Research On Phase Behavior Change And Seepage Characteristics Of CO₂and Oil In Porous Media

Carbon dioxide enhanced oil recovery (CO2-EOR) is the important technique for the global environment improvement and domestic oil energy supply increasement. It is the key for CO2-EOR to study on the phase state change and seepage characteristics of CO2/oil system in porous media. Nevertheness, for the fluids invisibility in porous media, the phase state and seepage parameters can not be measured precisely. Therefore, Magnetic Resonance Imaging (MRI) and X-ray CT were applied for visualization measurement of the phase state change and seepage characteristics of CO2/oil system, based on the traditional minimum miscibility pressure (MMP) measuring methods.According to MRI theory and gas/oil miscible principle, MRI technique was proposed in this study for determination of the MMP. MRI technique was verified by comparing the MMP values with those obtained by traditional methods. The MMP of CO2/n-decane system was modeling. The correlation between MMPs of CO2/n-alkane and the average carbon number was obtained. The results presented that the interface pattern was hemispherical at normal pressure and then changed from dish like to flat as pressure increased, and at last disappeared. The contact angle of the two phases and the volume of liquid phase enlarged as pressure increased.The phase state of CO2/n-decane system in glass bead packed column was visualized by using X-ray CT. In bulk fluid, the gray values of the images are proportional to the fluid densities. Mutual solubilities of CO2/n-decane system were calculated by modified PR EOS. In the porous media, the the gray values difference between the gas and liquid phases reduces as the pore scale decreases, which indicates a delay diffusion process and larger MMP of CO2/n-decane system.Near miscible displacement of CO2/n-decane in the single and parallel narrow channels, glass bead packed column and synthetic sandstone cores were conducted. In the single and parallel narrow channel, the injection rate should be concrolled more than a critical value to ensure the migration velocity of the interface of CO2and oil more than the oil expansion velocity. In the glass bead packed column, the transition zone area of the two phases decreases with the growing pressure. The ratio of the velocities of the transition zone and the "volumetric front" grows with the increaseing pressure to a certain value. In the synthetic sandstone cores, the recovery efficiency increases with the larger injection rate in the same permeablity cores. The cross pressure of the fit line of the recovery efficiency is larger than the MMP. Also, the ratio between the velocities of the transition zone and the " volumetric front" grows with the increaseing pressure to a certain value which increases with the permeability. Fingering performs during the miscible displacement because of the fluids properties, small permeability, large ratio between the diameter and the length of the core, along with the small ratio between the diameter of inlet/outlet and the core. The wavelength values estimated are just in good agreement with the experimentally observation values.