Insights Into Reducing Minimum Miscibility Pressure of CO2-Crude Oil System by Gas-Soluble Surfactants: Molecular Dynamics Simulation and Experimental Analysis

Abstract The effective miscibility of CO2 in the oil phase is a critical scientific concern for achieving the synergistic utilization of high-efficiency CO2 enhanced oil recovery (EOR) and carbon sequestration. In this study, the solubility behaviors of 2,2,4-trimethyl-1,3-pentanediol diisobutyrate (TXIB), C12EmPn, and their composite system TC in CO2 were first investigated by combining cloud point pressure (CPP) experiments and molecular dynamics (MD) simulations. The experimental results showed that at a mass fraction of 0.5%, TXIB exhibited the highest solubility with a CPP of 36.5 MPa. Furthermore, the MD simulation results further confirmed this solubility trend: TXIB displayed the strongest interaction with CO2, characterized by the lowest diffusion coefficient, the highest radial distribution function (RDF) peak value, and the largest coordination number. Due to its molecular structure, C12EmPn showed the fastest diffusion rate. The composite system TC exerted a synergistic effect, possessing both moderate diffusivity and strong structural stability. On this basis, an in-depth analysis of the mechanism of surfactants in the CO2–oil system via MD simulations revealed that the mixing degree (Dmix) of TC reached as high as 55.92%, which was significantly higher than that of TXIB and C12EmPn, thereby substantially enhancing the miscibility between CO2 and the oil phase. Combined with interaction energy analysis, it was found that TC could construct a more stable solvation structure, which not only improved the compatibility of CO2 but also achieved a significant reduction in minimum miscibility pressure (MMP). This study provides theoretical and engineering support for MMP regulation, surfactant molecular design, and high-efficiency carbon utilization in CO2-EOR.