Phase Behavior and Viscosity Modeling of Athabasca Bitumen and Light Solvent Mixtures

Abstract

Abstract This paper describes a systematic approach to model the phase behavior and viscosity of Athabasca bitumen and light- solvent mixtures for a wide range of temperatures. A cubic equation of state (EOS) is first developed using Athabasca crude assay data. We use a modified Jacoby correlation to describe the relationship of specific gravity and molecular weight for the bitumen sample. A gamma molar distribution model is used to fit the Athabasca crude assay data, then single-carbon-number (SCN) fractions are defined out to C90+. The Twu correlation is used for estimating SCN critical properties, including C90+, resulting in an EOS with 89 components (EOSSCN). Pure solvent-crude oil mixture PVT data were tuned to the EOSSCN model by adjusting a fixed set of BIPs (binary interaction parameters) between pure solvent components (N2, CO, CO2, C1, C2) and all C7+ components. For viscosity modeling, the LBC (Lorenz-Bray-Clark) correlation is used, with SCN critical volumes modified individually to ensure that the LBC correlation estimates SCN viscosities as given by a modified Twu correlation, based on specific gravity and normal boiling point. The EOSSCN model was lumped into five pseudo-fractions (EOS5), the heaviest being C90+. The resulting model reproduces accurately all phase and volumetric behavior of pure-solvent-crude mixtures. Initial viscosity prediction of the Athabasca crude by the EOSSCN/LBC and EOS5/LBC models is satisfactory for dead- bitumen with viscosity only affected by temperature. However, for viscosities of pure-solvent-saturated bitumen at varying temperatures, the EOS5/LBC model did not perform well. Our solution was to split the heaviest fraction C90+ into two sub- fractions, where only critical volumes differ, resulting in "lower-viscous" and "higher-viscous" C90+ fractions (C90+L into C90+H). The fraction of C90+L (fL) was found to correlate with pure solvent solubility and temperature, resulting in a quite- accurate overall viscosity fit. This final model has, in reality, six components, even though the two heaviest fractions are identical for EOS calculations – we call this final model EOS6/LBC. The final EOS6/LBC model was checked against measured PVT and viscosity data for mixtures of the same Athabasca bitumen using synthetic combustion gas solvents made up of C1, CO2, and N2.