The Topology of Phase Boundaries for Oil/Brine/Surfactant Systems and Its Relationship to Oil Recovery

Surfactant/oil/water phase diagrams have become the most important screening tool used to select microemulsion systems for enhanced oil recovery. The number of phases coexisting at a given salinity, the extent of the single-phase region, and the position of the phase boundaries all have relevance with respect to oil displacement efficiency. It is shown that the phase diagrams can be made to take on different configurations depending on the alcohol cosurfactant, the salinity, the impurities present in the surfactant, and the dispersity of the surfactant mixture. Besides the importance of the phase boundary shape, this study provides further insight into factors determining the height of the binodal surface on the pseudoternary phase diagram. Results show the effect of salinity as well as the surfactant, alcohol, and hydrocarbon types on the height of the binodal surface. It is shown that salinity is the main factor; other parameters have little or no influence once a surfactant has been selected. Finally the microemulsion viscosity is shown to be related to the proximity of the formulation to phase boundaries. Extensive data for one system are presented. Introduction It is now recognized that formulating surfactant/oil/brine systems that exhibit desirable phase behavior is an important step in optimizing performance of microemulsion systems for enhanced oil recovery. Oil is displaced by a combination of mechanisms-miscible displacement, swelling of the oil phase, and low tension displacement all of which are related to the topology of the phase boundaries in composition space. To predict the outcome of a particular project, a representation of the phase boundaries and their evolution when diluted with oil or brines having various proportions of divalent ions is required. For example, successful application of the salinity gradient concept demands phase relationships specially structured to accommodate the variations in salinity experienced by the surfactant slug during the course of the flood. Recent publications have dealt with the optimal salinity as a function of total amphiphile concentration (surfactant plus cosurfactant), and reported trends that are quite different from those found if the cosurfactant (alcohol) concentration is held constant. One purpose of this paper is to demonstrate that contorted phase boundaries found by Glover et al are caused by the variation of alcohol concentration when the concentration of total amphiphile is varied and because the direction that the phase boundaries twist or rotate is controlled by the nature of the alcohol. Another important factor is the extent of the single-phase region. More precisely, the height of the demixing curve in the pseudoternary representation should be minimized. This would permit, in principle, the amount of surfactant and cosurfactant in the micellar slug to be minimized. A correlation permitting the determination of the oil, salinity, alcohol, and surfactant at which the height of the demixing curve is minimized has been reported, but few data giving the value of the minimum height have been presented. This height is an important feature of the phase boundary topology and extensive measurements are reported here. The microemulsion viscosity must be high enough to help maintain mobility control. It is sometimes difficult to achieve the required levels of viscosity. Studies of microemulsion viscosity have been reported. We provide further data here and have related the microemulsion viscosities to phase behavior. Materials and Experimental Techniques The phase diagrams have been established by two techniques: a titration procedure and a grid-point technique. SPEJ P. 28^