Assessing 4D seismic signal and its value across saline aquifer CCS sites in the UK

Monitoring of CO2 operations is a regulatory requirement, and time-lapse seismic monitoring is a widely used technology to ensure success and safety of CCS operations. This study focuses on injection of CO2 into saline aquifers across key UKCS locations, and the detectability of 4D signal resulting from water displacement. We analyse the effect that changing CO2 saturation and dissolution of CO2 in brine have on the signal NRMS, as well as the effect of vertical heterogeneity on the resolution of 4D signal. A typical 4D towed streamer survey is used as a guide to set up detection thresholds for signal NRMS, and minimum thicknesses detectable is assessed across CO2 saturation levels. This study utilises normal incidence reflectivity to calculate the signal NRMS for particular saturation, dissolution and intra-reservoir layer thickness scenarios. 25 locations in the UKCS are assessed, and cumulative density functions are created for various acquisition surveys in order to determine the minimum thickness of intra-reservoir layers, that result from reservoir heterogeneity. Ultimately, a detection table is created for the 4D signal, which utilises the calculated signal NRMS and detection thresholds based on the probability distribution of the NRMS in a typical 4D towed streamer survey.