An extended numerical manifold method for two-phase seepage–stress coupling process modelling in fractured porous medium

Abstract

To investigate the two-phase seepage–stress coupling process in fractured porous medium, this study extends the cohesive element-based numerical manifold method (Co-NMM) by incorporating a two-phase seepage–stress coupling model considering the effect of matrix-fracture interface on the two-phase flow and fracture propagation induced by the two-phase seepage pressure. The proposed two-phase flow solving framework implicitly calculates the fluid pressure and saturation of the two-phase flow based on a two-phase unified pipe network method. Furthermore, to more realistically model the hydraulic behaviour of two-phase flow in fractured porous medium, a matrix-fracture interface condition called the extended capillary pressure condition is incorporated into the two-phase flow solving framework to capture the interactions among fluid flow in the fractures and matrix. Due to the inheritance of the Co-NMM, one key advantage of the extended method is the simulation of complex multi-fracture propagation caused by the two-phase seepage–stress coupling. The two-phase flow solving framework is first validated by reproducing the water displacing oil in a single fracture and the gas displacing water in a single-fractured porous medium against analytical and numerical solutions, respectively. The two-phase seepage–stress coupling procedure is then verified by performing a one-dimensional consolidation problem of soil column, in which comparisons between the numerical and analytical results regarding the pore pressure and compression displacement are presented. Finally, with the extended method, CO2-enhanced oil recovery in fractured reservoir is preliminarily studied by considering the effect of gas injection rate and capillary pressure on the evolution of two-phase flow and fracture propagation. The results elucidate that high CO2 injection rate can lead to fracture propagation in the reservoir, and both capillary pressure and fractures have a significant effect on the CO2 distribution.