The effects of nonionic surfactants on enhancing miscibility between oil and CO2: A molecular dynamics study

The harsh underground conditions often lead the CO2-oil minimum miscible pressure too high to be attainable. In previous studies, nonionic surfactants polyoxypropylene alkyl ether (CiPOj) and tri-isobutyl citrate (TBC) were utilized to enhance CO2-crude oil miscibility. Both of them have a hydroxyl at the terminal. The effects of surfactant terminal hydroxyl acetylation on CO2-crude oil miscibility deserves investigation and microscopic mechanisms need to be explained. In this work, four surfactants, C6PO3, acetylated C6PO3, TBC and acetylated TBC, were selected for the CO2-surfactant-crude oil system and dynamics simulations were carried out at 358.15 K with NVT ensemble. The density profile and fluid properties such as interfacial tension (IFT), minimum miscible pressure (MMP), CO2 solubility in alkane, oil volume expansion factor and CO2 extraction efficiency were calculated for systems with and without surfactants. The study found that those surfactants can reduce CO2-crude oil MMP although they prefer dissolving into the oil phase. The addition of surfactant enhances CO2 solubility in the alkane phase, thus resulting in a larger oil volume expansion factor. There exists an approximate linear relationship between the volume expansion factor and CO2 solubility. Compared with C6PO3 and TBC, the surfactant with terminal hydroxyl acetylation further reduces MMP by 1.18 MPa and 0.43 MPa, respectively. Among the four surfactants selected in this research, acetylated TBC outperforms the other three in increasing CO2 solubility due to the reinforcement of CO2-alkane interaction and the reduction of alkane-alkane interaction. The system with acetylated C6PO3 yields the largest CO2 extraction efficiency because of the weakened CO2-CO2 interaction.