Recovering Residual Heavy Oil after CO₂ Flooding by Middle-Phase Microemulsion

A light portion with a carbon number below 25 can generally be extracted by supercritical carbon dioxide during the EOR process, leaving behind residual heavy components that reduce oil recovery and cause irreversible reservoir damage. To address this, middle-phase microemulsion is introduced in this work to solubilize the accumulated heavy portion after CO2 flooding. The microemulsion system was formulated with sodium 4-dodecylbenzenesulfonate and alcohol propoxypropylene ether sulfate under simulated reservoir conditions, and its bicontinuous microstructure was verified using cryo-electron microscopy and small-angle X-ray scattering. With 2D microfluidics, the solubilization process of the heavy portion in situ was visualized. Further experiments in a 3D microchip show that in situ microemulsification can further reduce remaining oil saturation by 26% after CO2 flooding, and core flooding tests indicated a 20% increase in recovery factor. These results highlight the significant potential of middle-phase microemulsions in enhancing heavy oil recovery, offering a promising strategy for improving oil recovery after CO2 flooding.