Merging Discrete Fracture Network Meshes With 3D Continuum Meshes of Rock Matrix: A Novel Approach

Most geologic media exhibit an extreme heterogeneity with high variability of permeability and structural complexity. Modeling subsurface heterogeneity and simulating fluid flow and transport through variably fractured rocks as continuous processes is crucial for multiple geological applications. However, the complexities of subsurface media often cause difficulties in the simulation process, in particular, in high quality computational mesh generation. In the current work, we present a new methodology for constructing a mesh, where a topological 2D mesh in 3D space of a discrete fracture network is seamlessly connected with a continuum 3D volume mesh. The combination of a 2D conforming triangulation with a continuum 3D volume mesh, which represents rock matrix, enables control volume flow calculations. This new capability enables modeling of subsurface flow controlled by diffusivity and transport dominated by advection as a seamless computational process. The new meshing algorithm is implemented and successfully verified for geologic CO₂ storage applications. The CO₂ migration starts from a primary storage reservoir (continuum), moving through a fractured caprock (Discrete Fracture Network, DFN) to an upper reservoir (continuum). The reservoirs are modeled using a continuum volume mesh, and the caprock, where fractures are naturally present, is modeled using a discrete fracture network. The constructed model allows one to capture the effect of fracture network complexity on CO₂ migration.