A study of the mechanism of enhancing oil recovery using supercritical carbon dioxide microemulsions

Supercritical carbon dioxide (scCO₂) microemulsion was formed by supercritical CO₂, HA sodium bis(2-ethylhexyl) sulfosuccinate (AOT, surfactant) and C₂H₅OH (co-surfactant) under pressures higher than 8 MPa at 45°C. The fundamental characteristics of the scCO₂ microemulsion and the minimum miscibility pressure (MMP) with Daqing oil were investigated with a high-pressure falling sphere viscometer, a high-pressure interfacial tension meter, a PVT cell and a slim tube test. The mechanism of the scCO₂ microemulsion for enhancing oil recovery is discussed. The results showed that the viscosity and density of the scCO₂ microemulsion were higher than those of the scCO₂ fluid at the same pressure and temperature. The results of interfacial tension and slim tube tests indicated that the MMP of the scCO₂ microemulsion and crude oil was lower than that of the scCO₂ and crude oil at 45 °C. It is the combined action of viscosity, density and MMP which made the oil recovery efficiency of the scCO₂ microemulsion higher than that of the scCO₂ fluid.