Diffusion of CO2 molecules in the carbonated water-crude oil system

The mass transfer by diffusion of CO2 molecules between oil and aqueous phases is of great significance to the enhanced oil recovery by CO2 flooding in tight reservoirs. Through combining experimental measurement data with theoretical diffusion model, this paper simultaneously determines the diffusion coefficients of CO2 molecules in oil and aqueous phases in the carbonated water-crude oil binary system using the trial-and-error method. This study focuses on the influence of initial pressure of the oil-water mixing system on the diffusion coefficient, and deeply explores the variations of the density of oil-aqueous phases, CO2 concentration, oil-water interface movement, and CO2 diffusion front position in the oil phase during diffusion. The results show that: (1)As CO2 diffuses from the aqueous phase into the oil phase, the pressure of the carbonated water-crude oil system increases. After 10 hours of diffusion at 60℃, the pressure is increased by 24% to 31% for the initial pressures of 15.39, 19.25 and 22.82 MPa (Experiments 1 to 3). (2)Near the two-phase interface, the density of the aqueous phase and CO2 concentration in the aqueous phase gradually decrease, while the density of the oil phase and CO2 concentration in the oil phase gradually increases, causing the volume of the oil phase to expand and that of the aqueous phase to shrink, and the two-phase interface moves toward the aqueous phase. (3)The higher the initial pressure, the greater the diffusion coefficient. The CO2 diffusion coefficient in the aqueous and oil phase in experiments 1 to 3 is increased by 52% and 9.2%, respectively, indicating that the CO2 diffusion coefficient in the aqueous phase is more sensitive to the initial pressure. (4)The higher the initial pressure, the farther the CO2 diffusion front migrates in the oil phase within the same time period. After 100 hours of diffusion, the migrating distances of the front positions in experiment 1 to 3 are 4.11 cm, 4.32 cm and 4.43 cm, respectively. © 2021, Editorial Office of ACTA PETROLEI SINICA. All right reserved.