CO₂ has been found to be an efficient agent for recovering heavy oil resources worldwide through an immiscible displacement process. One major disadvantage of CO₂ immiscible process is the limited solubility of CO₂ in heavy oil, resulting in limited enhanced oil recovery. Addition of light alkane solvents to CO₂ stream may provide a better recovery efficiency of heavy oil, though it is not well understood how addition of solvents will affect the phase behaviour and mass transfer of CO₂-heavy oil systems. Therefore, it is of fundamental and practical importance to study phase behaviour and mass transfer of the solvent(s)-CO₂-heavy oil systems under reservoir conditions.

In order to improve the phase behaviour modeling of highly asymmetric systems, e.g., solvent(s)-CO₂-heavy oil systems, a new alpha function for the Peng-Robinson equation of state (PR EOS) is first developed. In comparison with the existing alpha functions evaluated in this study, the modified alpha function with the redefined acentric factor at a reduced temperature of 0.6 provides more accurate prediction of vapour pressures of nonhydrocarbon and hydrocarbon compounds, especially heavy hydrocarbons. Subsequently, the enhanced swelling effect and viscosity reduction of CO₂-heavy oil systems with the addition of solvent C₃H₈ or n-C₄H₁₀ are experimentally measured and theoretically determined. An increased swelling effect of heavy oil is obtained by adding gas solvent C₃H₈ or n-C₄H₁₀ into the CO₂ stream, while an enhanced viscosity reduction of the CO₂-heavy oil system is also achieved in the presence of either solvent C₃H₈ or n-C₄H₁₀. By treating the heavy oil sample as a single pseudocomponent, three binary interaction parameter (BIP) correlations have been proposed for respectively characterizing CO₂- heavy oil binaries, C₃H₈-heavy oil binaries and n-C₄H₁₀-heavy oil binaries. The PR EOS with the modified alpha function and BIP correlations can be used to predict the saturation pressures and swelling factors of the aforementioned systems with a generally good accuracy.

Equilibrium interfacial tensions (IFT) between solvent(s)-CO₂ mixture and heavy oil have also been experimentally measured with an axisymmetric drop shape analysis (ADSA) setup. Addition of C₃H₈ and/or n-C₄H₁₀ into CO₂ stream leads to an obvious reduction of IFT between heavy oil and CO₂, though the degree of reduction depends on the added amount of the light alkane solvent(s). Theoretically, an optimized mechanistic parachor model provides a qualitative agreement with the measured equilibrium IFTs between solvent(s)-CO₂ mixture and heavy oil.

The liquid-liquid-vapour (L₁L₂V) phase boundaries of solvent(s)-CO₂ heavy oil mixtures in the pressure-temperature (P-T) diagram are also experimentally and visually determined with the PVT setup. The addition of an alkane solvent to the CO₂-heavy oil system tends to expand the pressure span of the L₁L₂V phase boundary, while the L₁L₂V phase boundary of solvent(s)-CO₂-heavy oil system shows its tendency to move towards the high-temperature and low-pressure side of the P-T diagram.

Experimental and theoretical methods have been performed to determine the diffusion coefficient of each component in the solvent(s)-CO₂ mixture or the apparent diffusion coefficient of the mixture in heavy oil. It is found that the gas-phase solvent fraction decreases as diffusion proceeds, while the gas-phase CO₂ fraction decreases during the diffusion test. As for the solvent(s)-CO₂ mixtures tested, the molecular diffusion coefficient of an individual solvent in heavy oil is found to be significantly larger than that of CO₂ in heavy oil. Also, at the same pressure, the C₃H₈-CO₂ mixture leads to an accelerated growth in swelling-factor compared to pure CO₂.