Compositional modeling of hybrid CO2 EOR with intermediate hydrocarbon solvents to analyze the effect of toluene on asphaltene deposition

Once asphaltenes have been precipitated from oil due to pressure, temperature, or oil composition changes, they can affect reservoir performance by reducing porosity and permeability and altering wettability. Moreover, they can cause plugging of wellbore and surface production facilities. The addition of liquefied petroleum gas (LPG) into the CO2 stream during CO2 flooding reduces oil viscosity, interfacial tension (IFT), and oil swelling, resulting in improved displacement efficiency, but also causes asphaltene precipitation because of a change in oil composition. When toluene is introduced into the LPG and CO2 stream to overcome the limitations of a CO2-LPG flood, it acts as an inhibitor of asphaltene precipitation. This research systematically examines the mechanism of asphaltene precipitation and how to mitigate problems associated with asphaltene deposition based on compositional simulations. Solid deposition model that included adsorption, pore throat plugging, and re-entrainment was considered. A compositional equation of state (EOS) model was developed to analyze the effects of LPG and toluene on asphaltene deposition and formation damage. At the center of reservoir, the amount of asphaltene deposition reached the maximum for 10% LPG-added CO2-LPG flooding, which was 12% higher than that from CO2 flooding. When 1% toluene was added into the CO2-LPG flood, the amount of asphaltene deposited declined by 2%. Oil resistance factor had the same tendency because of porosity reduction due to decreased deposition of asphaltene. Oil recovery for the CO2-LPG flood was enhanced by 4% compared to the CO2 flood, despite an increment in asphaltene deposition, and by 9% for the CO2-LPG-toluene flood. Results from this study indicate that LPG addition causes increased asphaltene deposition, which can be mitigated by using toluene as an inhibitor.