Effects of Hardness and Cosurfactant on Phase Behavior of Alcohol-Free Alkyl Propoxylated Sulfate Systems

Abstract

Summary The effect of hardness was investigated on equilibrium phase behavior in the absence of alcohol for blends of three alcohol propoxy sulfates (APSs) with an internal olefin sulfonate (IOS) with a C15–18 chain length. Hard brines investigated were synthetic seawater (SW), 2*SW, and 3*SW, the last two with double and triple the total ionic content of SW with all ions present in the same relative proportions as in SW, respectively. Optimal blends of the APS/IOS systems formed microemulsions with n-octane that had high solubilization suitable for enhanced oil recovery at both ≈25°C and 50°C. However, oil-free aqueous solutions of the optimal blends in 2*SW and 3*SW, as well as in 8 and 12% NaCl solutions with similar ionic strengths, exhibited cloudiness and/or precipitation and were unsuitable for injection. In SW at 25°C, the aqueous solution of the optimal blend of C16–17 7 propylene oxide sulfate, made from a branched alcohol, and IOS15–18, was clear and suitable for injection. A salinity map prepared for blends of these surfactants illustrates how such maps facilitate the selection of injection compositions in which injection and reservoir salinities differ. The same APS was blended with other APSs and alcohol ethoxy sulfates (AESs) in SW at ≈25°C, yielding microemulsions with high n-octane solubilization and clear aqueous solutions at optimal conditions. Three APS/AES blends were found to form suitable microemulsions in SW with a crude oil at its reservoir temperature near 50°C. Optimal conditions were nearly the same for hard brines and NaCl solutions with similar ionic strengths between SW and 3*SW. Although the aqueous solutions for the optimal blends with crude oil were slightly cloudy, small changes in blend ratio led to formation of lower phase microemulsions with clear aqueous solutions. When injection and reservoir brines differ, it may be preferable to inject at such slightly underoptimum conditions to avoid generating upper phase, Winsor II, conditions produced by inevitable mixing of injected and formation brines.