Microscale insights into gas–water interface movement in rough fractures with geometric heterogeneity

Rough fractures with geometric heterogeneity significantly affect the movement characteristic of the gas–water flow interface. However, studies on gas–water flow in smooth or reconstructed fractures face challenges in clarifying the specific effect of fracture roughness on gas–water microflow. This study employs the Weierstrass–Mandelbrot function to model rough fractures and develops a theoretical model for gas–water interface movement by coupling the Navier–Stokes and phase-field equations. This model provides a robust framework for analyzing gas–water microflow in rough fractures and the model is validated through contact angle measurements of coal, shale, sandstone, and salt rock. The model investigates the microscopic flow characteristics of gas and water in rough single fracture and fracture networks. The simulation results show that water accumulates in asperities of rough fractures, with its residual volume fraction increasing with fractal dimension but decreasing with larger contact angles. The vortices within the asperities of rough fractures are the fundamental reason preventing the transport of residual water. As the fractal dimension of rough fractures increases, both gas breakthrough time and residual water volume fraction increase, indicating that greater fracture roughness reduces gas–water transport speed and limits effective flow space. Additionally, four typical residual water distribution patterns (H-channel, parallel channel, dead-end channel, and rough fracture asperities) are identified. This study offers critical insights into how fracture roughness influences gas–water microflow, potentially guiding improved energy recovery and storage strategies in fractured formation.