Phase Behaviour and Mass Transfer of Solvent(s)-CO2-Heavy Oil Systems Under Reservoir Conditions
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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.