The supercritical CO2 (scCO2), as an environmentally friendly and inexpensive chemical, has been considered as an alternative solvent for the water flooding in enhanced oil recovery (EOR) processes. CO2-foam flooding can effectively avoid low injectivity issues surrounding water flooding in tight oil reservoirs with extremely low permeability, while facilitating geological carbon sequestration (GCS). However, due to low surfactant solublity in scCO2, the applicability of CO2-foam flooding has been hindered. In the past, extensive efforts have been devoted to increase surfactant solubility in scCO2. In 2020, Zhang et al. reported that adding ethanol can effectively increase the sodium bis(2-Ethylhexyl) sulfosuccinate (AOT) solubility in scCO2, leading to an EOR[1]. While this finding renders a promising solution to CO2-foam 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 investigate the role of alcohols in the solubility increment of AOT in scCO2 under typical tight oil reservoir conditions (333 K and 200 bar). We randomly inserted AOT molecules in two different systems (A, B) as initial configurations: System A consists of AOT and scCO2; System B consists of AOT, scCO2, and 10 wt.% ethanol. After 600-ns runs, the AOT molecules aggregate together and form rod-like reverse-micelles (RMs) in System A. On the other hand, AOT molecules form several small sphere-like RMs in System B. In System A, Na+ ions aggregate in the cores of RMs, while they are bonded to the head groups of AOT molecules. By introducing ethanols (system B), the Na+ ions are exposed to and solvated by the ethanol molecules, which prevent further AOT aggregations. The potential of mean force (PMF) calculation also reveals that small clusters formed by 4 AOT molecules attract each other in System A, while the opposite is true in System B. Our work should provide important insights into the design of scCO2 soluble surfactant formulas and chemical blending for EOR and GCS.

Why alcohols can improve reverse-micelle dispersity in supercritical CO2?: A molecular-level understanding

ACS Fall 2021

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