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 Pressure-Volume-Temperature (PVT) system is used to measure the equilibrium parameters including oil volume, gas volume and equilibrium pressure. In consequence, equilibrium properties including CO2 solubility, oil swelling factor and extraction pressure can be calculated and observed. In non-equilibrium condition, CO2 condensation and light oil component extractions are observed, as well as pressure decay data due CO2 dissolution is recoded. Furthermore, diffusion coefficient is calculated based on pressure data. Hence, the mechanisms 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 liquid oil-Vapor CO2 (LV), liquid oil-liquid CO2-Vapor CO2 (L1L2V), and liquid oil-liquid CO2 (L1L2), 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, swelling factor as well as extraction pressure can be calculated and determined. 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. During the non-equilibrium process, the pressure increases continuously by moving the piston at the various rates until the pressure reaches the desired pressure at different temperatures. 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 LV condition, where the EOR mechanism is mainly oil swelling effect. However, the swelling factor decreases with pressure during L1L2V and L1L2 conditions, indicating a change in the main controlling mechanism to CO2. As for the non-equilibrium process, the extraction is found to be closely related to the CO2 physical state. 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 orders larger than that in Period II, which is mainly driven by molecular diffusion.

In this work, both equilibrium and non-equilibrium properties have been measured and observed by using a piston-equipped visual cell. The mechanisms of CO2 EOR for Bakken oil have been comprehensively identified and analyzed at the different stages for the first time. This work sheds new light on the design of CO2-EOR application in unconventional oil reservoirs.

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