Surfactant Aging: A Possible Detriment to Tertiary Oil Recovery

Abstract

Introduction New processes for achieving improved oil recovery have been under development in the petroleum industry for many years. One such process, termed surfactant flooding uses surfactants to minimize oil/water capillary forces. The petroleum sulfonates are of interest as surfactants by virtue of their low cost. However, there are problems. Foster has shown that the low interfacial tensions required for efficient displacement are sensitive functions of surfactant concentration and salinity. Aqueous solutions of petroleum sulfonates appear to change with time. This tension aging has been found to occur in both the presence and absence of certain cosurfactants. Also visually striking are the metamorphic changes that slowly occur when the surfactant is carefully contacted with an aromatic hydrocarbon without mixing the two phases. The aging processes described here evolve through nonequilibrium states, some of which may be detrimental to oil-recovery processes. These nonequilibrium aging states develop slowly (over a period of months, perhaps even years) and would not influence model displacements lasting only a few days or weeks; but they would be significant in the time frame over which a tertiary oil-recovery program is conducted. Experimental Procedure All the pure hydrocarbon oils studied were spectroscopic grade. Further purification had no effect on their interfacial tensions and, therefore, they were used as received from the manufacturer, Chem Samples. The hydrocarbon mixture designated as pseudocrude is described elsewhere. Witco Chemical Co. supplied the petroleum sulfonate designated as TRS 10-80, and Shell Development Co. provided the Martinez 380 and Martinez 470. The aqueous surfactant/salt solutions were prepared using the surfactant as received, always adding the sodium chloride last. All the interfacial tension measurements were made without pre-equilibrating the two phases. The values of the tensions for these systems were obtained at 25 deg. C using a spinning-drop interfacial tensiometer. Results and Discussions Low Tensions and Surfactant Aging A substantial difference in some interfacial tensions was discovered when two surfactant solutions of differing ages were used to measure the interfacial tensions of three homologous hydrocarbon series. These three hydrocarbon series were (1) n-alkanes from pentane to heptadecane, (2) n-alkylbenzenes or 1-phenylalkanes from benzene to heptadecylbenzene, and (3) n-alkylcyclohexanes or 1-cyclohexylalkanes from cyclohexane to n-tridecylcyclohexane. Both aqueous phases conMined 0.2-percent surfactant/1.0-percent salt and were prepared using the same technique. prepared using the same technique. JPT P. 985