Post-breakthrough evolution of viscous fingering in micromodel porous media

When a less viscous fluid displaces a more viscous one in a porous medium, the interface becomes unstable, forming viscous fingers. While viscous fingering has been studied in depth, previous work has focused on the pre-breakthrough dynamics – that is, before the injected fluid reaches any outlet boundary. Here, we investigate post-breakthrough behaviour in a radial micromodel through experiments and pore-network simulations. Post-breakthrough displacement is key for practical applications, as it governs how much of the defending fluid is ultimately recovered. We show that the displacement pattern either ‘freezes’ or continues to grow, depending on the capillary number italic Ca Ca Ca and viscosity ratio upper M M M . A pore-network model predicts a sharp drop in growth rate at breakthrough and a geometry-controlled cross-over italic Ca Subscript times Ca × Ca_× below which the displacement pattern is arrested. These results highlight the role of domain size, pore geometry, fluid properties and injection rate in regulating the post-breakthrough dynamics and provide a framework for optimising displacement efficiency.