CVD-MPFA Full Pressure Support Scheme for Discrete-fracture Matrix Simulations on Unstructured Grids

We investigate cell-centred finite-volume, control-volume distributed multipoint flux approximation (CVD-MPFA), with full pressure support (FPS), for discrete-fractures on unstructured grids. Fractures are modelled by lower-dimensional interfaces between matrix cells in the physical mesh but expanded to equi-dimensional cells in the computational domain; known as the hybrid-grid method. We present a simple process to form a precise hybrid-grid locally for a dual-cell. We also present a novel robust method for intersecting fractures, for FPS method, which reduces the condition number of the global linear system and leads to larger time steps for tracer transport. TPS and FPS hybrid-grid formulations are compared with corresponding fine-scale explicit equi-dimensional formulations. The results show that the FPS scheme provides robust and improved approximations for weakly anisotropic fields compared to the TPS scheme. The FPS hybrid-grid method is also beneficial for fractured domains with very strong anisotropic full-tensor permeability fields where the TPS scheme exhibits spurious oscillations. Furthermore, we present FPS coupled with a lower-dimensional fracture model, where fractures are strictly lower-dimensional in the physical mesh as well as in the computational domain. We present a comparison of the hybrid-grid FPS method and the lower-dimensional fracture model for several cases of isotropic and anisotropic fractured media.