Pore-scale study of the effects of surface roughness on relative permeability of rock fractures using lattice Boltzmann method

Abstract

Two-phase flow in fractured geo-material plays a significant role in various subsurface processes. Despite some success in studying two-phase flow in ideal smooth-walled fractures, the more realistic issues regarding two-phase flow in rough-walled fractures are still not well understood. In this study, an improved approach based on free energy lattice Boltzmann model is proposed to analyze the two-phase immiscible flow in the rough-walled fracture and to further estimate the relative permeability curves. The proposed model is validated by comparison with analytical results of individual fracture, and good agreement is achieved in general. Then it is used to investigate the effects of wall roughness and the initial phase distribution patterns on the co-current flow of two immiscible fluids. The simulation results demonstrate that roughness impacts phase distribution, flow patterns and relative permeability curves in a single rough-walled fracture. Rough surface increases two-phase interference and flow resistance, and an unsteady flow state appears for the non-wetting phase (NWP) when the wetting phase (WP) saturation is small. In addition, the NWP flow can either be lubricated or be resisted by the WP depending on the flow patterns and the WP saturation.