Multiscale fractures integrated equivalent porous media method for simulating flow and solute transport in fracture-matrix system

Abstract

The fracture–matrix system, in which water is stored and transported, has a significant impact on the hydraulic behaviors of fractured geologic media (FGM). However, it is very challenging to accurately simulate flow behavior in FGM due to the difficulty of characterizing dual-permeability media, including multiscale fractures with high permeability and porous matrix with low permeability. In this study, a multiscale fracture integrated equivalent porous medium (MFEPM) method is proposed for simulating fluid flow and solute transport in a fracture–matrix system. The synthetic enhanced matrix (SEM) is compounded by integrating small-scale fractures into the porous medium, and the medium- and large-scale fractures are mapped to refined grids of the MFEPM to increase the characterization of the fracture geometry and orientation. Then, the equivalent hydraulic properties are calculated according to the properties of the fractures. Finally, a finite difference method (FDM) based on the MFEPM is used to simulate flow and solute transport in coupled multiscale fractures and rock matrix. The case study illustrates that compared with the traditional equivalent porous medium (EPM) method, the MFEPM manifests an efficient effect in the characterization of preferential flow; compared with the discrete fracture network (DFN) method, the MFEPM can characterize flow and solute transport in the SEM and represent mass interactions between the fractures and matrix. The results of the numerical case studies of flow and solute transport also show that the MFEPM method has a high computational accuracy and good reliability, while maintaining an appropriate computational burden.