(155a) Equilibrium and Non-Equilibrium Phase Behavior of CO2 and Crude Oil Interactions

The visualization and quantification of CO₂ and oil interactions give insight into the multiple mechanisms controlling CO₂ enhanced oil recovery processes. In this work, a high pressure high temperature full visual PVT system is used to observe and measure the equilibrium properties of CO₂ solubility in Bakken oil, swelling factor, and minimum miscibility pressure (MMP), together with the non-equilibrium properties of CO₂ condensation, light oil components extraction, and diffusion coefficient at elevated temperatures. The mechanisms at the different stages of CO₂ EOR process are identified and analyzed.

Firstly, excessive gaseous CO₂ is charged into the piston-equipped view cell coexisting with the pre-loaded Bakken oil. Three types of phase behavior can be captured under certain conditions including vapor-liquid (VL), vapor-liquid-liquid (VLL), and liquid-liquid (LL), which are common fluid types in the formation during CO₂ EOR process. In equilibrium process, the cell is pressurized stepwise. The volume of the swollen oil caused by the dissolution of CO₂ is recorded at each equilibrium pressure step, at which gas solubility and swelling factor can be calculated. Once the light components in Bakken oil start to be extracted into the liquid CO₂ phase, the volume of oil-rich phase decreases and a couple of extraction columns can be observed. The heavy components in oil are harder to be extracted so that the oil volume eventually reaches a plateau, where the pressure is recorded as the pseudo MMP and the system is at the near miscible condition. During the non-equilibrium process, the pressure increases continuously by moving the piston at the various rates until the pressure reaches the MMP. Finally, the pressure decay method is used to determine the diffusion coefficient between CO₂ and Bakken oil. It has been found that oil can be swollen by dissolving CO₂ at high pressures. The swelling factor increases with pressure during VL condition, where the EOR mechanism is mainly oil swelling effect. However, the swelling factor decreases with pressure during VLL and LL conditions, indicating a change in the main controlling mechanism to CO₂ extraction at the near miscible condition. As for the non-equilibrium process, the extraction pressure is found to be closely related to the moving rate of the piston. The condensing flow from CO₂ rich liquid phase to oil phase and the extracting flow from oil phase to CO₂ rich liquid phase have been filmed to demonstrate the EOR mechanisms. The effective diffusion coefficient in Period I, which is dominated by natural convection, is found to be three orderslarger than that in Period II, which is mainly driven by molecular diffusion.