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

Understanding the mechanisms of CO2 extraction or flooding are vital for enhancing oil recovery (EOR) in tight reservoirs. In this study, the CO2 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 CO2 flooding oil in the single/ double nanopores of DPAA and CO2 extraction oil in dead-end nanopores of the DFAA with and without the water film. Then, tight oil potential energy, threshold capillary pressure, CO2 solubility, and oil swelling in nanopores were calculated to clarify the effects of CO2 on oil transport. Moreover, different CO2 injection/ flowback rates and different water film thicknesses on dead-end nanopores on oil recovery were discussed. In the DPAA, the CO2 solubility and the oil swelling factor gradually decreased with distance from the CO2-oil interface (Y = 0 nm), where the higher the injection rate, the more easily the CO2 dissolved in the oil. However, the injection rate of CO2 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.