The design and optimization of solvent assisted thermal recovery processes for heavy oil and bitumen require accurate predictions of viscosity as a function of temperature, pressure, and composition. In this case study, the performance of the Expanded Fluid (EF) viscosity model is tested on viscosity data for an Alberta bitumen diluted with carbon dioxide (5.2 wt%), ethane (5.1 wt%), propane (7.6 and 16 wt%), n-butane (14.5 wt%), n-pentane (15 and 30 wt%) and n-heptane (15 and 30 wt%) at temperatures from 20 to 175°C and pressures up to 10 MPa. The main input to the EF model is the density of the fluid and densities were measured at the same conditions as the viscosity measurements.
The viscosity of the bitumen was fitted with average absolute relative deviation (AARD) of 8%. The viscosities of the diluted bitumen mixtures were predicted without tuning with an overall AARD of 17% when using measured densities as an input. The viscosity predictions were improved to an AARD of 7% using generalized viscosity binary interaction parameters. When using densities calculated with an excess volume based mixing rule, the viscosity predictions were slightly more deviated with an overall AARD of 10%.
The EF model predictions were used to evaluate the effectiveness of n-alkane solvents in reducing bitumen viscosity at in situ steam-solvent process conditions. The solubility of the solvent in bitumen was found to be the main factor controlling the mixture viscosity. The less volatile the solvent is, the greater is the viscosity reduction at a given pressure and temperature. As the process temperature increases, the greater is the viscosity reduction from a given solvent due to increased solubility at higher steam saturation pressures.