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A comparative study was carried out examining some coupled geomechanics and reservoir fluid flow simulations. The study attempted to answer the questions: "Why do we need coupled geomechanics and reservoir fluid flow analyses" and "How do we do the coupled work"? It is common in reservoir engineering to expect that volumetric deformation in the reservoir may change rock properties such as porosity and permeability, and hence change reservoir flows. However, the effects of shear or deviatory deformation, and stress re-distribution in the reservoir, on the fluid flow have been often neglected. These latter effects will be examined in this paper. This paper clarifies the notion that shear or non-uniform deformation and stress re-distribution in the reservoir may have significant effects on reservoir fluid flows. Thus, a "fully" coupled or "two-way" coupled analysis is a "must have" approach in a reservoir study where the shear deformation or changes in total stresses in the reservoir can not be ignored. Shear deformation or changes in total stresses, and the resulting stress re-distribution in the reservoir, can be caused by non-uniform reservoir flows; boundary, corner and arching constraints; material nonlinearity; sliding along interfaces or faults; irregular reservoir geometry; among other causes leading to non-uniform deformations. There are only a few cases in which a reservoir flow analysis can be conducted independently, i.e., without coupled geomechanical calculations, such as when the reservoir flow is occurring under uniaxial deformation or isotropic deformation conditions. The compressibility of the rock phase in these uncoupled reservoir flow simulations has to be selected with caution according to the deformation pattern that will occur in the reservoir. In the cases where no simple deformation pattern can be defined, a "fully" or "two-way" coupled geomechanical analysis has to be conducted along with the reservoir flow analysis. Otherwise, significant errors may occur in prediction of reservoir performance, not only for reservoir deformation, but also for reservoir fluid flows. |
