In our pore-level simulations, we have observed and characterized a crossover from fractal viscous fingering to standard compact flow with a rough interface. For more than a half century, it has been "standard practice of the petroleum industry" to use a "quarter power mixing rule" to characterize the effective viscosity of a mixture of two fluids. In attempting to apply the "quarter power mixing rule" to our simulation results, we find that it adequately represents our data. However, we find that a smaller power law better represents the behavior of larger (later time) flows.

The interfacial width is observed to obey the same crossover to a rough interface. In the compact regime, i.e. well past crossover, the width of this rough interface increases with time. Our results for interfacial roughness are compared

Our characterizations of the crossover predict the viscosity ratio dependence of various aspects of the flow behavior (interfacial advance, interfacial width, etc.).

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