Modeling fracture transient flow using the Embedded Discrete Fracture Model with nested local grid refinement

Abstract

Transient flow around hydraulic fractures is an important topic in the study of tight reservoirs. In this work, the Embedded Discrete Fracture Model (EDFM) is combined with nested local grid refinement (LGR) to improve its accuracy for simulating near-fracture transient flow. An automatic grid refinement procedure is developed to iteratively conduct LGR around complex fractures. In each iteration, the cells intersected by or close to fractures are selected and refined. The size of the refinement region and the level of refinement can be adjusted. Through this process, the cell size in the near-fracture region is reduced rapidly, while the total cell count remains at a relatively low level. An analysis of the optimal refinement ratio for regular Cartesian grids is also conducted. Furthermore, the EDFM is implemented in grids with nested LGR, and connections between the matrix and fractures are properly determined to compute the transmissibility factors as input for reservoir simulators.The developed methodology is first verified against a logarithmic LGR model for different fracture orientations, and the results show that different flow regimes can be reliably simulated. Subsequently, the flow regimes for non-orthogonal, non-parallel, and nonplanar hydraulic fractures are studied with this approach to illustrate the unique patterns of transient flow around complex fractures. This work provides a general and convenient method to perform transient analysis for reservoirs with complex fracture geometries.