Physsicochemical Aspects of Microemulsion Flooding

Abstract

Abstract Whenever water, an oil, and a surfactant equilibrate at concentrations of surfactant in excess of critical micelle concentrations, one or more microemulsions form.. In view of this, all surfactant flooding processes may involve microemulsions in situ. Ternary diagrams have been constructed for three specific microemulsion systems showing The effects of salinity and cosurfactant on phase behavior, viscosity, resistivity, optical birefringence, and interfacial tension. Using these data, micellar structure maps were prepared for the single-phase region. In this connection, Winsor's concept of intermicellar equilibrium was found consistent with microemulsion systems of interest for tertiary oil recovery. Experimental techniques are described for minimizing the extent of the multiphase region and measuring the low interfacial tensions that obtain there. Introduction GENERAL CONSIDERATIONS It is sometimes difficult to sort out the multiplicity of terminology concerning various kinds of micellar solutions and the structural states in which they exist. For example, Schulman introduced the term "micro emulsion." Winsor objects on the grounds that all emulsions are unstable and a microemulsion must be some kind of an emulsion. Tosch et al. use the phrase "micelle-containing solutions," whereas Shinoda and Kunieda prefer "swollen micellar solution." Nevertheless, the term microemulsion is convenient, is in common use, and it is only necessary to understand precisely what is meant by the term. In this paper a microemulsion is defined to be a stable, translucent micellar-solution of oil, water that may contain electrolytes, and one or more amphiphilic compounds (surfactants, alcohols, etc.). It should be noted that according to this definition a microemulsion is not an emulsion. A microemulsion may have distinct internal and external phases, but in many cases there is no identifiable external phase. phase. Since a microemulsion has at least three components -- oil, water, and surfactant - the compositional state of the system must be specified with at least three numbers. It is therefore convenient and instructive to employ a ternary representation as shown in Fig. 1. The simple situation will involve three pure components, and the multiphase region will be bounded by a continuous binodal curve. Everywhere above the binodal curve a single phase exists that undergoes transitions among various structural states as the compositional point moves about the diagram. These transitions may be gradual, reflecting an equilibrium in which there is significant coexistence of different micellar configurations, as proposed by Winsor. proposed by Winsor. In the multiphase region, the most simple, three-component system involves only two phases throughout; one is oil-external and the other water-external. SPEJ P. 491