limit in the seismic monitoring and key factors controlling it need to be quantitatively understood. The CO2CRC Otway Project attempts to improve this understanding by exploring the capability of seismic reflection method to detect and monitor a 15-kt injection of supercritical CO2/CH4 mixture in a saline aquifer at a depth of 1500 m. The monitoring program consists of TL 3D seismic surveys using a buried geophonearray, TL 3D vertical seismic profiling(VSP), and offset VSP. Seismic acquisition was carried out at injection intervals of 5, 10, and 15 kt over a 5-month period and also, 9 and 23 months after the end of injection. The TL seismic images clearly show the distribution and evolution of the stored CO2/CH4 plume. The results demonstrate the potential of TL reflection seismic to provide key information to both operators and regulators for confirming the security and behavior of stored CO2 at very small volumes.