1150 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Ind. Eng. Chem. Res. 1994,33, zyxwvu 1150-1158 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPON Effect of Added Surfactant zyxw on Interfacial Tension and Spontaneous Emulsification in Alkali/Acidic Oil Systems Jeff Rudin, Camille Bernard, and Darsh T. Wasan' Department zyxwvutsrqp of Chemical Engineering, Illinois Institute of Technology, Chicago, Illinois 60616 An experimental investigation of the buffered surfactant-enhanced alkaline flooding system chemistry was undertaken to determine the influence of various species present on interfacial tension as a function of pH and ionic strength. Phase behavior testa that monitor the extent of emulsification are sufficient to determine the region of low interfacial tension. Optimization of interfacial tension by adjustment of the ionic strength alone may not necessarily provide the lowest interfacial tension under the best conditions. The pH should be simultaneously optimized along with ionic strength to allow better control over attainment of low interfacial tension. The dominant mechanism by which added surfactant aids in the reduction of interfacial tension is the formation of mixed micelles with the ionized acid. Although added surfactant partitioning from the influence of the un-ionized acid and ionic strength will affect interfacial behavior, the formation of mixed micelles plays a dominant role. Middle-phase formation is possible with a low acid oil using a petroleum sulfonate at a proper pH and ionic strength. Introduction Rudin and Wasan (1993) have shown in the absence of added surfactant that the interfacial tension is sensitive to the pH, and when ionic strength is high, interfacial tension goes through a deep minimum. This minimum is caused by the simultaneous adsorption of ionized and un- ionized acid upon the interface. With this understanding that the pH is just as important as ionic strength in affecting interfacial tension and extent of emulsification, a practicalmethodwas developed (Rudinand Wasan, 1993) to cost-effectively buffer the aqueous alkaline pH in an intermediate range (pH of 9-12) by using mixed alkalis. The investigation of surfactant-enhanced alkali/acidic crude oil interactionswith relation to improved oil recovery has been addressed by several investigators (Martin et al., 1985; Nelson et al., 1984; French and Burtchfield, 1990; Schuler et al., 1986). It has been shown that small zyxwvut amounta of surfactant, normally less than 0.5 wt 96, together with an alkaline additive can produce ultralow interfacial tension (i.e., a high interfacial activity) against an acidic crude oil, improving oil recovery. Many investigators (Martin et al., 1985; Nelson et al., 1984; French and Burtchfield, 1990; Schuler et al., 1986) have shown that there is a beneficial synergistic effect of combining alkali with surfactant in lowering interfacial tension and im- proving oil recovery. It is still unknown whether transient (initial) interfacial tension or equilibrium interfacial tension or both are more important in improving oil recovery. What is known is that oil recovery is higher with the combination of surfactant and alkali than with either taken alone, and the addition of a mobility control polymer can make the alkalilsurfactantlpolymer process several times less expensive than the micellar/polymer flood, recovering the same amount of oil (Surkalo, 1990). The surfactant-enhanced alkaline flooding process and the alkaline flooding process are classically interpreted on the basis of salinity. However, both fail to consider the effect of pH, and as a result, the process is explained solely in terms of the effect of ionic strength. This reasoning can lead the researcher to misinterpret the results or cause confusion in designing meaningful formulations to be injected into the oil reservoir. * Author to whom correspondence should be addressed. 0888-5885/94/2633-1l50$04.50/0 The alkali, as explained by Krumrine et al. (1982a,b), serves a dual role. First, interfacial tension is lowered by the generation of soaps, which reduce capillary forces and facilitate oil displacement. Second, surfactant retention is reduced by the alkali by imparting a negative charge on the rock surface, thus providing a larger supply of surfactant for sustaining high interfacialactivity. Falcone et al. (1982) has shown that not only is surfactant adsorption reduced, but it is minimal in the intermediate pH range of 9-12. When divalent ions are present, the alkali precipitates as a multivalent hydroxide, reducing the detrimental effect of divalent ions on surfactant solubility and at the same time plugging the pores with improvement in areal sweep efficiency. The addition of surfactant to the alkaline solution certainly improves oil recovery. However, the exact role that the surfactant plays in its synergism with the alkali and how to take advantage of this synergism in designing floods for maximum oil production is still unknown. In this paper, alkali/acidic oil interactions are investigated in order to better understand the mechanisms involved in improving oil recovery by surfactant-enhanced alkaline flooding. Materials and Experiments In this study, two crude oils containing natural organic acids were contacted with an alkaline pH aqueous solution. One crude oil used in this investigation is a Long Beach crude oil obtained from THUMS Long Beach Co., having an acid number of 1.0 determined from ASTM procedure D-664 and an API gravity of 24.9. It was centrifuged at 4oooOg for 30 min to remove water and clays. The viscosity of the crude oil after centrifuging was 52 CP at 25 OC, and the interfacial tension against deionized water was 24.5 mN1m. The other crude oil used was obtained from SURTEK from Adena field located in Morgan County, Colorado. It is a light oil with an API gravity of 41.95, a viscosity of 3.75 CP at 25 OC, and an acid number less than 0.002. The alkaline solutions were a mixture of sodium hydroxide, sodium chloride, and Trona, which is an industrial grade chemical of 94% sodium bicarbonate. All alkalis were obtained from Fisher Scientific Co., except Trona, which was obtained from Kerr-McGee Chemical co. 1994 American Chemical Society