Experimental Observation of Two Distinct Finger Regimes During Miscible Displacement in Fracture

Miscible displacement of two-phase fluids in rough fractures is relevant to some industrial processes, including enhanced oil recovery and geological carbon sequestration. When a less viscous fluid displaces another more viscous fluid, finger instability occurs. Previous works focused on miscible displacement in porous media or Hele-Shaw, but the experimental study was rarely reported for rough fractures. Here, we perform visualization experiments of water displacing glycerol in a transparent fracture model to investigate the effects of flow rate and diffusion in miscible displacement. We quantify the displacement patterns using the sweep efficiency, the mixing length, and the relative contact area. We observe two distinct displacement regimes: dominant finger regime and multiple fingers regime. A critical Peclet number Pe is obtained to identify such two regimes. Below the critical Pe, the channel forms, and the displacement is the dominant finger regime, which results in low sweep efficiency and linearly growth of mixing length at late time. Above this critical Pe, intensive tip-splitting events result in the formation of dendritic displacement pattern, and the displacement is multiple fingers regime, slowing down the growth rate of mixing length at late time and contributes to the higher sweep efficiency. Our work shows a critical Pe that separates the two distinct regimes and improves our understanding of the evolution of the miscible displacement fronts in rough fractures.