The potential and mechanism of nonionic polyether surfactants dissolved in CO2 to improve the miscibility of CO2–hydrocarbon systems

Certain unique reservoir conditions and technical difficulties have greatly limited the use of CO2 miscible injection in unconventional reservoirs. Lowering the miscibility pressure to realize the miscibility of CO2 and crude oil is critical for achieving the most efficient recovery process. In this study, we investigated whether nonionic polyether surfactants could improve the miscibility of CO2 and hydrocarbons. And an improved experimental method for a quantitative evaluation of additive efficiencies in lowering the miscibility pressure was introduced for the first time. The dissolution behaviors of nonionic polyether surfactants in CO2 and phase behaviors of CO2–hydrocarbon systems with and without surfactant additives were conducted. The effects of surfactant structure and concentration and temperature on the solubility of nonionic polyether surfactants in CO2 along with the efficiency of miscibility pressure reduction for CO2–hydrocarbon systems were analyzed. The results showed that the polyoxypropylene alkyl ethers exhibited excellent potential for improving the miscibility of the CO2 and hydrocarbons, especially when low molecular weight surfactant molecules with high polyoxypropylene group content were used. The first-contact miscibility pressure (FCMP) of the mixed oil and CO2 could be reduced by as much as 22.51% with 3 wt% C4(PO)3 addition. Furthermore, a mechanistic understanding showed that the solubilization of the CO2 caused by polyether reverse micelles resulted in a reduction of miscibility pressures for CO2–hydrocarbon systems. The study results and the novel evaluation method provided guidance for designing and selecting appropriate additive structures to reduce miscibility pressures of CO2–oil systems and meet the CO2 miscible injection technology requirements.