Pore-network study of the mechanisms of foam generation in porous media.

Understanding the role of pore-level mechanisms is essential to the mechanistic modeling and simulation of foam processes in porous media. Three different pore-level events can lead to foam formation: snapoff, leave behind, and lamella division. The initial state of the porous medium (fully saturated with liquid or already partially drained), as surfactant is introduced, also affects the different foam-generation mechanisms. Bubbles created by any of these mechanisms cause the formation of new bubbles by snapoff and leave behind as gas drains liquid-saturated pores. Lamellae are stranded unless the pressure gradient is sufficient to mobilize those that have been created. To appreciate the roles of these mechanisms, their interaction at the pore-network level was studied. We report an extensive pore-network study that incorporates the above pore-level mechanisms, as foam is created by drainage or by the continuous injection of gas and liquid in porous media. Pore networks with up to 10 000 pores are considered. The study explores the roles of the pore-level events, and by implication, the appropriate form of the foam-generation function for mechanistic foam simulation. Results are compared with previous studies. In particular, the network simulations reconcile an apparent contradiction in the foam-generation model of Rossen and Gauglitz [AIChE J. 36, 1176 (1990)], and identify how foam is created near the inlet of the porous medium when lamella division controls foam generation. In the process, we also identify a new mechanism of snap-off and foam generation near the inlet of the medium.