FRACTIONATION AND CHARACTERIZATION OF UTAH TAR SAND BITUMENS - INFLUENCE OF CHEMICAL-COMPOSITION ON BITUMEN VISCOSITY

The viscosity of tar sand bitumen, among other properties such as thermal conductivity and specific heat, represents an important property that controls various recovery processes. Tar sand bitumens, particularly from Utah, have been shown to vary significantly in their physical and chemical properties from one location to another. The highly viscous nature of the bitumen was understood, based on a model developed by Nellensteyn and further revised by Pfeiffer et al. and Marvillet, to arise from a dispersion of asphaltenes in viscous oils with constituent aromatics and resins acting as suspension stabilizers. Based on this colloidal nature of bitumens, the differences in viscosities were accounted for by different degrees of peptization of the asphaltenes. In a recent related study by Christensen et al., it was shown that for an Asphalt Ridge (Utah) bitumen the viscosity increased with an increase in asphaltenes content. Variation in the asphaltene content of the bitumen was accomplished by a prior fractional separation of the bitumen into maltenes and asphaltenes and mixing these fractions together in various proportions. From the colloidal suspension model for bitumen and the results of Christensen et al., it may appear that asphaltenes alone are responsible for the observed high viscosity of bitumen. However, a comparative analysis of Athabasca (Alberta, Canada) and Asphalt Ridge bitumens, particularly in terms of their viscosities and asphaltene contents, reveals otherwise. Asphalt Ridge bitumen, though containing only 7% asphaltenes, has a viscosity value of 80 Pa.s at 50-degrees-C whereas Athabasca bitumen, which has 17% asphaltenes, has a viscosity of only 5 Pa.s at 50-degrees-C. These results indicate that in addition to asphaltenes there are other compositional properties of the bitumen which influence its viscosity. In this paper a detailed analysis consisting of fractionation and characterization of Asphalt Ridge and Sunnyside bitumens was undertaken in order to determine what compositional properties other than asphaltene content may influence bitumen viscosity. The results suggest that a discrete group of compounds present in the polar fractions of the bitumen may have a significant influence on bitumen viscosity and may be of equal, if not greater, importance than the asphaltene content itself. The interpretation of the observed results not only lends support to the model of Nellensteyn, but also identifies another compositional factor which determines bitumen viscosity.