Estimation of SARA Fraction Properties With the SRK EOS
Authors: Greaves M. , Ayatollahi S. , Moshfeghian M. , Alboudwarej H. , Yarranton H.W. 
Published: March 2010

One approach to modelling asphaltene solubility is regular solution theory. The key parameters for this approach are the molar volume and solubility parameters of each constituent. However, these parameters are largely unknown for crude oils. Some authors have used cubic equations of state (CEOS) to estimate the solubility parameters and molar volumes of solvents and C7+ fractions, but CEOS have yet to be applied in this way to asphaltenes due to their high molar mass and unknown critical properties.

In this work, a modified Soave-Redlich-Kwong EOS with the Peneloux correction is used to estimate the molar volumes and solubility parameter of the four solubility classes (saturates, aromatics, esins, and asphaltenes) of bitumens. The EOS is modified for the asphaltenes, which are assumed to be polymeric-like compounds consisting of aggregates of monodisperse asphaltenemonomers. Correlations are developed for the critical properties and acentric factor of each solubility class. The EOS-predicted roperties are tested against density measurements of SARA fractions from several bitumens. The predicted parameters areused to determine the onset of asphaltene precipitation from bitumen upon the addition of heptane and the predictions are compared with measured onsets.

Introduction

Asphaltenes are defined as the crude oil fraction that precipitates upon the addition of an n-alkane (usually n-pentane or n-heptane) but remains soluble in toluene(1). Asphaltene precipitation can occur upon a change in pressure, temperature, or composition and can be a major problem for oil producers. For example, asphaltene precipitation in the reservoir or wellbore, triggered by a drop in pressure, can significantly reduce production. Asphaltene deposition in surface facilities and pipelines can occur upon the addition of condensate diluent. Treatment to remove the deposits increases operating costs. In order to prevent or mitigate asphaltene deposition, it is necessary to predict asphaltene precipitation.

To choose an appropriate precipitation model it is necessary to consider asphaltene chemistry. Asphaltenes are mixtures of many thousands of chemical species but these species share some common features. They are polynuclear aromatics and have the highest molar mass, aromaticity, and heteroatom content of all the crude oil components(1). Asphaltenes are also known to self-associate into aggregates consisting of approximately 2 to 6 molecules per aggregate. The aggregates have been considered as colloidal particles(2) or macromolecules(3). With the colloidal view, the associated asphaltenes are considered to form a stack, which is surrounded and dispersed in the oil by resins. Precipitation is believed to occur when the resins are stripped from the colloid allowing aggregation and phase separation to occur. With the macromolecular view, the associated asphaltenes are considered to be independent molecules along with the resins and other crude oil constituents. Precipitation is considered to be a liquid-liquid or liquid-solid phase transition. Recent molar mass and calorimetry experiments favour the macromolecular view(4, 5). Hence, a traditional thermodynamic approach is recommended for asphaltene precipitation.

Of the many thermodynamic approaches to modelling asphaltene precipitation(6), the two most prevalent are equations of state(7-9) and regular solution theory(2, 6, 10-12).