Geomechanical Modelling and Time-Shift Detectability for CO2 Injection in Depleted Gas Reservoirs, North Sea

Geomechanical monitoring using time-lapse (4D) seismic data contributes to ensuring the effective and safe injection of CO2 into subsurface reservoirs, playing a crucial role in Measurement, Monitoring, and Verification (MMV) programs. This research focuses on the application of time-shift signals to monitor pressure build-up during CO2 injection in the Southern North Sea (SNS) depleted gas reservoirs. These reservoirs are characterized by complex fault networks that cause compartmentalization, leading to non-uniform pressure distribution and compromised seal integrity during injection and post-closure. Therefore, monitoring stress changes helps reduce containment risks. We build a detailed 3D reservoir and geomechanical model for the Chiswick field as a case study based on the National Data Repository dataset. This model is employed to simulate strain changes and estimate time-shift signals using a post-stack R-factor model for depletion and CO2 injection scenarios. Our analysis indicates that the time-shift signals at the top of the reservoir are detectable using typical seismic acquisition methods like towed streamer, with a -1.3 ms speedup observed after a 26 MPa pressure increase. However, the presence of salts and the intricate geometry of the Zechstein formation cause issues with seismic imaging, reducing detectability in the immediate overburden below the threshold in observations.