A numerical framework for two-phase flow of CO2 injection into fractured water-saturated reservoirs

Abstract

A novel numerical framework is presented for simulation of compositional compressible gas-water phase flow in fractured porous media based on a fully-implicit cell-centred finite-volume method on unstructured grids. We employ a discrete-fracture model where fractures are modelled lower-dimensionally, within a rock matrix. A mass balance equation is solved over the rock-matrix while a lower-dimensional flow equation is solved over the lower-dimensional fractures, with coupling terms to account for the flux transfer from the surrounding rock-matrix. The discretisation of the Darcy-fluxes is based on the control-volume distributed multi-point flux approximation (CVD-MPFA) coupled with a lower-dimensional fracture model. We solve a non-linear system for the primary variables that are the phase pressure and the component molar densities while the secondary variables are updated depending on the updated values of the primary variables at each iteration of the non-linear solver. We use the cubic type equation-of-state (EOS) to model the physical properties of gas components, e.g. CO2. The water phase is multi-component as we allow solubility of gas components in the water phase. Solubility of H2O in the gaseous phase is also included in the developed method. We compare compositional simulation results, using unstructured grids, obtained via the lower-dimensional fracture model and the results obtained by the equi-dimensional fracture model for a case of CO2 injection into a water saturated reservoir with intersecting fractures. The results show good agreement between the two models while the lower-dimensional fracture model is also computationally cost effective. Results are also presented for the evolution of injected CO2 under gravity through a water saturated reservoir with and without fractures that demonstrate the profound effects of discrete-fractures that can lead to CO2 leakage. The presented method is also applied to a 3D simulation of injection of a multi-component gas with CO2 and CH4 into a saline water saturated reservoir with a surface 2D fracture network.