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.