Assessing the effects of CO2/methane mixtures on gas-oil interfacial tension and fluid flow using compositional simulation

In CO2-enhanced oil recovery (CO2EOR) processes, the gas-oil interfacial tension (IFT), which is a key property to determining oil recovery factor, affects rock wettability, capillary pressure, relative permeability, oil flow, and oil recovery factor (RF). The IFT is a key property when addressing the flow assurance issues in CO2EOR processes, in which methane-rich gases mix with CO2 for injection. Although the literature reports some studies on the effect of methane concentration on the properties of CO2-oil systems, investigating methane content effects on IFT and fluid flow is still lacking, which is the primary motivation for the present work. Compositional simulations were carried out to evaluate the behavior of density, pressure, phase composition, oil saturation, IFT, and oil recovery to verify the effect in the IFT considering flow for different scenarios with varying methane concentration, pressure, and flow rate. The main focus is on the variation of these properties with IFT and their mechanisms. The results show how the methane content alters the CO2 dissolution in oil, the light hydrocarbon extraction from the oil, the reservoir pressure, the phase densities, the oil saturation, the gas-oil IFT, and the RF. At the beginning of the process, an optimal methane concentration provides the maximum RF. However, the EOR process achieves the highest recovery in the long term by injecting only CO2.