Evaluating Crude Oils by SARA Analysis

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.

Correlations between SARA Fractions, Density, and RI to Investigate the Stability of Asphaltene

“…Leontaritis and Mansoori [17] recommended using the ratio of resins to asphaltenes as an indicator of asphaltene stability, based on the hypothesis that resins confer asphaltene stability by peptizing (or coating) asphaltenes. Fan et al [27] observed that the proportions of each of the SARA fractions in a crude oil are related to the stability of asphaltenes in that oil. Alkafeef et al [28] define the stability of a colloid dispersion as its resistance to flocculation, and the degree of "resistance" is used as a measure of the dispersion stability.…”

Prediction of crude oil refractive index through optimized support vector regression: a competition between optimization techniques

“…Taylor et al demonstrated that the onset of asphaltene precipitation could be detected from RI measurement (Taylor et al 2001). Fan et al (2002) examined the relation between crude oil RI and asphaltene stability in crude oil. They suggested that the difference between crude oil RI and crude oil RI at the onset of asphaltene precipitation could be employed as a decisive factor for the assessment of asphaltene stability in crude oils (Fan et al 2002).…”

“…Fan et al (2002) examined the relation between crude oil RI and asphaltene stability in crude oil. They suggested that the difference between crude oil RI and crude oil RI at the onset of asphaltene precipitation could be employed as a decisive factor for the assessment of asphaltene stability in crude oils (Fan et al 2002). Buckley and Wang (2002) showed that the wettability altering tendency is related to crude oil RI.…”

“…Due to drawbacks encountered in various methods of experimental measurement, development of a crude oil RI model could facilitate exact calculation of crude oil RI from readily available experimental data. Several researchers have tried to develop a predictive model for determining the RI (Chamkalani et al 2014;Fan et al 2002;Buckley and Wang 2002;Chamkalani et al 2012;Chamkalani 2012;Gholami et al 2014a, b;Vargas and Chapman 2010;Zargar et al 2015;Tatar et al 2015). The published models available in the literature for estimating the value of crude oil RI can be divided into two main categories.…”

“…In the second group of models, the RI of crude oil is calculated using SARA (saturate, aromatic, resin, and asphaltene) fraction data. The concept of the mutual dependence of crude oil RI and SARA fraction data was introduced by Fan et al (2002). They developed an empirical correlation between the SARA fraction data and RI of crude oil.…”