Molecular simulation of CO2–hydrocarbon minimum miscibility pressure in tight reservoir nanopores under varying wettability conditions

Abstract

Tight oil reservoirs are characterized by low porosity, low permeability, and the development of micro- and nano-scale pore structures. Conventional depletion and water flooding methods are generally ineffective, making CO₂ flooding a widely adopted approach to enhance oil recovery. The minimum miscibility pressure (MMP) is a key parameter in determining whether CO₂ can achieve miscibility with crude oil. However, due to the strong heterogeneity in the wettability of tight oil reservoirs, the mechanism by which different wettability conditions affect the MMP of CO₂–oil systems remains unclear.In this study, quartz nanopore models with varying wettability were constructed, and Non-Equilibrium Molecular Dynamics (NEMD) simulations were performed to calculate the MMP within nanopores. The effects of temperature, pore size, wall wettability, and crude oil composition on the MMP and CO₂ storage efficiency were systematically analyzed. The results show that the MMP within nanopores is lower than that in the bulk phase. MMP increases with temperature and exhibits a non-monotonic trend with decreasing pore size—initially decreasing and then increasing. As the wettability shifts from water-wet to oil-wet, the MMP tends to increase. Due to the strong interactions between the pore wall and CO₂ molecules, water-wet reservoirs are more favorable for CO₂ storage and result in a lower MMP in CO₂–oil systems.This study provides new insights into the microscopic miscibility mechanisms of CO₂ and oil under varying wettability conditions in tight reservoirs and offers theoretical support for enhancing oil recovery and CO₂ sequestration through CO₂ injection in tight oil formations.