Fragmentation of viscous fingers in porous media: A three dimensional video microscopic study

Abstract

A compact viscous finger in a porous medium destabilizes to form a droplet phase at large flow rates. We present a three dimensional video imaging to study this destabilization phenomenon. The images show that a system-spanning finger is a connected network of a large number of pore-scale fingers which fragment at high flow rates to destabilize the network. The volume occupancy of the droplet distribution in the steady state, arising out of events of break-up and coalescence in the pore scale, shows a non-monotonic behavior, naturally separating sub-pore scale droplets from those which span many pores. An analysis of the motion of the interface between the displacing and the displaced fluids provides evidence that the frequency of the pore scale break-up events increases with the flow rate. For low flow rates, these events are mainly localized near the leading edge of the front. However, for higher flow rates, the pore-scale break-up events extend well-behind the leading edge of the front which prevents the growth of a compact finger and instead promotes the formation of a droplet phase.