Foam Mobility Control for Surfactant EOR

Abstract

Abstract Foam generated in situ by surfactant alternated with gas injection is demonstrated as a substitute for polymer drive in the alkaline-surfactant-polymer (ASP) EOR process. Foam is also effective in a similar process for a 266 cp crude oil, even though the system did not have enough polymer for favorable mobility control. Foam is shown to greatly enhance sweep efficiency in a layered sandpack with a 19:1 permeability ratio. Foam diverted surfactant solution from the high-permeability layer to the low-permeability layer. Ahead of the foam front, liquid in the low-permeability layer crossflowed into the high-permeability layer. A layered system with a 19:1 permeability contrast could be completely swept in 1.3 TPV (total pore volume) with foam while waterflooding required 8 PV (pore volume). Introduction Foam as a means for mobility control of surfactant flooding was introduced 28 years ago by Lawson and Reisberg (1980). This concept was not immediately adopted because of the lack of understanding of the mechanism of mobility control with foam. Since that time there have been many advances in the understanding of foam mobility control. There have been many field tests of steam foam (Hirasaki 1989; Patzek 1996) and CO2 foam. One of the most successful field demonstrations of foam mobility control was in the Snorre field (Blaker 2002). Foam was used as mobility control for surfactant aquifer remediation at Hill AFB in Utah (Hirasaki 1997, 2000). Foam was used as mobility control for alkaline surfactant flooding in China (Zhang 2000; Wang 2001). The most important advance in understanding that has made foam mobility control practical is the understanding of the condition necessary to generate "strong" foam. There is a critical pressure gradient that must be exceeded to generate strong foam during the flow of surfactant solution and gas through homogeneous porous media (Falls 1988; Gauglitz 2002; Kam 2003; Rossen 1996, 2007; Tanzil 2002a). Below this pressure gradient gas may flow as a continuous phase with only modest mobility reduction. Above this pressure gradient, stationary bubbles are mobilized such that bubble-trains have multiple branch points. A flowing bubble divides into two bubbles at each branch point and thus regenerates bubbles that are lost to coalescence. Foam bubbles can also be regenerated (independent of pressure gradient) when gas and surfactant solution flow across a step increase in permeability with a ratio greater than 4 (Falls 1988; Tanzil 2002a). If one recognizes the critical pressure gradient necessary for strong foam, experiments can be conducted at high enough flow rate or pressure-drop such that the critical pressure gradient is exceeded. The other important advance in understanding is the observation that when the foam is flowing with conditions where it is regenerated in situ, the gas mobility is determined by a "limiting capillary pressure" above which the lamellae become unstable and bubbles coalesce (Khatib 1988). This understanding explains why in this regime, the pressure gradient is a function of the liquid flow rate but independent of the gas flow rate. Also, foam mobility can be modeled by "fractional flow theory" in this flow regime (Gauglitz 2002; Rossen 1996). In this regime, gas mobility increases with increasing gas fractional flow and decreasing permeability. This permeability dependence makes foam especially useful for improving sweep in layered systems (Heller 1994; Bertin 1998; Kovscek 2002). The dependence of foam mobility on fracture aperture has been demonstrated to be beneficial in the sweep of fracture systems (Yan 2006).