Crude oils can be described compositionally by a number of methods. SARA analysis is widely used to divide crude oil components according to their polarizability and polarity using a family of related analytical techniques. Problems arise because the analytical techniques do not necessarily produce identical results. Users of the data, however, rarely distinguish between the different techniques, assuming that SARA fraction values generated by any of the commonly used methods are essentially interchangeable. We examine this assumption for medium gravity crude oils and 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. An improved HPLC method is introduced that gives analyses comparable to the ASTM-recommended chromatographic method in less time than that required for TLC analysis. An internal consistency test is recommended for evaluating SARA fraction data.

Summary This paper clarifies the rheology of xanthan and partially hydrolyzed polyacrylamide (HPAM) solutions in porous media, especially at low velocities. Previous literature reported resistance factors (effective viscosities in porous media) and an apparent shear thinning at low fluxes that were noticeably greater than what is expected on the basis of viscosity measurements. The polymer component that causes the latter behavior is shown to propagate quite slowly and generally will not penetrate deep into a formation. Particularly for HPAM solutions, this behavior can be reduced or eliminated for solutions that experience mechanical degradation or flow through a few feet of porous rock. Under practical conditions where HPAM is used for enhanced oil recovery (EOR), the degree of shear thinning is slight or nonexistent, especially compared to the level of shear thickening that occurs at high fluxes.

Here, we present a case study on a Wyoming well with known asphaltene deposition issues as a result of natural depletion. Field deposits and crude oil from the same well were collected for analysis. Compositional differences between field deposits, lab-generated capillary deposits, and C 7 -precipitated asphaltenes were determined by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), and all three samples show similar trends in composition, displayed as plots of aromaticity versus carbon number. An enrichment of highly condensed aromatic molecules for the field deposit is detected with both ultrahigh-resolution mass spectrometry and thermal cracking experiments and could predict asphaltene deposition. FT-ICR mass spectral analysis of solvent-extracted fractions suggest different deposition mechanisms for field deposits (slow deposition) compared to rapid precipitation in standard asphaltene preparation protocols that contain trapped maltenes.

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.

Summary For hydrophobically associative polymers, incorporating a small fraction of hydrophobic monomer into a hydrolyzed polyacrylamide (HPAM) polymer can promote intermolecular associations and thereby enhance viscosities and resistance factors. In this paper, we investigate the behavior of a new associative polymer in porous media. The tetra-polymer has low hydrophobic-monomer content and a molecular weight (Mw) of 12–17 million g/mol. Total anionic content is 15–25 mol%, including a few percent of a sulfonic monomer. This polymer is compared with a conventional HPAM with 18–20 million g/mol Mw and 35–40% anionic content. Rheological properties (viscosity vs. concentration; and shear rate and elastic and loss moduli vs. frequency) were similar for the two polymers [in a 2.52% total dissolved solids (TDS) brine at 25 °C]. For both polymers in cores with permeabilities from 300 to 13,000 md, no face plugging or internal-filter-cake formation was observed, and resistance factors correlated well using the capillary-bundle parameter. For the HPAM polymer in these cores, low-flux resistance factors were consistent with low-shear-rate viscosities. In contrast, over the same permeability range, the associative polymer provided low-flux resistance factors that were two to three times the values expected from viscosities. Moderate shear degradation did not eliminate this effect—nor did flow through a few feet of porous rock. Propagation experiments in long cores (up to 157 cm) suggest that the unexpectedly high resistance factors could propagate deep into a reservoir—thereby providing enhanced displacement compared with conventional HPAM polymers. Compared with HPAM, the new polymer shows a significantly higher level of shear thinning at low fluxes and a lower degree of shear thickening at high fluxes.