In the Sleipner field (Norwegian North sea), Statoil has injected for 15 years more than 11 Mt of CO2 into a saline aquifer. To monitor the CO2 migration inside the aquifer, time-lapse seismic acquisitions have regularly been applied and seven pre-stack PP seismic vintages are now available. The time pre-stack stratigraphic inversion currently used by the oil industry has been applied to the Sleipner-CO2 storage to better characterize the aquifer. The methodology developed by IFP is based on Bayesian formalism, the workflow consists of 3 steps: (1) a seismic to well-log calibration, (2) the building of an a priori model and (3) the inversion process. The last step consists in simultaneously inverting a few limited incidence angle stacks in order to estimate an optimal multi-parameter earth model, parameterized in P- and S-wave impedances, by an iterative process. Such values are crucial attributes for both reservoir characterization and CO2 monitoring. With several vintages available, like the Sleipner-CO2 case, a complementary study has been done with a time lapse (4D) inversion which consists in simultaneously inverting several vintages. The results of this study are focused on the 1994 (before injection) and 2006 (after an injection of 8.4 Mt of CO2) vintages. The IFP stratigraphic inversion methodology was applied: from offset gathers of 1994 and 2006 vintages, we have built 3 limited angle substacks, then the well-to-seismic calibration step was achieved for each of them. Finally pre-stack inversion (one inversion is performed for each vintage) and 4D joint inversion has also been performed. Considering the elastic impedances results inside the CO2 plume, P-wave impedances decrease drastically due to the presence of CO2. S-wave impedances are much less affected since there are no major changes in the rock matrix: the variations are essentially due to density variations. Outside the plume (around and in the shale overburden), the variations are quasi-nonexistent, especially for the 4D inversion results, providing at least a good delineation of the CO2 plume. In agreement with seismic amplitude analyses, the stratigraphic inversion results tend to show the efficiency of the shale overburden at the resolution level attained after inversion, which is a key issue for the long-term storage of CO2. These results are helpful inputs for both reservoir simulations and petro-elastic considerations to try to target quantifying the CO2 mass.


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