Fast, immiscible fluid-fluid displacement in three-dimensional porous media at finite viscosity contrast

The crossover from capillary to viscous flow during immiscible fluid-fluid displacement has been studied in transparent, three-dimensional porous media. The defending and the invading fluids have equal densities, so that gravity effects are eliminated. The viscosity ratio of defending fluid to injected fluid m is moderate m{}14. Dense, ball-like displacement structures are generated at high injection rates. Surprisingly, the flow appears to be nearly completely stabilized even though the viscosity of the invading fluid is significantly lower than that of the defending fluid. For the physical system studied here, fully developed ball-like patterns arise for capillary numbers Ca{≥}5×{}10^-3. A similar behavior is found in experiments using the same grains and fluids in a two-dimensional system.