Novel Insight into Foam Mobility Control

Abstract

Abstract A laboratory study of foam propagation in natural sandstones in the absence of oil is reported. The goal of this study was to elucidate further the mechanisms of foam mobility control. The C14-16 Alpha-Olefin Sulfonate (AOS) surfactant was selected to stabilize nitrogen foam. X-ray CT scan images were taken during foam propagation to map liquid saturation over time. The effects of surfactant concentration and of total injection velocity were examined in detail as these are key parameters for controlling foam strength and foam propagation under field conditions. The experiments revealed that foam mobility decreases in two steps: during the initial forward foam propagation it decreases by an order of magnitude compared to water mobility andduring the secondary backward liquid desaturation it decreases further by one to two orders of magnitude for sufficiently high surfactant concentrations.The steady-state mobility reduction factor increases considerably with both surfactant concentration and total injection velocity. A hysteresis was observed for a cycle of increasing and decreasing surfactant concentration or total injection velocity. The observed effects could be interpreted mechanistically in terms of surfactant adsorption and foam rheology. Implications for field application of foam for immiscible and miscible gas EOR are discussed. Introduction Foam is generated whenever gas is forced through porous medium containing a surfactant solution. The gas breaks into bubbles that are stabilized by the surfactant in a liquid continuum (Holm, 1968; Schramm and Wassmuth, 1994). The occurrence of foam is discernible in the drastic lowering of gas mobility (Kovscek and Radke, 1994; Rossen, 1996; Zitha et al., 2006). Such mobility reduction is essential for improving the volumetric sweep efficiency during immiscible or miscible gas flooding of oil reservoirs. It has thus obvious potential benefits not only for enhanced oil recovery (EOR) (Patzek, 1996; Turta and Singhal, 1998; Yan et al., 2006; Farajzadeh et al., 2010) but also for near-wellbore applications such as acid diversion, gas-shutoff, and water-shutoff (Hanssen and Dalland, 1994; Kibodeaux et al., 1994; Behenna, 1995; Zhdanov et al., 1996). This study will focus on the effects of surfactant concentration and of total injection velocity on foam propagation and foam strength in the absence of oil: related studies of foam behavior in the presence of oil have been reported recently (Andrianov et al., 2011; Guo et al., 2011). The emphasis on the surfactant concentration and injection velocity arises from two main reasons. Firstly, the surfactant concentration is the main physical parameter that can be used for switching foam on or off and for tuning foam strength to the requirements for a specific EOR field application. Secondly, the total injection velocity is a key parameter for controlling foam injection and propagation. The mobility reduction by foam conceals the interplay of two distinct but intimately related effects (Schramm and Wassmuth, 1994; Kovscek and Radke, 1994; Rossen, 1996). The first is the mobility reduction of the liquid phase and the second is that of the gaseous phase. Bernard et al. (1965) have shown long time ago that the relative permeability function for aqueous phase during foam flow is identical to that for the free (unfoamed) gas. Hence, since the liquid viscosity is constant, the reduction of the liquid mobility is simply due to the lowering of its relative permeability with increasing gas saturation. This has been largely confirmed by others (Sanchez et al., 1986; Friedmann et al., 1991; Vassenden et al., 1999). The lowering of gas mobility is much subtler and has been a subject of extensive research. Many authors (Bernard et al., 1965; Kovscek et al., 1997) distinguish two contributions to the foam mobility reduction: (a) the lowering of the foam relative permeability compared to the free gas and (b) the increase in the foam effective viscosity. Although separating relative permeability and viscosity effects experimentally remains an unresolved issue, the investigation of the two effects individually provided valuable insight on foam behavior in porous media.