Development of an Embedded Discrete Fracture Model for Field-Scale Reservoir Simulation With Complex Corner-Point Grids

Abstract

Summary Corner-point grids have the capability to model complex geological features, such as faults and irregular reservoir boundaries. As an industry standard, they are widely used to simulate different types of reservoirs, including conventional and unconventional reservoirs. It is necessary to effectively simulate natural or hydraulic fractures in such reservoirs. In this work, a discrete fracture model is developed to conveniently simulate fractures in geologically complex reservoirs represented by corner-point grids. The method is an extension of the embedded discrete fracture model (EDFM). We first present the difficulties in terms of geometrical calculations pertaining to corner-point grids, including the irregularity and degeneracy of block geometry and irregular connections between fracture segments. A general-purpose geometrical algorithm is developed to find the intersections between the matrix and fractures in corner-point grids. This algorithm properly handles the intersection between a general polyhedron and a general polygon, in which both the polyhedron and the polygon can be convex or concave. Transmissibility-factor formulations are also further developed for connections and intersections between fractures. The calculation of effective well indices in different situations is also discussed in detail. Several case studies are presented to illustrate the accuracy and applicability of the developed model in standard black-oil or compositional simulators. The accuracy of the developed model is demonstrated by comparing its simulation results with those of local-grid-refinement (LGR) models. It is also found that the accuracy of the EDFM is not sensitive to matrix gridding when the average size of gridblocks is similar. Field-scale studies using synthetic and realistic reservoir models are presented to illustrate the significance of fractures during secondary recovery. Existing simulators can directly be used in conjunction with the proposed approach with slight modification in simulation input, if the simulators have nonneighboring-connection (NNC) functionality. Through the case studies, the algorithms and methodology developed in this work are shown to be highly effective for the modeling of fractures in field-scale reservoir-simulation studies with complex corner-point grids.