Modeling Crude Oils for Low Interfacial Tension
Authors: Cayias J.L. , Schechter R.S. , Wade W.H. 
Published: December 2007

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.

Abstract
Using the correlation between interfacial-tension behavior for three homologous series of hydrocarbons and a simple, mole-fraction averaging procedure, it was found possible to predict interracial tensions for complex hydrocarbon oil phases/aqueous surfactant Phases. This leads to an extension of the equivalent alkane carbon-number (EACN) concept to a mixed hydrocarbon oil phase. The EACN for eight crude oils was determined and it was found that the interfacial tension of crudes can be best modeled by alkanes in the range of hexane to nonane.

Introduction
Previous publications, have demonstrated that low interfacial tensions can be attained with pure hydrocarbons as the oil phase. The aqueous phase contained surfactants that were proposed as candidates for tertiary oil recovery of crude oils, namely petroleum sulfonates and alkyl xylene sulfonates. These observations demonstrate that complex hydrocarbon mixtures are not a necessary requirement for low tensions and, as can be attested by any worker in the field, are certainly not a sufficient requirement for attaining low tensions. The question is, then, how can tensions for complex hydrocarbon mixtures (crude oils) be modeled given the tension of their components?
Systematic trends in the tension have been observed for various hydrocarbon homologous series. It was found, for example, that an alkyl benzene gave the same interfacial tension as an alkane having the same number of carbon atoms as the alkyl side chain. The alkyl side chain is therefore the only contributor to the tension. Likewise, it was found that a cyclohexyl ring was approximately equivalent to four carbons. The hydrocarbon giving minimal tension can be varied by changing either the salinity or the surfactant concentration, but the above-mentioned scaling rules were found to apply. These observations have resulted in the concept of an equivalent alkane carbon number (EACN) for binary mixtures of alkanes, alkyl benzenes, and alkyl cyclohexanes. Hydrocarbon behavior was found to be additive and mole-fraction weighted by the simple relationship,
(1)
where the Ci are EACN values for the individual components, the Xi are mole fractions, and Cavg is the EACN for the mixture. For example, an equimolar mixture of butylcyclohexane (C1 = 4 for butyl groups + 4 for cyclohexyl = 8, X1 = 0.5) and propylbenzene (C2 = 3 for the propyl group + 0 for propylbenzene (C2 = 3 for the propyl group + 0 for benzene, X2 = 0.5) has a Cavg of 5.5. This mixture then would be predicted to yield a minimum interfacial tension against a surfactant solution that gives low tensions against pure alkanes intermediate between pentane and hexane. Eq. 1 has been verified for a wide variety of binary mixtures. Crude-oil behavior could be predicted by expanding Eq. 1 to the general form,
(2)
where the running index i extends over all the crude oil components. Of course, this cannot be accomplished in practice since all the components of a specific crude oil have never been identified. However, if Eq. 2 applies to complex but known-composition hydrocarbon mixtures, then applicability to crude oils can be inferred. Tests verifying Eq. 2 are reported in this paper. Eq. 2, in a variety of situations, requires that effects resulting from alkane isomerization be investigated. In addition, the role of sulfur compounds needs to be assessed. These studies also are reported.
Finally, two different experimental techniques are used to validate the EACN concept as applied to crude oils, and values for eight samples are reported.

EXPERIMENTAL PROCEDURE
All interfacial tensions were measured without pre-equilibration at 27 degrees C using the spinning drop pre-equilibration at 27 degrees C using the spinning drop technique. Five sodium sulfonate surfactants were used: Witco TRS 10-80, Shell Martinez 380 and Martinez 470, and Exxon Chemical Dodecyl and Pentadecyl Xylene sulfonates. Pentadecyl Xylene sulfonates. SPEJ
P. 351