Abstract
• A quantitative evaluation method of additive promoting miscibility was proposed. • Polyoxypropylene alkyl ethers significantly improved oil–gas miscibility. • Low molecular weight surfactants with high PO groups exhibited the best effect. • Solubilization of CO 2 in oil resulted in a reduction of miscibility pressures. Certain unique reservoir conditions and technical difficulties have greatly limited the use of CO 2 miscible injection in unconventional reservoirs. Lowering the miscibility pressure to realize the miscibility of CO 2 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 CO 2 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 CO 2 and phase behaviors of CO 2 –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 CO 2 along with the efficiency of miscibility pressure reduction for CO 2 –hydrocarbon systems were analyzed. The results showed that the polyoxypropylene alkyl ethers exhibited excellent potential for improving the miscibility of the CO 2 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 CO 2 could be reduced by as much as 22.51% with 3 wt% C 4 (PO) 3 addition. Furthermore, a mechanistic understanding showed that the solubilization of the CO 2 caused by polyether reverse micelles resulted in a reduction of miscibility pressures for CO 2 –hydrocarbon systems. The study results and the novel evaluation method provided guidance for designing and selecting appropriate additive structures to reduce miscibility pressures of CO 2 –oil systems and meet the CO 2 miscible injection technology requirements.
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