Estimation of thickness and velocity changes of injected carbon dioxide layers from prestack time-lapse seismic data

In this study, we bring together the two main categories of time-lapse seismic analysis-amplitude analysis and time-shift analysis-to estimate simultaneously the changes in thickness and velocity of a 4D seismic anomaly. The methodology is applied to time-lapse seismic monitoring of carbon dioxide (CO2) storage at Sleipner field, Norway, that shows significant 4D effects. The 4D anomalies resulting from CO2 injection appear as a multilayer reflection pattern within the relatively shallow Utsira Sand. This multireflective appearance within the sand layer is interpreted as CO2 layers trapped below thin shale layers. Because most of the CO2 layers are believed to be thin (0-20 m), the interference between top and base of these layers needs to be taken into account in 4D seismic analysis. By studying the reflected event from a horizon below the Utsira Sand, we estimate 4D traveltime shifts caused by the presence of the CO2 layer above this horizon. We then combine these traveltime shifts with measured amplitude changes for the top and base of the CO2 layer to estimate velocity and thickness changes for the thin CO2 layer. In 1999, after three years of injection, the most likely velocity change was around 200 m/s and the thickness of the CO2 layer was around 15 m. In 2001, the corresponding velocity change and thickness estimates were 400 m/s and 15 m, respectively. Finally, in 2002, the most likely velocity change was 500 m/s and the thickness of the CO2 layer was 15 m. It is not straightforward to apply this method to a stack of CO2 layers because 4D time shifts below the Utsira Sand only provide information about the average time shift for all layers. The amplitude information for each individual CO2 layer cannot be resolved without knowing the velocity change within each layer. However, our result from a single CO2 layer may be used to constrain the velocity changes for the multilayer CO2 case.