Abstract

Time-lapse seismic ā€œ4Dā€ close-the-loop (CtL) technology can play a significant role in creating robust static and dynamic models for deepwater reservoirs, as demonstrated by a case study in Equatorial Guinea, West Africa. Initial static and dynamic models for the field were built using well data and seismic attribute volumes, from prestack 3D and 4D simultaneous angle versus offset (AVO) inversions, in conjunction with rock-physics analysis. We then used seismic inversion technology to estimate static and dynamic reservoir properties over the field's life, including porosity, net-to-gross (NTG), and changes in both fluid saturations and pressures. To model changes, we used a baseline and two high-repeat 4D seismic surveys as input to the inversion work. The study shows how the many competing production effects and their combined nonunique 4D signatures posed a serious challenge to achieving history-matched dynamic models consistent with 4D seismic data. However, 4D CtL technology served as a guide for reservoir engineers updating the dynamic model simulations by providing direction and constraints for matching pressures, waterfronts, and gas breakout areas. Moreover, 4D CtL helped refine existing mental sweep models based on historic field performance and available tracer data. Previous models largely matched this data but used some undesirable manual edits in the simulation model. This has allowed for more geologically sound model adjustments, especially to the water-injection strategy, which obtained promising results. A number of targets were dropped based on 4D matched simulation models that showed elevated water saturations with a high chance for early water breakthrough and reduced well recovery. On the other hand, new targets similar to the one in this case study were identified and/or derisked. This successful case study demonstrates the potential of rock physics, especially when integrated with prestack AVO inversion techniques, to produce a set of attributes that accurately explain the time-lapse production effects observed on seismic.

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