Integration of 4D seismic into the dynamic model: Girassol, deep offshore Angola

In the deep-water environment of West Africa, 3D seismic information is a key factor for exploration, appraisal, development, and monitoring of hydrocarbon fields. To obtain a better understanding of the reservoir, modeling and simulation work was performed, demonstrating that high-resolution (HR) seismic data would provide a more accurate image of the reservoir. The results of this HR survey (Beydoun et al., 2002) had a significant impact on the definition of the Girassol reservoir model by permitting clearer identification of the stacked turbidite channels (Navarre et al., 2002). Girassol was discovered in 1996 off Angola, in water depths up to 1400 m. The field was initially close to the bubble point pressure with no gas cap. After three appraisal wells, the decision was made to launch a fast track development. 4D HR seismic was planned as a reservoir-monitoring tool. The 4D HR data currently consists of a base 3D HR seismic survey shot in 1999 and a monitor 3D HR shot in the two last weeks of 2002. Many people usually think about 4D at the end of a field life, as a way to identify possible by-passed oil. Nevertheless it is now frequently used to acquire 4D early in the field life for monitoring purposes (Goto et al. 2004). In the Girassol case, it was decided to shoot the repeat 3D HR survey after only one year of production and about six months after the start of gas injection. The first reason was to monitor the effect of gas injection in an extremely heterogeneous turbidite environment. The second reason was that, in such a deep offshore environment, the monitoring through re-entry for log measurement is prohibitively expensive. The first results confirmed the ability of 4D to contribute to field monitoring (Dubucq et al., 2003) only four weeks after the last shot had been completed (Lefeuvre et al., 2003). Therefore, based on the excellent quality of the 4D response and on further processing, it was decided to incorporate information from the 4D data into an updated reservoir model. This was done qualitatively in a first phase, which is described in this paper. However, the ultimate goal to use seismic-derived saturation and pressure change distributions to constrain the reservoir model during the history matching process (Gosselin et al., 2003) is not addressed in this paper.