Optimized CO ₂ Injection Methods for Enhancing Oil Recovery of Low-Permeability Reservoirs

Premature CO2 breakthrough in production wells during CO2 flooding is a significant factor that limits the enhanced oil recovery (EOR) effect in low-permeability reservoirs. To improve oil production efficiency, experiments on CO2 immiscible injection and in situ nuclear magnetic resonance (NMR) analysis were conducted to identify the dynamic patterns of oil production in different locations and various pore sizes in low-permeability cores under several CO2 displacement methods. The findings indicated that continuous CO2 immiscible flooding (CO2–IMF) primarily extracts oil from the larger pores of the cores. Increasing the displacement pressure reduces the pore radius limit for oil production, which can also lead to earlier CO2 breakthrough from larger pores or fractures. The CO2-water alternating injection (CO2 WAG) and the combination of CO2–IMF with CO2 Huff-and-Puff (HnP) effectively suppress earlier CO2 breakthrough in production wells. However, the combination of CO2–IMF and HnP shows a more significant improvement in oil production, particularly for oil trapped near the production wellbore and in smaller reservoir pores. When comparing the oil production effects of CO2–IMF at 18 MPa, the pore radius limit for oil production decreased from 0.06–0.17 to 0.04–0.14 μm, resulting in an increase in oil recovery by 5–20% with the combination of CO2–IMF and HnP. The study results may guide the optimal scheme design of controlling CO2 premature breakthrough and recovering residual oil near the wellbore of low-permeability reservoirs.