Abstract
The immiscible displacement of a viscous fluid by a less viscous one in a porous medium is known to be unstable and often results in dendritic fingering patterns, where the pressure gradient is screened internally and only advanced invading parts grow. We present experiments in porous media, systematically varying the imposed injection pressure ($P$) and viscosity ratio ($M$) of the invading to defending fluid. A crossover to more stable and compact invasion structures is found for higher $P$ and $M$. This compact invasion regime onset is observed even for $M<{10}^{\ensuremath{-}3}$. We show that the pressure gradient is not screened in this regime and invasion patterns exhibit rich internal flow dynamics.
Highlights
Two-phase flow in porous media is of central importance in a wide range of fields from fundamental research to applied areas and practical daily life problems like making a cup of tea
We describe a crossover from the viscous fingering instability to a compact invasion regime during viscously unstable drainage of porous media, and we investigate the underlying mechanisms of this compact fluid displacement
We show that above a threshold of injection pressure and viscosity ratio a more stable and compact invasion structure emerges within the viscous fingering patterns, i.e., a roughly circular displacement with viscous fingers on the outside
Summary
Two-phase flow in porous media is of central importance in a wide range of fields from fundamental research to applied areas and practical daily life problems like making a cup of tea. The most unstable wavelength given by the theory by Saffman and Taylor [17] is governed by the competition between viscous and capillary forces and predicts the onset of instability but does not predict the coexisting growth of a compact internal zone and viscous fingers. This compact growth is observed in our experiments even when the viscosity of the displaced fluid is 1000 times higher than that of the invading fluid
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