Mixing Rules for Optimum Phase-Behavior Formulations of Surfactant/Oil/Water Systems
Authors: Salager J.L. , Bourrel M. , Schechter R.S. , Wade W.H. 
Published: December 2007

Abstract
Many formulations used in surfactant flooding involve blends of surfactants designed to glue the best oil-recovery efficiency. Because oil-recovery efficiency usually is presumed to relate closely to surfactant/brine/oil phase behavior, it is of interest to understand the effect of mixing surfactants or of mixing oils on this phase behavior.We show that a correlation defining optimal behavior as a function of salinity, alcohol type and concentration, temperature, WOR (water/oil ratio), and oil type can be extended to mixtures of sulfonated surfactants and to those of sulfonates with sulfates and of sulfonates with alkanoates, provided the proper mixing rules are observed. provided the proper mixing rules are observed. The mixing rules apply to some mixtures of anionic and nonionic surfactants, but not to all. These mixtures exhibit some properties that may be of practical interest, such as increased salinity and practical interest, such as increased salinity and temperature tolerance.

Introduction
Recent studies have shown that formulation of the surfactant/brine/oil system is a key factor governing the performance of microemulsions designed to recover residual oil. These studies demonstrate that all optimal formulations exhibit characteristic properties that are remarkably similar. In general, properties that are remarkably similar. In general, the optimal microemulsion can solubilize large quantities of oil and connate water; in the presence of excess quantities of oil and water, a third surfactant-rich middle phase is formed. The interfacial tensions (IFT's) between the excess phases and the surfactant-rich phase are both low - about 10 dyne/cm (10 mN/m). Given an oil/brine system from a particular reservoir, one can achieve this formulation by varying the surfactant or the cosurfactant. Different oils, brines, or temperatures require formulations correspondingly altered to maintain optimal conditions.
Previous studies have shown that the three-phase region exists over a range of values when one parameter, such as cosurfactant concentration, parameter, such as cosurfactant concentration, salinity, temperature, etc., is varied systematically (often called a scan). Thus, some ambiguity may exist with regard to the selection of those parameters representing the optimal formulation. Clearly, the optimum is that which recovers the most oil. However, tests are laborious, difficult to reproduce precisely, and sensitive to other factors, such as precisely, and sensitive to other factors, such as mobility, surfactant retention, wettability, etc. Therefore, it is desirable to impose an alternative definition that can be used for screening, though the final design still is dictated by core floods.Healy and Reeds have shown that the optimal formulation for oil recovery closely corresponds to that for which the IFT's between the excess oil and water phases and the surfactant-rich phase are equal. An almost equivalent criterion also was shown to be that point in the three-phase region for which the volume of oil solubilized into the middle phase equals the volume of brine. Furthermore, Salager et al. have used still another criterion that seems to be essentially equivalent to those used by Healy and Reed - an optimal salinity is defined as the midpoint of the salinity range for which the system exhibits three phases.These criteria are useful because they permit the screening of microemulsion systems using simple laboratory tests.
SPEJ
P. 271