Foam Generation at Layer Boundaries in Porous Media

Foam can be created by snap-off in a strongly water-wet porous medium as gas and surfactant solution flow across a sharp transition in permeability. A simple pore-network model shows that this depends on a large viscosity contrast between gas and liquid and the ratio of gas and liquid flow rates, but not on the ratio of pore-throat to pore-body diameters. The model explains a variety of experimental observations.