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Time-lapse seismic monitoring has proven to be one of the most valuable CO2 storage monitoring methods, offering a non-invasive tool with the ability to detect the arrival of CO2 in specific units, and allowing quantitative information about the storage site to be inferred. 4D seismic can be used to detect saturation changes, pressure changes and rock deformation, and the same sensors can be used for active and passive seismic monitoring.
Here, we review this progress in seismic monitoring technology within the frame of rethinking the objectives of monitoring. What is the purpose and value of seismic monitoring? To detect CO2, to validate storage concepts and models or simply to understand what is happening in the storage unit? We conclude that smart detection of thin layers of CO2 will always be important. However, a deeper understanding of the CO2 plume evolution in complex geological stores may be far more important than many assume. FWI inversion of good multi-offset seismic acquisition datasets can really ‘open the box’ needed for explaining what is happening in the store, thereby demonstrating conformance and containment – which is ultimately the goal of safe CO2 storage operations.