Quantification of gas exsolution of alkane solvents-CO2 mixture in heavy oil with consideration of individual interfacial resistance under nonequilibrium conditions

Abstract

A novel and pragmatic technique has been developed and validated to quantify gas exsolution of alkane solvent(s)–CO2–heavy oil systems with consideration of interface mass transfer for each gas component under nonequilibrium conditions. Experimentally, constant composition expansion (CCE) tests of three alkane solvent(s)–CO2–heavy oil systems are conducted with a visualized PVT cell under equilibrium and nonequilibrium conditions. The liquid height and pressure of the system are continuously monitored and recorded during experiments to measure, respectively, the bubblepoint pressure, pseudo-bubblepoint pressure, and the entrained gas volume. With the assumption of instantaneous nucleation, a mathematical model which integrates Peng-Robinson equation of state (PR EOS), Fick's second law, and nonequilibrium boundary condition has been developed to quantify not only the amount of the evolved gas and entrained gas, but also the dynamic composition of gas phase as a function of time. Once the deviations between the measured gas volumes and the calculated ones have been minimized, the mass transfer Biot number, individual diffusion coefficient, and interface mass transfer coefficient of each gas component as well as the gas bubble number are determined. Increases in experimental temperature and pressure are found to impose opposite effects on diffusion coefficient during gas exsolution processes. The diffusion of each gas component is found to be faster when the temperature becomes higher or the initial pressure becomes lower. Either CO2 or C3H8 diffuses faster than CH4 in the liquid phase under the same condition. In addition, the interface mass transfer coefficients, with an order of CO2 > CH4 > C3H8, obtained in this study are much higher than those collected in the literature since the nonequilibrium conditions greatly facilitate gas exsolution. The determined mass transfer Biot numbers in this study are large, indicating that the bulk resistance due to molecular diffusion inside the heavy oil dominates the gas exsolution process compared to the interfacial resistance.