Effects of CO2-philic nonionic polyether surfactants on miscibility behaviors of CO2-hydrocarbon systems: Experimental and simulation approach

Accomplishing the high miscibility of CO2 and crude oil under reservoir conditions is critical for achieving the most efficient recovery process by CO2 injection into low-porosity and low-permeability reservoirs. In this study, the effects of nonionic polyether surfactants on improving the miscibility of CO2-hydrocarbon systems were investigated. The influence of polyether structure on the reduction of miscibility pressure between CO2 and hydrocarbons was evaluated by experiments. The results showed that the miscibility pressure of CO2 and C16 could be reduced by 4.8% to 10.2% after adding 0.04 mol polyethers with different structures per kg CO2. The capability of polyethers to improve oil-gas miscibility exhibited a first growing then diminishing trend with increasing polyoxypropylene chain length and decreased with increasing alkyl chain length. Moreover, the microscopic influence mechanism of polyether on the miscible processes of CO2-hydrocarbon systems was investigated by molecular dynamics simulations. The miscible degree parameter was defined in the simulation to quantitatively estimate oil-gas miscibility. The distributions of different molecules and potential interactions during the miscible process were analyzed. The simulation results showed that the solubilization of CO2 molecules among polyoxypropylene chains in the oil phase caused by the local aggregation of nonionic polyether surfactants enhanced the vdW interactions between hydrocarbons and CO2 and then improved the oil-gas miscibility. Furthermore, simulation results suggested that the number of polyoxypropylene groups in the nonionic polyether surfactant should be maintained at 8 to achieve the highest amount of CO2 solubilization and the best potential in improving the oil-gas miscibility. The study results provide guidance for understanding the miscible behavior among CO2, additives, and hydrocarbons, as well as designing the appropriate additive structures for improving oil-gas miscibility.