Summary
Gravity segregation of reservoir fluids is mainly controlled by density. Although most gases used in the field for enhanced oil recovery (EOR) result in a reduction in density upon mixing with the oil, carbon dioxide (CO2) can result in an increase of the density upon mixing. Experimental observations confirmed this behavior. In addition, field operations report an early breakthrough for CO2 flooding, which is related to the associated gravity segregation caused by the abnormal density behavior. However, the molecular interactions at play that have an impact on the observed macroscopic behavior have not been well-understood or deeply investigated. Molecular simulation of methane, propane, and CO2 mixtures with octane, benzene, pentane, and hexadecane is studied up to the miscibility limit at temperatures up to 260°F (400 K), and pressures up to 6,000 psi (400 bar). There is a proximity between the values of density obtained through molecular simulations and those obtained through experimental work and equation-of-state (EOS) methods. It is evident that oil/CO2 mixtures sustain their density to a higher gas mole percentage compared with other gases, with the density in some cases exceeding the pure liquid-hydrocarbon density even when gas density at those conditions is lower. Our results have demonstrated that the proposed mechanisms in literature—namely, intermolecular Coulombic and induced dipole interactions and the stretching of the alkane molecules—might not be the key to understanding the oil/CO2 density behavior. However, the molecular size of the gas seems to play an important role in the density profile observed.
