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[Display omitted] • Revealing the effect of CO 2 on oil potential energy in nanopores. • CO 2 solubility and oil swelling factor explained within nanopores. • Influence of CO 2 on the recovery law of different components in oil. • Analysis of different water film thickness on CO 2 enhanced oil recovery. Understanding the mechanisms of CO 2 extraction or flooding are vital for enhancing oil recovery (EOR) in tight reservoirs. In this study, the CO 2 EOR mechanism in the displacement-affected area (DPAA) and diffusion-affected area (DFAA) of quartz nanopores were thoroughly investigated using molecular dynamics simulation techniques. First, the following two contents were mainly simulated, namely CO 2 flooding oil in the single/double nanopores of DPAA and CO 2 extraction oil in dead-end nanopores of the DFAA with and without the water film. Then, tight oil potential energy, threshold capillary pressure, CO 2 solubility, and oil swelling in nanopores were calculated to clarify the effects of CO 2 on oil transport. Moreover, different CO 2 injection/flowback rates and different water film thicknesses on dead-end nanopores on oil recovery were discussed. In the DPAA, the CO 2 solubility and the oil swelling factor gradually decreased with distance from the CO 2 -oil interface (Y = 0 nm), where the higher the injection rate, the more easily the CO 2 dissolved in the oil. However, the injection rate of CO 2 was inversely proportional to oil recovery. In addition, it took longer for the displacement efficiency in the 6 nm pore of double pores to reach the same displacement efficiency as in the single 6 nm pore. In the DFAA, the effect of flowback rate on the displacement efficiency of oil was relatively low. However, the thickness of the water film was a key factor that affected the oil displacement efficiency in the DFAA. [ABSTRACT FROM AUTHOR]

CO 2 -enhanced oil recovery (CO 2 -EOR) technology has shown great application potential in the development of tight reservoirs. Since the proportion of nano-pores in tight oil reservoirs exceeds 77%, it is necessary to further study the occurrence state of crude oil in nano-pores. In this paper, the molecular dynamics simulation was applied to study the adsorption and diffusion behaviors of multi-component crude oil in quartz (hydrophilic) nano-pores. Besides, the density discretization method was used to investigate the occurrence characteristics of crude oil, the proportion of movable fluid in nano-pores, and the effects of CO 2 and polar molecules (C 3 H 6 O) on the adsorption characteristics of crude oil. The simulation results showed that the adsorption state of crude oil in the quartz nano-pores was in the form of 4 adsorption layers with a thickness of 0.45 nm each. In nano-pores, the oil molecules with longer molecular chains were more likely to aggregate and adsorb on the quartz surface. Among them, polar oil molecules have the strongest adsorption capacity on the quartz surface. Moreover, the crude oil potential energy and the self-diffusion coefficient gradually increased from the vicinity of the quartz wall and tended to be stable in the free layer. Meanwhile, the content of movable crude oil gradually augmented with the increasement of nanometer pore width and temperature. Furthermore, the pressure had little effect on the density distribution of crude oil in the pores. In contrast, the temperature had a more significant effect on the density distribution of the adsorption layer. At last, due to the more substantial adsorption capacity of CO 2 on the rock surface, the crude oil adsorbed initially on the rock surface would be stripped off by CO 2 , converting from irreducible oil to moveable oil. [Display omitted] • Distribution profile of adsorbed and free crude oil in various nanoslits. • The self-diffusion coefficient and potential energy of crude oil at different positions. • Temperature has a great influence on the occurrence state of crude oil. • Effects of different proportions of CO 2 on the occurrence state of crude oil in nanoslits. [ABSTRACT FROM AUTHOR]