The generation of foam in porous media remains a controversial issue. It has been reported that foam is formed only above a minimum flow rate (or capillary number), above a minimum pressure gradient, or below a critical capillary pressure. In this paper we investigated experimentally how these and other parameters affect the formation of N2 and CO2 foams in Berea sandstones.
We found that foam was readily formed by co-injecting gas and surfactant solution whenever the core was presaturated with surfactant, regardless of flow rate or pressure gradient In general, foam generation can be initiated by first creating regions of high surfactant saturation in situ and then draining those regions; the onset of foam generation coincided with the onset of drainage. Somewhat surprisingly, it was easier to generate a stronger foam than a weaker foam, and fewer pore volumes of injection were needed to form foam in a longer core than in a shorter core.
A consistent mechanism that emerges from these observations is that foam is formed by raising capillary pressure in a surfactant-saturated region. As gas enters such regions, individual lamellae are formed by snap-off in certain pores. These lamellae must be sufficiently stable to allow the local capillary pressure to increase in time, forcing gas to enter increasingly smaller pores and generate more lamellae. Presaturating the porous medium with surfactant ensures that stable lamellae are formed whenever snap-off occurs. In contrast, if continuous gas channels exist over sample-spanning distances, the injected gas preferentially flows through such channels, capillary pressure cannot rise significantly, and foam generation is delayed. Practical implications of these findings on applications of foam are discussed.