Characterization of Petroleum Sulfonates

The majority of surfactant systems considered for enhanced oil recovery include petroleum sulfonates as the primary component. Previous work has shown a marked dependence of petroleum-sulfonate performance upon its composition. petroleum-sulfonate performance upon its composition. Numerous analyses for sulfonate surfactants are described in the literature, but care must be exercised in applying these procedures to oil-recovery agents. In general, published procedures have been developed for sulfonates with relatively narrow equivalent-weight distributions and can cause significant errors when applied to petroleum sulfonates. This paper includes techniques for characterization of laboratory- or plant-manufactured samples as well as samples produced from laboratory core or field tests. Steps described in characterization include separation and purification, gravimetric analysis, colorimetric analysis, determination of average equivalent weight and equivalent-weight distribution, and estimation of relative content of mono-, di-, and polysulfonates. For some analyses, procedures are described to minimize errors caused by changes in composition resulting from preferential separation of sulfonate components in displacement tests. A procedure is described for separation of a manufactured sulfonate into equivalent-weight fractions. These fractions may be recombined in different ratios to examine behavior of sulfonates with various compositions. Analysis of petroleum sulfonates made by different reaction schemes shows that sulfonate composition may be influenced substantially by choice of sulfonation conditions. Introduction Most surfactant-based enhanced oil recovery processes include natural petroleum sulfonates as processes include natural petroleum sulfonates as the primary component. Natural petroleum sulfonates are defined as those manufactured by sulfonation of crude oil, crude distillates, or any portion of these distillates in which hydrocarbons present are not substantially different from their state in the original crude oil. These natural materials, then, are quite different from synthetic sulfonates, which are derived most commonly from sulfonation of olefinic polymers or alkyl aromatic hydrocarbons. In general, polymers or alkyl aromatic hydrocarbons. In general, natural petroleum sulfonates are much more complex mixtures than synthetics. The major reason for this difference in complexity is that the natural materials contain condensed-ring, as well as single-ring, aromatics that permit multiple sulfonation to occur. These di- and polysulfonated materials cause the equivalent-weight distributions of natural sulfonates to be much broader than those of monosulfonated synthetics. It is important to point out that equivalent weight of a material means nothing so far as specific structure is concerned, but it has been shown to be a measure of surfactant effectiveness. Additionally, sulfonate equivalent weight and equivalent-weight distribution, and how they affect and are affected by adsorption, have been explored in detail. Characterization of such complex mixtures is extremely difficult. Standard methods exist that are purported to characterize natural petroleum purported to characterize natural petroleum sulfonates (for example, ASTM Procedures D2548-69, D855-56, D2894-70T, and D1216-70) but these procedures are, for the most part, not suitable for procedures are, for the most part, not suitable for defining surfactants of interest in enhanced oil recovery processes. Brown and Knobloch clearly showed the difficulties in trying to determine molecular species present in natural petroleum sulfonates with broad equivalent-weight spectra. Problems are even more severe when sulfonates Problems are even more severe when sulfonates present in laboratory core effluents or production present in laboratory core effluents or production well samples from field trials are to be characterized. Complications are caused by salt from the aqueous phase as well as crude oil contamination. Salt must be removed scrupulously for accurate equivalent-weight measurements, and crude oil must be removed since it interferes with colorimetric techniques as well as use of light absorbance for concentration determinations. The purpose of this paper is to present several methods for characterizing natural petroleum sulfonates. SPEJ P. 184