Evaluation of Signals from Monitoring CO2 Injection in the North Sea Using 4D Seismic

In the North Sea 4D seismic will remain a key tool in monitoring CO2 injection projects, and a major contributor to MMV due to the proven track record on producing fields. In this setting, deep saline aquifers and abandoned gas reservoirs are amongst the viable geologic storage options, with the former having the largest storage capacity. However, re-purposing of O&G 4D quantitative techniques for cost-effective monitoring requires careful evaluation of the expected characteristics of the 4D signals. In this work we describe and analyse several key signals that are expected for saline aquifers and depleted gas reservoirs. For saline aquifers the 4D fluid-related signals are large and visible using low-cost acquisition during the injection phase, although intra-reservoir complexity requires more attention. Both amplitude and time-shift signals may be observed. For gas reservoirs, a strong pressure response will be readily visible and swamp the saturation changes, rendering the more subtle fluid signals harder to observe. A base reservoir response may be ‘just’ detectable on dedicated towed streamer data. Comprehensive simulation studies are required in both cases to calibrate the anticipated 4D signals further.