Seismic amplitude analysis provides new insights into CO2 plume morphology at the Snohvit CO2 injection operation

CO2 has been injected at the Snohvit Field since 2008, with the storage operation split between two distinct injection phases. Until 2011, CO2 was sequestered in the deeper Tubaen Formation before problems with increasing pressure necessitated moving the injection to the overlying Sto Formation. A comprehensive time-lapse seismic monitoring programme has been undertaken over the injection site throughout this period. Uniquely, this study examines four separate seismic vintages starting with the 2003 baseline data and ending with the 2012 repeat survey. The 3D seismic reflection data reveal the seismic character of the anomalies imaged in the Tubaen and Sto Formations to be dissimilar. Time domain analysis and spectral decomposition are used to investigate the CO2 plume morphology in both cases. The seismic response during the initial phase is complex, showing contributions from both fluid and pressure changes. The majority of the reflectivity is ascribed to a build-up of pore-water pressure in the wider reservoir. Seismic analysis of the second phase reveals a simpler distribution, consistent with a conical plume formed by buoyancy-driven upward advection of CO2. The thickness of the spreading layer is calculated, and a maximum temporal thickness of 22 ms is derived from both time and frequency analysis. Direct comparison of the two methodologies reveals good agreement over the central parts of the layer where spectral techniques are applicable. Results are then used to determine the total mass of CO2 in the Sto Formation as 0.51 million tonnes. This is consistent with the true injected mass of 0.55 million tonnes.