4 March 2025
The Culzean field, in the North Sea, has been producing since 2019 gas condensate from fluvial sandstones located within dipping rotated fault blocks at approximately 4km of depth. Two surveys have been acquired with ocean bottom sensors to image and then monitor the evolution of the reservoir during production. In addition to classical time-lapse seismic processing, a time-lapse FWI has been performed to estimate the velocity variation over the production time. Due to the thick chalk layer located just above the target structure and the dipping nature of the reservoir, 4D FWI is the ideal tool compared to more conventional 1D approach based on time-shift estimations. This fast velocity layer represents a challenge for velocity model building and processing in general as it prevents the penetration of diving waves even with 7km of offset and also generates strong multiple curtains covering the reservoir interval. Despite the shallow water environment and complex geology, the 4D FWI implemented in this project was able to recover velocity variations as weak as 1% after only 3 years of production, providing crucial information that can help reservoir evolution assessment.
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