Manipulation of Viscous Fingering in a Radially-Tapered Cell Geometry

When a more mobile fluid displaces another immiscible one in a porous medium, viscous fingering propagates with a partial sweep, which hinders oil recovery and soil remedy. We experimentally investigate the feasibility of tuning such fingering propagation in a non-uniform narrow passage with a radial injection, which is widely used in various applications. We show that a radially converging cell can suppress the common viscous fingering observed in a uniform passage, and a full sweep of the displaced fluid is then achieved. The injection flow rate, $Q$, can be further exploited to manipulate the viscous fingering instability. For a fixed gap gradient $\alpha$, our experimental results show a full sweep at a small $Q$ but partial displacement with fingering at a sufficient $Q$. Finally, by varying $\alpha$, we identify and characterize the variation of the critical threshold between stable and unstable displacements. Our experimental results reveal good agreement with theoretical predictions by a linear stability analysis.