Determination of mutual diffusivity of CO2/C3H8-heavy oil systems with consideration of natural convection under reservoir conditions

Abstract

By performing dynamic volume analysis, we developed and applied an experimental and theoretical technique to determine mutual diffusivity (i.e., diffusivity of a gas component in an oil phase and that of the extracted oil components in the gas phase) of CO2/C3H8-heavy oil systems in the presence of natural convection under reservoir conditions. Experimentally, diffusion tests are conducted for CO2/C3H8-heavy oil systems at high pressures and elevated temperatures with a PVT setup, while swelling factors of the oil phase are measured and recorded continuously. We have collected gas and oil samples at end of the test for gas chromatography (GC) and oil compositional analysis. Theoretically, mutual diffusivity between a solvent and heavy oil can be determined once the discrepancies between the measured and calculated parameters, including swelling factors and gas compositions, have been minimized. The diffusivities of both solvents (i.e., CO2 and C3H8) and oil components are found to increase with pressure and temperature. Except for the dissolution of solvent(s) into heavy oil due to concentration difference, there exists an obvious extraction process from the oil to gas phases during each experiment at elevated temperatures as light-medium components (i.e., C1-C4 and C6-C8) have been detected in the collected gas samples at the end of the test, during which the oil swelling effect is diminished at its late period when the solvent-heavy oil system approaches its equilibrium state. Compared to the CO2-heavy oil systems, more light components have been detected for the C3H8-heavy oil systems, indicating that stronger extraction occurs when adding C3H8 to the heavy oil at elevated temperatures. Since natural convection has an opposite direction from molecular diffusion, separating the former from the latter results in larger diffusion coefficients of CO2 and C3H8. In addition to new experimental data, both diffusivity of a solvent/CO2 in heavy oil and that of the extracted oil components in the gas phase are quantified in a more complex solvent(s)/CO2-heavy oil system at high pressures and elevated temperatures, not only serving as the benchmark for future studies to consider the contribution of temperature and porous media, but also allowing us to evaluate and optimize the hybrid solvent-steam method in a heavy oil reservoir within a unified and consistent framework.