Fluid-Fluid Displacement Patterns in Microfluidic Analogues of Rough Fractures with Controlled Roughness

We report results of immiscible fluid-fluid displacement experiments on 3D-printed artificial fractures with controlled roughness. Our fracture analogue consists of a microfluidic cell with an irregular post pattern and a “free gap” between the top of the posts and the flat surface. We inject a low-viscosity nonwetting fluid to displace a more viscous wetting fluid and observe a transition from porous media flow to flow between parallel plates as a function of free-gap height. When the free gap is smaller than or comparable to the post height, the drainage pattern is similar to that in a 2D porous micromodel. In contrast, when the free gap is significantly larger than the post height, drainage in the gap dominates, and the displacement pattern is controlled by the capillary number. At high Ca, a film of the defending phase is left on the rough surface behind the front; this film then redistributes and forms discrete wetting clusters. At low Ca, the invading fluid occupies the full height of the free gap, with little wetting phase trapped even under unfavorable viscosity ratio. Our observations point to the mechanistic interplay of roughness and capillarity on trapping of the wetting phase in a rough fracture, a process that significantly affects its multiphase-flow properties.