Microscopic production characteristics of tight oil in the nanopores of different CO2-affected areas from molecular dynamics simulations

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 diffusionaffected 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.