Application of Full Wavefield Inversion to Deep-water 4D Seismic Surveys - Workflow and Analysis of the Results

4D seismic, by taking snapshots of a producing oil/gas field with two seismic surveys (baseline and monitor), provides an effective method for field-wide surveillance and monitoring of the subsurface hydrocarbon production. When production induces large acoustic velocity changes in the reservoir interval, for example, due to gas exsolution and/or gas migration, large time shifts between the seismic reflection events of the baseline survey and those of the monitor survey lead to imaging repeatability problems. Modifications of the baseline velocity model are needed for use to migrate the monitor seismic data (Chen et al., 2014). In this paper, we applied full wavefield inversion (FWI) technology (Virieux and Operto, 2009; Vigh et al., 2011) to two vintages of 4D seismic data. Such an application not only yields time lapse changes of the velocity models (Routh et al., 2012), but also the velocity perturbations needed to modify the baseline velocity model to migrate the monitor seismic data. The results were analyzed and evaluated through comparison with well log data, fluid substitution modeling, and reservoir simulation models from a deep-water West Africa field.