Foam Propagation at Low Superficial Velocity: Implications for Long-Distance Foam Propagation

Since the 1980s, experimental and field studies have found an anomalously slow propagation of foam (Friedmann et al. 1991, 1994; Patzek 1996), a phenomenon that cannot be fully explained by surfactant adsorption. Friedmann et al. (1994) conducted foam-propagation experiments in a cone-shaped sandpack and concluded that foam, once formed in the narrow inlet, was unable to propagate at all at lower superficial velocities near the wider outlet. They concluded that long-distance foam propagation in radial flow from an injection well is in doubt. Ashoori et al. (2012) provide a theoretical explanation for a slower and/or nonpropagation of foam front at decreasing superficial velocity. By linking foam propagation to the minimum superficial velocity u(t)(min) (or minimum pressure gradient del P-min) required for foam generation in homogeneous porous media (Rossen and Gauglitz 1990; Gauglitz et al. 2002), the study reveals that the minimum velocity for maintaining the propagation of foam is far less than that for creating foam but greater than the minimum velocity for maintaining foam in place. Lee et al. (2016) and Izadi and Kam (2019) find a minimum velocity for foam propagation from analysis of a similar population-balance model but associate it with the minimum velocity for foam stability. In this study, we extend the experimental approach of Friedmann et al. (1991) in the context of the theory of Ashoori et al. (2012). We observe dynamic propagation of foam in a cylindrical core with stepwise increasing diameter such that the superficial velocity decreases from inlet to outlet (in a ratio of 16:1). Previously (Yu et al. 2019), we mapped the conditions for foam generation (at large superficial velocities) in a Bentheimer sandstone core, in relation to surfactant concentration and injected gas fraction (foam quality). In this study, we enrich the map with the conditions for downstream propagation of foam (at significantly smaller superficial velocities). We also interpret our results for both foam generation and propagation in terms of local pressure gradient (following the implications of Ashoori et al. 2012), which plays a dominant role in the mobilization and creation of foam. Our results suggest that the minimum superficial velocities for both foam generation and propagation increase with increasing foam quality and decreasing surfactant concentration, in agreement with theory (Rossen and Gauglitz 1990). In addition, the minimum velocity for propagation of foam is much less than that for foam generation, as has been predicted by Ashoori et al. (2012). Implications of our laboratory results for field application of foam are briefly discussed.