Abstract

Brazilian carbonate reservoirs contain huge oil-in-place volumes and are likely to have an important economic impact in global oil industry. Such carbonate reservoirs, both Albian and Aptian, are located offshore in deep water, which has an important role in development costs. In siliciclastic plays it is standard practice to use time-lapse (4D) seismic to locate wells in unswept areas, maximizing the recovery factor. However, in carbonate plays, time-lapse seismic monitoring remains challenging, with few cases of success published around the world. Carbonates differ from siliciclastic rocks in many aspects. With a varied chemical composition and complex pore connectivity, carbonates bring on a large diversity of facies, each one showing a specific seismic response. Moreover, the usually high incompressibility of the carbonate matrix reduces sensitivity to pressure and saturation changes, which are the main effects expected on a hydrocarbon field undergoing production. Considering both the economic relevance and the difficulties of using time-lapse seismic to monitor carbonate reservoirs, Petrobras created a strategic program to face this challenge, a time-lapse seismic (S4D) study of an Albian carbonate reservoir located in the Campos Basin offshore Brazil. Three legacy data were available: streamer–1987, streamer–2002, and ocean-bottom cable–2010. Extensive feasibility studies are performed as well as a meticulous prestack 4D simultaneous inversion based on the seismic data, preliminary cross-equalized to partially overcome the initial low repeatability. A multidisciplinary interpretation conducted by a team of geophysicists, geologists, and reservoir engineers finally lead to results overcoming expectations. Saturation and pressure changes are detected using P- and S-impedance attributes, consistent with production data. Results are opening perspectives for further monitoring other carbonate reservoirs, especially with new presalt Brazilian discoveries in the Campos and Santos Basins, which might have a huge economic impact.

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