We present a successful case study in which prestack 3D and 4D simultaneous AVO inversion is used in conjunction with rock-physics analysis to estimate saturation and pressure changes in a West Africa brownfield using time-lapse (4D) seismic as input. We show that, in 4D seismic, there can be many competing production effects that can be difficult to disentangle using traditional 4D interpretation methods, such as amplitude differences between the base and monitors. This begs the need for a more sophisticated approach to decouple these competing effects, such as the use of prestack simultaneous 3D and 4D inversions. Multiple substack seismic data are used to estimate a variety of 3D and 4D petroelastic attributes for mapping static and dynamic reservoir properties with the primary objective of influencing the continuous infill drilling and the overall reservoir management strategy. Facies-specific low-frequency models were used as priors for the 3D inversion, while velocity changes from time-lapse time shifts were used as priors for the 4D inversion. We also demonstrate the use of rock-physics templates coupled with a lithology-specific Gassmann fluid-substitution method to establish a nonlinear regression-based rock-physics model that obeys bound theory from classical rock physics and honors single and multimineral fluid-substitution theory. The resulting templates, when integrated with the prestack AVO-inversion technique, produce a set of attributes that accurately explain the time-lapse production effects observed on seismic.

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