A 2D MPLBM-FDEM method for modeling two-phase displacement in porous media under fluid-solid interaction conditions

This paper develops a two-dimensional (2D) numerical framework by coupling the multi-component pseudopotential lattice Boltzmann method (MPLBM) and the combined finite-discrete element method (FDEM) to investigate the process of two-phase displacement in porous media under fluid–solid interaction (FSI) conditions. The MPLBM is used to simulate two-phase displacement processes, while the FDEM simulates the mechanical response of solids, and the immersed moving boundary (IMB) scheme is employed to achieve bidirectional coupling between MPLBM and FDEM. To prevent artificial pore-channel closure under confining pressure in 2D simulations, an algorithm for equivalent 3D pore connectivity in 2D simulations (E3PC-2D) is developed via the introduction of an outer ghost layer (OGL) element strategy. The MPLBM-FDEM coupling framework is verified through four benchmark cases, while the mechanical and hydraulic consistency of the E3PC-2D algorithm is further assessed using stress-dependent porosity and permeability responses. Then, the MPLBM-FDEM method is adopted to investigate the evolution of water–oil displacement under varying capillary numbers and confining pressures. Numerical results indicate that the saturation ( S w ) and fractal dimension ( D f ) of the invading water phase decrease as confining pressure increases, and S w is more sensitive to changes in confining pressure than D f . At the pore scale, quantitative analysis of throat-width reduction and capillary-entry-threshold variation shows that increasing confining pressure enhances local entry resistance and invasion selectivity, thereby producing narrower preferential invasion pathways. In addition, this study proposes a theory of displacement pattern transition that accounts for the effects of confining pressure and develops a corresponding phase diagram. The phase diagram is generally consistent with the simulated regime distribution and predicts a widening trend of the crossover-zone interval with increasing confining pressure. Overall, this study provides a robust methodology for analyzing multiphase displacement mechanisms under FSI conditions.