The presence of injected CO2 in the Utsira Sand at the Sleipner site, Norway, is associated with a high negative P-wave velocity anomaly; that is, a low postinjection velocity and a strong seismic response. Time-lapse seismic imaging of CO2 injection at Sleipner is thus a viable monitoring tool of the injected CO2. The work flow usually involves conventional seismic processing, including stacking, and results in seismic images. Multiple reflections, interference effects such as tuning, and the velocity pushdown effect due to CO2 injection render these seismic images ambiguous in terms of the localization and the quantification of the CO2 in the Utsira Sand. Nonetheless, seismic images often form the basis for analyses that aim to quantify the injected CO2. We employed elastic 2D full waveform inversion to invert prestack seismic Sleipner data from preinjection (1994) and postinjection (1999) and compared the resulting postinjection P-wave velocity model with the corresponding seismic image. We found that the high-amplitude reflections in the seismic image do not everywhere coincide with low postinjection P-wave velocities. Drawing extensive and integrated conclusions is out of our scope, because this would require full control over the seismic data processing and a more comprehensive forward modeling. For instance, modeling should be done in 3D and an adequate anelasticity formulation should be added. However, the waveform inversion scheme we used accounts for all the aforementioned elastic propagation effects. The results therefore suggested that the exclusive use of seismic images to quantify CO2 could be revised and full waveform inversion should be added to the analysis toolbox.
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