Pore-level modeling of wetting

The displacement of one fluid by another in a porous medium is influenced by the competition between advance through the centers of the pore space and flow of a wetting phase along crevices; different pore filling mechanisms whose threshold capillary pressures depend on the number of filled neighbors; and the perturbative effects of viscous and capillary forces. We present a three-dimensional pore-level model that represents these effects, including flow in crevices, and discuss the types of behavior that can result. Viscous and buoyancy forces introduce a finite correlation length for both the advancing front and for the trapped nonwetting phase. We use the percolation theory to derive expressions for the correlation length and the shift in trapped saturation for different flow regimes. When cooperative pore filling is significant, the average finger width is much larger than a single pore length, and percolation theory cannot predict the trapped saturation, even if asymptotically the fluid pattern is percolationlike. The type of fluid pattern changes with flow rate because of the competition between flow in crevices and frontal advance. This in turn can lead to a significant decrease in trapped nonwetting phase saturation with flow rate. This is a mechanism for a reduction in trapped saturation at much lower capillary numbers than that predicted using percolation theory or from considering mobilization of ganglia. (c) 1995 The American Physical Society