Simulation of a large hydrocarbon reservoir that interacts with other producing reservoirs through a common aquifer with a single regular fine-grid model can be very difficult if not entirely impossible. Such a model will require a very large number of cells in order to account for all interactions among the producing reservoirs and still provide enough resolution in the window area being studied. Due to the limitations in computer hardware that can handle the enormous resources required by the model and the solution algorithm, this task is found to be impractical.

A two-model approach has been used to overcome this difficulty. In this approach, a coarse-grid, areal model of all reservoirs and the aquifer and a fine-grid, three-dimensional model of the window area of interest are used. The two models are run alternately to feed each other with reservoir rates and boundary conditions while the convergence of boundary conditions is maintained. This approach is found to be resource intensive and requires excessive human effort.

This paper discusses the application of the local grid refinement approach as an alternative to the two-model approach to simulate a large hydrocarbon reservoir. The local grid refinement approach involves use of only one model of the reservoir and its associated aquifer system. It allows a finely gridded multi-layer model of the reservoir of interest to be imbedded into a coarsely gridded, single-layer areal model of its associated aquifer system. Consequently, the number of cells is reduced significantly and it becomes practical to simulate the large hydrocarbon reservoir and its associated aquifer system in one model.

For this purpose, therefore, a locally refined grid model was constructed and initialized using data from the already existing models used in the two-model approach. History match runs were made successfully. If compared with the two-model approach, the LRG model history match results are favorable.

The LRG model approach is found to be more advantageous than the two-model approach in terms of eliminating the inaccuracies, the excessive resources, and the intensive human efforts associated with the two-model approach. The simulation results has proved that this approach is feasible and more efficient than the two-model approach. Therefore, the use of this approach will greatly simplify the simulation of the large hydrocarbon reservoir and its associated aquifer system.

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