Laboratory measurements monitoring supercritical CO2 sequestration using ghost reflections retrieved by seismic interferometry

Monitoring of seismic changes inside the reservoir layer during CO2 sequestration can be valuable for extraction of reservoir quantities like saturation and pore pressure. The accuracy of the monitoring could be deteriorated due to nonrepeatability errors in the source and receiver geometry. Applying seismic interferometry (SI) to permanent networks of seismic stations to retrieve virtual sources at the positions of the stations eliminates the non-repeatability in the source positioning. SI is traditionally applied using crosscorrelation. We show results from application of SI to ultrasonic data for layer-specific monitoring of sequestration of supercritical CO2. The data are recorded on a two-layer sample consisting epoxy (caprock) and Bentheimer sandstone (reservoir). We apply SI by crosscoherence, which has the potential to retrieve results with higher temporal resolution that SI by crosscorrelation. Using SI, we retrieve non-physical reflections from the bottom of the sandstone as if source and receiver were placed at the top of the sandstone. The velocities we estimate from the non-physical reflections during injection of brine aiming to displace supercritical CO2 and during injection of supercritical CO2 aiming to displace brine indicate rather similar saturation for both injection cases. We confirm the latter by transmission measurements, but with lower resolution.