Numerical investigation on immiscible displacement in 3D rough fracture: Comparison with experiments and the role of viscous and capillary forces

Immiscible fluid-fluid displacement in rough fractures is important in many subsurface processes, including enhanced oil recovery and geological carbon sequestration. Numerous previous works experimentally investigated the dynamics of multiphase flow in fractures, but direct numerical simulation for fluid-fluid displacement in 3D rough fractures and the validation with experiments were rarely reported. Here we perform 3D direct numerical simulations for drainage processes of water displacing oil in a real rough fracture with capillary number log(10)Ca ranging from -3 to -5. Comparison between experiments and simulations shows that the simulated results can generally reproduce the dynamic invasion morphologies at the scale of fracture. The discrepancy of the invasion morphologies, however, is observed at the local scale, mainly because the mesh resolution is not small enough compared with the scale of the interface. We then perform quantitative analysis of the simulated results to investigate the role of capillary and viscous forces. We show that Haines Jump events in the flow passage of void neck connecting to a wider void occur in the capillary-dominated flow regime. Statistical analysis of the velocity fields under various flow rate conditions shows that as the effect of viscous force becomes more important and eventually dominates that of capillarity, void-filling toward the outlet is continuously enhanced, with the velocity vector angles being more probably localized in the zone around the bulk flow direction. The direct evidence provided by the 3D numerical simulations improves our understanding of the competition of viscous and capillary forces controlling the immiscible displacement in rough fractures.