Viscosity Models For Gas-Free Athabasca Bitumen
Authors: Khan M.A.B. , Mehrotra A.X. , Svrcek W.Y. 
Published: March 2010

Athabasca bitumen is a viscous mixture of high molecular weight polycyclic compounds. Rheologically, this bitumen is considerably more viscous than most conventional crude oils. The viscosity-temperature relationship for the bitumen is such that many of the correlations used to predict viscosity of conventional crudes do not provide satisfactory estimation of the bitumen viscosity.

The viscosity equations based on the model theories by Eyring and Hildebrand were modified to describe the temperature dependence of the dead-bitumen viscosity. These modified Eyring's and Hildebrand's equations correlated the existing viscosity data to an average deviation of 13% and 7.5%, respectively. A brief discussion of the theories of liquid viscosity and their limitations when applied to complex hydrocarbon mixtures is also presented.

Finally, two empirical correlations employing double-logarithmic viscosity functions have been developed. These correlations provide a good description of the viscosity temperature relationship of gas-free Athabasca bitumen. The non-linear viscosity model yielded slightly better results with an average deviation of 7.1% compared to 8.2% with the linear model. The temperature range for the applicability of these correlations is from 20 °C to 130 °C.

Introduction

The Athabasca bitumen is a mixture of high molecular weight polycyclic compounds. The average chemical composition(1) of the Athabasca bitumen along with the bitumens from other regions in Alberta is given in Table 1. The structural models for fractions of the Athabasea bitumen were determined by N.M.R. spectroscopic techniques(2) and are shown in Figure 1. Using the same techniques, the average molecular formulae for the Athabasca and Cold Lake bitumens have been found to be, C36.7 H55.6 N0.19 S0.82 O0.51 and C34.5 H53.5 N0.11 S0.72 O0.47' respectively (2).

The Athabasca bitumen consists of about 20% asphaltenes, distributed in about 80% of a dispersing medium known as the maltenes. The maltenes can be further subdivided into two components: a light component known as the oil phase (saturates and aromatics) and a heavy phase known as the resins. From the above comments, it can be inferred that the Athabasca bitumen is very different from normal crude oils both in structure and in molecular size and hence in physical properties. For example, the Athabasca bitumen has a far higher viscosity and density than normal crude oils. In fact bitumens are obtained as the heavy residues or "bottoms" in the distillation of crude oils. Rheologically, the Athabasca bitumen has been found to be essentially Newtonian in chataeter at low shear rates ? 1.0 s-')(3). The purpose of this paper is to provide a brief review of the theories of liquid viscosity and their present state of development; and to present the viscosity models that have been developed for the viscosity of the gas-free Athabasca bitumen.

Theories of Liquid Viscosity

The theories may be classified as:Rigorous Statistical Mechanical TheoriesCorresponding States TheoryModel Theories

A complete theory to describe the liquid state, based on the kinetic theory of matter, is still under development. Likewise, the theories of liquid viscosity are still in their early stages of development.