Analisis Sara

SPE 75228
Evaluating Crude Oils by SARA Analysis
Tianguang Fan SPE, Jianxin Wang SPE, and Jill S. Buckley SPE/New Mexico Tech
Copyright 2002, Society of Petroleum Engineers Inc.
This paper was prepared for presentation at the SPE/DOE Improved Oil Recovery Symposium
held in Tulsa, Oklahoma, 13–17 April 2002.
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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 dividescrude 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. Weexamine 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 toassessment 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 nonpolarmaterial 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 classificationsystem

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 chromatographymethod 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...