Robust implementations of the 3D-EDFM algorithm for reservoir simulation with complicated hydraulic fractures

Abstract The geometry of the subsurface hydraulic fracture is complicated which deviates from the conventional single pan shape. It can be in multi-wing shape by the formation heterogeneity and/or 3D fracture networks caused by the pre-exising natural fractures. Effective and flexible handling of hydraulic fractures is crucial for the reservoir simulation of unconventional reservoirs. In this paper, we discuss the robust implementations of the 3D-EDFM algorithm, which is applicable to numerical simulation of fluid flow in unconventional reservoirs with complicated, arbitrary 3D hydraulic fracture networks. Key features of this 3D-EDFM methodology consist of vertex-based geometric calculations, and polygons with three to six vertices from box-plane intersections can be captured with this algorithm. The 3D-EDFM has been implemented in reservoir simulators via the integral finite difference (IFD) method. Its numerical scheme and implementation are verified by comparisons with analytical solutions and other established numerical solutions. This method not only provides an effective approach of handling realistic hydraulic fractures, but also significantly improves computational efficiency, while keeping sufficient accuracy. For some extreme cases that the transient state flow lasts long (i.e., large-size matrix blocks with extremely low permeability), our numerical analysis indicates only one level of local grid refinement (LGR) is needed to retain the required accuracy. We also provide an analytical-solution based method to optimize this LGR grid size if needed. This enhanced method is able to handle multiple 3D hydraulic fractures with arbitrary strikes and dip angles, shapes, curvatures, conductivities, and connections. Therefore, the 3D-EDFM provides great flexibility to handle hydraulic-fracture input data, interpreted from geomechanics modelings as well as microseismic data, for reservoir simulation. We present illustrative application examples for three unconventional reservoir engineering problems of interest in practice: (1) a single fracture with a complicated shape due to multi-layer geological features, (2) multiple isolated fractures with curvatures due to the stress shadow effect, and (3) multiple fractures with complicated orientations caused by natural fracture interactions. The 3D-EDFM is demonstrated to capture all the key flow patterns dominated by the discrete fractures in these three cases.

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