Comprehensive History Match and Forecast Work for an Oil Reservoir with Uncertainty in Original Gas Oil Contact

Okpani, Olu (Chevron Nigeria Ltd) | Ambastha, Anil (Chevron Nigeria Ltd)



A comprehensive reservoir simulation study was recently carried out for the A4 reservoir located in the Niger Delta. The A4 reservoir is divided into two fault blocks (Main and West) that are connected in the down-dip part of the reservoir both in the oil leg and aquifer. Original Oil-Water Contact (OOWC) was logged in the reservoir, but no Original Gas-Oil Contact (OGOC) was logged by the wells that penetrated the reservoir. Thus, there existed uncertainty in the OGOC from the Highest Known Oil (HKO) to the crest of the reservoir.

During the period of the simulation project, two oil producers (A4-1p and A4-2p) were producing from the Main Block, while one water injector (A4-1i) was providing pressure support. Two additional oil producers were then being planned to increase the recovery from the reservoir. One of the wells was planned to be drilled up-dip of the existing two producers in the Main Block, while the other well was planned to drain the West Block.

Base model deterministic history-match and sensitivity studies were conducted to gain insight into the reservoir performance and parameters that affect history match, especially the OGOC. Then, probabilistic history-matching was carried out to assess the full range of uncertainties of the different history-match parameters with special consideration to the OGOC.

Probabilistic history-matching shows that the P10 OGOC for the Main Block is about 24ft shallower than the HKO, which was also supported by the base deterministic model. The simulation models were then used to forecast the performance of the two additional planned development wells to validate the planned landing depth of the completions. The two additional development wells were drilled and brought online. Initial test results from the new development wells were consistent with the pre-drill base deterministic simulation predictions.