Competitive Behavior of Polar and Nonpolar Shale Oil During CO2-Induced Mobilization in Water-Bearing Nanopores

Abstract The efficiency of CO2-enhanced oil recovery (EOR) in shale reservoirs is critically influenced by formation water and the complex chemistry of the crude oil. However, the roles of components in shale oil and its underlying mechanisms are lacking. In this work, molecular dynamics simulations were performed to investigate the CO2 flooding process in water-bearing shale nanopores, comparing representative nonpolar and polar multicomponent oil models. The results reveal that nanopore oil displacement efficiency is governed jointly by shale oil composition and water content, with polarity being the predominant factor. Specifically, nonpolar oil shows a monotonic decrease in efficiency with water content, whereas polar oil exhibits persistently lower efficiency that declines gradually with increasing water content, attributed to the tenacious hydrogen-bonding adsorption of heteroatomic species. Water reduces CO2 adsorption near the quartz surface and promotes clustering of oil components such as hexadecane, thereby increasing transport resistance. For polar oil, recovery is consistently lower and also declines with increasing water content, governed by the tenacious adsorption of heteroatomic compounds via hydrogen bonding. Amphiphilic molecules such as 1-nonanethiol act as molecular bridges. The thiol groups anchor strongly to the surface, while their alkyl chains link to nonpolar components via van der Waals interactions, forming a resistant network. Additionally, hydrogen bonding among polar components leads to stable aggregates that further restrict CO2 access to the oil phase. This study offers a theoretical basis for optimizing CO2-based recovery processes in water-bearing shale reservoirs rich in polar components.