Mechanism of CO2/N2 stripping shale oil based on molecular dynamics simulation

Abstract:[Objective]The aim of this research is to investigate the storage state of tight oil and the mechanism of its replacement by CO2 using molecular dynamics simulations.[Methods]The Monte Carlo method and molecular dynamics simulation algorithms were employed to model the storage state of alkanes of varying molecular weights on rock surfaces. These models helped to examine the storage characteristics of alkane molecules on different types of rock surfaces and to analyze the micro-mechanisms of tight oil replacement by CO2 and N2. The simulated temperature and pressure conditions were selected to tight reservoir in the Sichuan basin (343.13K, 20MPa).[Results]The measured diffusion coefficients of C7 in CO2 were 1.88×10-5 and 1.83×10-5 on quartz and calcite surfaces, respectively. In contrast, the coefficients were lower in N2, at 6.4×10-6 and 9.01×10-6, respectively.[Conclusion]The findings indicate that CO2 is significantly more effective than N2 in replacing tight oil. The difficulty of displacing alkane molecules from rock surfaces increases with the relative molecular weight. As the relative molecular weight increases, it becomes more challenging to displace alkane molecules from the rock surface, and the adsorption of alkane molecules on the calcite surface is stronger than on the quartz surface. Based on the experimental results presented in this paper, the CO2 replacement mechanism can be broadly categorized into four stages: molecular diffusion, competitive adsorption, emulsification and dissolution, and a mixed-phase stage (involving low molecular weight alkanes).