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

Premature CO 2 breakthrough in production wells during CO 2 flooding is a significant factor that limits the enhanced oil recovery (EOR) effect in low-permeability reservoirs. To improve oil production efficiency, experiments on CO 2 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 CO 2 displacement methods. The findings indicated that continuous CO 2 immiscible flooding (CO 2 –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 CO 2 breakthrough from larger pores or fractures. The CO 2 -water alternating injection (CO 2 WAG) and the combination of CO 2 –IMF with CO 2 Huff-and-Puff (HnP) effectively suppress earlier CO 2 breakthrough in production wells. However, the combination of CO 2 –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 CO 2 –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 CO 2 –IMF and HnP. The study results may guide the optimal scheme design of controlling CO 2 premature breakthrough and recovering residual oil near the wellbore of low-permeability reservoirs.