Search for a command to run...
Abstract In this study, full-waveform inversion (FWI) was applied to a subset of nodes from a 4-component (4C), full-azimuth, ocean-bottom-node seismic survey deployed over the Sleipner site in 2023. The inversion uses frequencies up to 70 Hz, which are the highest frequencies used at inverting for the velocity structure of the Sleipner plume. For the first time, a velocity model was obtained that resolves all the known carbon dioxide (CO2) accumulations at Sleipner (9 layers) and many of the vertical/subvertical feeders that connect the CO2 accumulations. The derived velocity model enables the discovery of new feeders and confirms other feeders and small accumulations that were suspected from previous 4D reflection studies and recent FWI analyses. By comparing our FWI results using the 2023 data with high-quality FWI results using the 2010 data, the evolution of the CO2 plume in the past decade was mapped, enabling the assessment of CO2 layers and feeders over time. Our FWI velocity model gives new insights into the nature of CO2 migration at this storage site, confirming that geologic heterogeneity is the main feature controlling the migration pathways. Major factors influencing the vertical migration of CO2 are channel margins and faults developed over the channel margins due to differential compaction of the channel sands compared to the background sediments. These migration pathways connect the observed fill-to-spill microaccumulations found within the Utsira Sandstone. Our analysis suggests that, by August 2023, the CO2 plume occupies 0.1546 × 10⁹ ± 0.0192 × 10⁹ m3, comprising 19.63 ± 4.05 Mt. This value agrees closely with 19.1 Mt reported by the Norwegian Offshore Directorate, highlighting the value of FWI as a tool for CO2 storage verification. As a contribution to a cost-effective monitoring toolbox, this study demonstrates the value of high-frequency, 3D FWI for monitoring CO2 migration in shallow, heterogeneous aquifers.