A Hybrid Continuum-Discrete Scheme for Simulating CO2 Migration and Trapping in Fractured Sandstone Reservoirs

Supercritical CO2 (scCO2) injected into fractured sandstone reservoirs, such as at the In Salah site in Algeria, flows preferentially through the fracture network and invades into the rock matrix if matrix permeability is sufficiently high and capillary entry pressure is sufficiently low. The invading scCO2 displaces resident brine in the matrix downgradient, allowing for additional scCO2 matrix storage. The displaced brine crosses matrix bridges and brine-filled small-aperture regions within fractures between neighboring matrix blocks. To account for preferential scCO2 flow through fractures, scCO2 invasion into the rock matrix, and cross-fracture brine flow, we propose a hybrid continuum-discrete scheme with one fracture continuum (F) and multiple matrix continua (M1 through Mn). The scheme allows for global F-F flow through fractures, local F-M1 flow for scCO2 invasion, local Mi-Mi+1 brine and scCO2 flow, and global Mn-Mn brine flow. The proposed scheme differs from existing continuum-based dual-porosity, dual-permeability, and multiple interacting continua (MINC) schemes in that the matrix-matrix connectivity via bridges and brine-filled regions is considered explicitly, while the existing schemes allow either 0 (e.g., dual-porosity) or 100% (e.g., dual-permeability) matrix-matrix connectivity. The proposed scheme is implemented in the multi-phase multi-component simulator TOUGH2 and demonstrated on a hypothetical, large-scale scCO2 injection and storage case. A sensitivity analysis on different driving forces for fracture-matrix interactions is conducted in both homogeneous and heterogeneous fractures and matrix. It is found that in a thick reservoir, buoyancy enhances scCO2 invasion from fractures into the rock matrix, driving global matrix-matrix brine flow. Without scCO2 invasion into the matrix, the storage capacity and efficiency of CO2 in fractured sandstone, which are proportional to formation compressibility and pressure increase, is small.