Imaging the Aquistore reservoir after 36 kilotonnes of CO2 injection using distributed acoustic sensing

Aquistore is a geologic CO2 storage project that is using a deep saline formation as a storage reservoir. From April 2015 to February 2016, approximately 36 kilotonnes of CO2 were injected into the reservoir at a depth of 3130-3350 m. We have developed an analysis of distributed acoustic sensing (DAS) 3D vertical seismic profiling data acquired in February 2016, marking the first seismic survey since injection began. The VSP data were processed in parallel with baseline preinjection data from a November 2013 survey, with the objective of detecting and characterizing the subsurface CO2 plume and evaluating the repeatability of DAS in a reservoir monitoring project. A single processing sequence was devised that (1) accurately imaged the reservoir for the baseline and monitor data and (2) attained adequate repeatability to observe time-lapse differences related to the presence of CO2. Repeatability was somewhat compromised by the less advanced noise cancellation methodology of the DAS system used for the baseline survey. In the final cross-equalized migrated data volumes, normalized root-mean-square (nrms) difference values of < 0.4 were attained at the reservoir level indicating good repeatability compared with most surface seismic studies. An injection-related amplitude anomaly with maximum nrms values of approximately 0.7 is apparent in the Deadwood Formation of the reservoir, whereas no significant nrms anomalies were observed near the injection and monitoring wells in the Black Island Member or above the reservoir.