On the Role of the Viscosity Ratio during Low-Capillary-Number Forced Imbibition in Porous Media

Abstract

Abstract The role of the viscosity ratio κ during forced imbibition in porous media is investigated theoretically using a new simulator, and experimentally based on displacements in model pore networks. Both theory and experiment show that κ is an important parameter of microdisplacement in porous media, not only for intermediate and large capillary number values, but also for small values, say Ca -6 . In the latter region the residual nonwetting saturation S or is virtually independent of Ca for κ 1. The unexpected result is that, even for very small values of Ca, S or decreases appreciably as κ decreases, especially in the case of very good wettability. Simulations indicate that the effect of κ on S or for low Ca values is enhanced as the contact angle decreases. The phenomenon is attributed to a synergistic effect between capillary microfingering and localized viscous forces. It must be emphasized that the velocity gradients which are created locally by the advance of a single meniscus, or of a wetting film, are sufficiently large to make viscous stresses important, even when the Ca value of the macroscopic flow is very low (say, of order 10 -8 ). A favorable viscosity ratio reduces the extent of capillary microfingering and thus increases the efficiency of microdisplacement, despite the fact that for Ca -6 the viscous stresses are negligible on a macroscopic scale. Further work is needed to analyze the synergism between κ and θ e at low Ca values.