Abstract

The effectiveness of some improved oil recovery schemes can depend on the composition of the target oil. Crude oils can be described compositionally by a number of methods. SARA analysis divides crude oil components according to their polarizability and polarity using a family of related analytical techniques. Problems arise because several of the analytical techniques in use do not produce identical results, although the users of the data rarely distinguish between them, assuming that SARA fraction values generated by any of the common techniques are essentially interchangeable. We examine this assumption for three SARA analysis methods: gravity-driven chromatographic separation, thin layer chromatography (TLC), and high pressure liquid chromatography (HPLC). Results for a suite of six crude oil samples show that a significant volume of volatile material that contains both saturates and aromatics is lost in the TLC analysis. Application of SARA fraction data to assessment of asphaltene stability is demonstrated.

Introduction

Analysis of the composition of crude oils can be endlessly complex; the amount of detail collected should be dictated by the application for which the data is needed. One simple analysis scheme is to divide an oil into its saturate, aromatic, resin, and asphaltene (SARA) fractions. The saturate fraction consists of nonpolar material including linear, branched, and cyclic saturated hydrocarbons. Aromatics, which contain one or more aromatic rings, are more polarizable. The remaining two fractions, resins and asphaltenes, have polar substituents. The distinction between the two is that asphaltenes are insoluble in an excess of heptane (or pentane) whereas resins are miscible with heptane (or pentane). This classification system is useful because it identifies the fractions of the oil that pertain to asphaltene stability and thus should be useful in identifying oils with the potential for asphaltene problems.

SARA analysis began with the work of Jewell et al.1 Three main approaches have been used to separate crude oils and other hydrocarbon materials into SARA fractions. A claygel adsorption chromatography method is the basis of ASTM D2007. This method requires a fairly large oil sample, is time consuming and difficult to automate, and requires large quantities of solvents.

Improved methods fall into two groups. In the first group are high pressure liquid chromatographic (HPLC) methods, first introduced by Suatoni and Swab.2 Early HPLC techniques used silica or alumina columns to separate lighter petroleum fractions. The development in preparation of bonded phase of HPLC columns—especially NH2-bonded materials— made it practical to separate heavier fractions of petroleum samples.3–7 HPLC techniques are faster, more reproducible, and more readily automated than the ASTM column technique. In both cases, however, it is necessary to remove the asphaltene fraction before proceeding with the chromatography. Asphaltenes are either irreversibly adsorbed or precipitated during the saturate elution step and quantitative recovery cannot be achieved.8

The fastest separation method uses thin-layer chromatography (TLC) with quartz rods that are coated with sintered silica particles. Unlike column and HPLC techniques, asphaltenes need not be separated from other crude oil components before chromatographic analysis. A popular technology known as the Iatroscan that combines TLC with flame ionization detection (TLC-FID) was first applied by Suzuki9 to automate quantitative SARA separations, a method which has since been used extensively.10–11 Barman12 compared SARA analyses of heavy hydrocarbon distillates by the clay-gel and TLC-FID methods. TLC-FID uses very small amounts of sample. SARA fractions in a crude oil sample are often well resolved using established development procedures and quantitative results are obtained by the measurement of peak areas, assuming that each SARA fraction has an identical FID response factor.

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