Conditions for Foam Generation in Homogeneous Porous Media

Abstract

Abstract The generation of strong foam in homogeneous porous media has been reported to be a function of flow rate, pressure drop, permeability, and system length. This study seeks to explain foam generation in terms of dimensionless groups and establish the criteria for foam generation based on the mobilization of stationary foam lamellae. Steady-state theories and experiments in the literature suggested the existence of a critical pressure gradient for foam mobilization that is inversely proportional to permeability. Experiments involving transient displacement of surfactant solution with gas injection, however, revealed a critical pressure drop that is inversely proportional to the square-root of permeability. The observation led to a new criterion for mobilization and foam generation in transient experiments, that the pressure drop across the displacement front must exceed the pressure drop to mobilize a single lamella. Introduction Due to foam's effect in reducing gas mobility, it has been used for a wide range of subsurface applications ranging from enhanced oil recovery1–3 and oil well stimulation4,5 to aquifer remediation. 6–9 Nevertheless, how foam generation is affected by different factors is not yet fully understood. Depending on the conditions for foam generation, the mobility of foam in porous media can differ by more than a factor of 100. For homogeneous porous media, various factors have been recognized to influence foam generation and the transition from weak foam to strong foam. They include flow rate, pressure drop, pressure gradient, permeability, and system length. 10–16 However, there have also been different observations on how these factors affect foam generation. Hence, several different hypotheses have been proposed,11,13,16 each differing in their underlying mechanisms and/or resulting in different predictions. Thus, this study attempts to reconcile the different observations and search for the dimensionless groups that govern foam generation in homogeneous media. Background Foam generation mechanisms. Researchers have identified three underlying mechanisms for foam generation, namely leave-behind, capillary snap-off, and lamella division. 17 Foam lamellae can be generated by leave-behind when two gas fingers invade adjacent liquid-filled pore bodies. However, the left-behind lamella is oriented parallel to the direction of flow and does not make the gas discontinuous, thus resulting only in continuous-gas foam, also called weak foam. The generation of discontinuous-gas foam, or strong foam, requires the occurrence of snap-off and/or lamella division. Snap-off may take place when a non-wetting phase (gas in the case of foam) enters a pore constriction initially filled with wetting liquid. 18 Sufficient amount of the wetting phase is necessary for snap-off. In addition, Pc at the invading gas front must be below a critical value experimentally demonstrated to be about half of the capillary entry pressure for a given porous medium,. 12Pore body radius of at least about twice the pore throat radius is necessary to create the required capillary pressure reduction for snap-off. 19 Lamella division, another mechanism that leads to strong foam, may take place only after some lamellae have been generated and when the pressure gradient becomes large enough to mobilize the lamellae. When a moving lamella train encounters a branch in the flow path, it may split into two, one in each branch of the path. Conditions for foam generation. For the remainder of this paper, we will use the term "foam generation" not only to describe the transition from no foam to strong foam, but also that from weak continuous-gas foam to strong discontinuous-gas foam. Weak foam is produced by the leave-behind mechanism while strong foam is usually obtained through some combination of all three mechanisms. Foam generation mechanisms. Researchers have identified three underlying mechanisms for foam generation, namely leave-behind, capillary snap-off, and lamella division. 17 Foam lamellae can be generated by leave-behind when two gas fingers invade adjacent liquid-filled pore bodies. However, the left-behind lamella is oriented parallel to the direction of flow and does not make the gas discontinuous, thus resulting only in continuous-gas foam, also called weak foam. The generation of discontinuous-gas foam, or strong foam, requires the occurrence of snap-off and/or lamella division. Snap-off may take place when a non-wetting phase (gas in the case of foam) enters a pore constriction initially filled with wetting liquid. 18 Sufficient amount of the wetting phase is necessary for snap-off. In addition, Pc at the invading gas front must be below a critical value experimentally demonstrated to be about half of the capillary entry pressure for a given porous medium,. 12 Pore body radius of at least about twice the pore throat radius is necessary to create the required capillary pressure reduction for snap-off. 19 Lamella division, another mechanism that leads to strong foam, may take place only after some lamellae have been generated and when the pressure gradient becomes large enough to mobilize the lamellae. When a moving lamella train encounters a branch in the flow path, it may split into two, one in each branch of the path. Conditions for foam generation. For the remainder of this paper, we will use the term "foam generation" not only to describe the transition from no foam to strong foam, but also that from weak continuous-gas foam to strong discontinuous-gas foam. Weak foam is produced by the leave-behind mechanism while strong foam is usually obtained through some combination of all three mechanisms.