Exploring 4D seismic potential for monitoring CO₂ injection in depleted North Sea gas fields

Carbon capture and storage (CCS) is crucial for meeting global CO₂ emissions reduction targets. Monitoring, measurement, and verification (MMV) plans are essential for the effective management of CO₂ storage sites, with 4D seismic data playing a key role. To aid this understanding, this study presents simulation to seismic modelling (Sim2Seis) case studies for CO₂ injection into depleted gas fields, focusing on the Goldeneye, Hamilton, and Viking fields in the North Sea. Using compositional reservoir flow simulation models, we conduct a forward modelling process to generate synthetic seismic at key time steps such as baseline (pre-CO₂ injection) and at the end of CO₂ injection (post-CO₂ injection). We demonstrate that 4D seismic signals, related to saturation changes, are strong and most visible at fluid contacts, particularly where CO₂-hydrocarbon gas mixture displaces the aquifer. By contrast, intra-reservoir signals arise primarily from fluid compositional changes with only limited contrast in acoustic properties from density and velocity variations. These findings suggest that 4D seismic is better suited for containment monitoring and leakage detection than conformance monitoring. They also highlight the need for tailored strategies based on compositional reservoir flow simulation models. Continued research into fluid flow physics and seismic interpretation is critical to optimizing monitoring strategies for CO₂ storage in depleted gas fields.