Effect of CO₂ mass transfer on rate of oil properties changes: application to CO₂-EOR projects

The performance of any CO₂-EOR project relies on the capability of CO₂ to change the oil properties at a reasonable period of time. The rate of mass transfer of CO₂ into a specific oil controls the rate of oil properties changes. Additionally, the rate of CO₂ transfer into an oil sample is dependent on CO₂ thermodynamic state which is a function of pressure and temperature. Hence, this study aims to investigate the mass transfer of CO₂ into a crude oil at gas and also supercritical state to estimate the rate of resultant changes of the oil properties at these two different states. To do so, two mass transfer pressure decay tests were conducted to study the molecular diffusion of CO₂ in a light crude oil at both supercritical and gas states. Later, a mathematical model was used to determine the mass transfer parameters, i.e. diffusion coefficient (D) and interface mass transfer coefficient (k), and the dynamic concentration distribution of CO₂ within the oil phase. As a part of this study, the time-dependent changes of the crude oil properties due to the molecular diffusion process of CO₂ were also obtained for both supercritical CO₂ and CO₂ at gas state. For this purpose, the results of pressure decay experiments were combined with the results of additional experiments carried out to measure PVT properties of the CO₂-oil system including CO₂ solubility, oil swelling, oil viscosity, and interfacial tension (IFT) at various pressures and a constant temperature. The measured mass transfer parameters, i.e. D and k, were found to be higher for CO₂ at supercritical state than that at gas state. The higher diffusivity of the supercritical CO₂ in the oil resulted in faster changes of the oil properties as compared to the rate of properties changes by CO₂ gas. For instance, it took 2 h for the oil to be swollen by 30% when it was in contact with CO₂ at supercritical state as compared to 10 h at gas state. In the case of oil viscosity, for example, during 9 h, 58% reduction in oil viscosity was resulted by supercritical CO₂ as compared to 25% reduction by CO₂ gas. The results of this study can be used for better planning of CO₂-EOR projects and particularly for CO₂ Huff and Puff where the soaking time needs to be optimized properly.