| dc.description.abstract | The Bakken Petroleum System (BPS) is one of the largest unconventional petroleum and natural gas resource and the most prolific tight oil plays in the North America. It is estimated that 10 to 400 billion barrels of oil has been generated and charged into the Bakken region. It is reported that the production rate drops drastically and stabilizes at a low value, which results in an unsatisfying recovery factor in the primary recovery stage. Either secondary recovery process or enhanced oil recovery (EOR) process is required since a small improvement in recovery factor can result in a huge improvement in oil production. CO2 based EOR process is the most favorable method for Bakken reservoirs due to favorable hydrocarbon properties. In consequence, it is important to quantify and investigate parameters such as CO2 solubility and oil swelling factor of this hydrocarbon system. In addition, experiments are designed to visualize phase behaviors at real reservoir conditions for comprehensive investigation and analysis of the CO2 EOR mechanism. CO2 dissolution and oil swelling effect are key mechanisms in the CO2 EOR process. CO2 solubility, oil swelling factor, and extraction pressure are systematically examined at elevated temperatures and pressures. In the phase behavior experiments, excessive CO2 and desirable Bakken crude oil is injected into a piston-equipped pressure/volume/temperature (PVT) cell. Different experiments on CO2 and Bakken oil system are performed at 27, 80, and 120 °C in order to visualize different phase behaviors. For each phase behavior experiment, the system is pressurized step by step by moving the piston forward. Each stabilized pressure is recorded as equilibrium pressure, and the oil phase volume is measured at each equilibrium pressure for swelling factor calculation. The pressurization process is repeated until CO2 and oil phases become miscible. In the simulation section, flash calculations are performed by using CMG WinProp. Bakken oil components are lumped into seven pseudo-components. Properties such as binary interaction coefficient, critical properties, and volume shift value of each pseudo-component are modified in order to match the oil swelling factor determined in the phase behavior experiment in order to develop an accurate equation of state (EOS) model. Then, the tuned EOS model is used in a cell-to-cell simulation for minimum miscibility pressure (MMP) calculation. The MMP determined in this section is compared with two pressures determined in the former section (the pressure results in a zero swelling factor and the pressure result in a zero interfacial tension). Overall, it is found that the extraction pressure decreases with system temperature. When the system temperature is below the CO2 critical temperature (31.1 °C), there are three phase behaviors, liquid-vapor (LV), liquid-liquid-vapor (L1L2V), and liquid-liquid (L1L2) observed in this system. However, there are only two types of phase behaviors observed when the system temperature exceeds the CO2 critical temperature. According to the CMG WinProp calculation at 80 and 120°C, the interfacial tension (IFT) between CO2 phase and Bakken oil phase is 0 at 1897 and 2732 psi, respectively, at which the super-critical CO2 starts to become liquid-like and a LV phase behavior in observed. Results from the cell-to-cell simulations show that the multiple contact MMP at 80 and 120°C are 2011 and 2812 psi, respectively, at which the interfacial tension across two phases reduces to zero. In conclusion, the multiple contact MMP is achieved when the super-critical CO2 behaves liquid-like. | |
