Drainage dynamics: From capillary to viscous fingering under different wettability

The displacement of two-phase fluids within porous media is a critical process in various engineering and industrial fields. However, the inherent instability of the fluid interface often leads to an irregular morphology of the displacement front, which can impact the efficiency of displacement and limit its application in engineering activities. In this study, the color gradient model based on the lattice Boltzmann method was employed to simulate displacement, with the aim of investigating the dynamic evolution of the displacement front under the combined effects of wettability and injection velocity, specifically during the capillary to viscous fingering transition. The results demonstrate that the impact of wettability on the displacement front morphology increases as the injection velocity decreases. The displacement process with the maximum fingering distribution range and compact morphology is found at the midpoint of the capillary-viscous fingering crossover zone. Furthermore, an in-depth analysis of the phase field evolution and pressure difference changes between the inlet and outlet unveiled the pore-filling mechanisms associated with the unique variation characteristics of the displacement front and the fluid topological structure. These comprehensive pore-scale findings offer theoretical insight supporting the engineering application of the two-phase flow in porous media from multiple perspectives.