Ethanol Blending to Improve Reverse Micelle Dispersity in Supercritical CO₂: A Molecular Dynamics Study

Despite a great promise in the enhanced oil recovery in tight formations, CO₂ flooding with surfactants is hindered due to the low surfactant solubility in supercritical CO₂ (scCO₂). Alcohol blending can increase the sodium bis(2-ethylhexyl) sulfosuccinate (AOT) solubility in scCO₂. While this finding offers a promising solution to CO₂ flooding in tight oil reservoirs, to the best of our knowledge, their working mechanism still remains elusive. Herein, we report a molecular dynamics simulation study to explore the working mechanism of alcohols in reverse micelle (RM) dispersity ("solubility") increment. The spontaneous aggregation process in two systems (System A consisting of AOT and scCO₂; System B consisting of AOT, scCO₂, and 10 wt % ethanol) are conducted under a typical tight oil reservoir condition (333 K and 200 bar). After 600 ns runs, the AOT molecules aggregate together and form rod-like RMs in System A, while form several small sphere-like RMs in System B. We observe that the aggregation process in System A occurs when two clusters approach each other end-to-end. More CO₂ molecules are around the Na⁺ ion at the end of the clusters, which can be readily replaced by AOT molecules. On the other hand, the ethanol molecules can better solvate and surround Na⁺ ions, preventing the further aggregation of AOT clusters in System B. The potential of mean force calculations also reveal that while two small clusters formed by four AOT molecules attract each other in System A, they repel each other in System B. Our work should provide important insights into the design of scCO₂-soluble surfactant formulas.