Use of a Hybrid Grid in Reservoir Simulation

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Abstract
Summary: A technique is presented to improve the treatment of wells in reservoir simulators by the use of an orthogonal curvilinear grid (cylindrical or elliptical) in well regions and a rectangular grid elsewhere in the reservoir. Special methods are developed to handle irregular blocks connecting the two types of regions. The nonlinear equations for the two Regions are solved with different levels of implicitness; the simultaneous solution (implicit) method is applied to the well regions, while an implicit pressure, explicit saturation (IMPES) technique is used for the reservoir region. An efficient approach, requiring no outer iterations, between well and reservoir regions is used to couple different regions. The scheme is validated by the simulation of the flow of a slightly compressible fluid in a square domain with one boundary open to flow. For this case, the numerical solution with the hybrid grid approach matches the analytical solution exactly. A computer model based on the approach was developed for simulating two-phase flow in multiwell horizontal reservoirs. We show that* the proposed technique provides much better prediction of the phase split in the producers than the conventional approach.

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