In-situ dynamic characterization of residual oil displacement mechanism by CO₂ after water flooding using online nuclear magnetic resonance

To enhance the oil recovery in the middle-deep reservoirs of the Jidong Oilfield, this study investigates the controlling mechanism of pore-throat structure on oil displacement efficiency during water flooding and miscible CO₂ flooding, providing a theoretical basis for optimizing development strategies. By integrating online NMR displacement and high-pressure mercury injection experiments, a conversion relationship between NMR T₂ spectra and mercury injection data was established. This allowed for the quantitative characterization of pore-throat structure using average pore-throat radius and sorting coefficient. Systematic water flooding and miscible CO₂ flooding experiments were conducted on core samples with high, medium, and low permeability. The results indicate that pore-throat structure parameters decrease with decreasing permeability. During water flooding, high- and medium-permeability cores primarily utilized micropores (utilization rates of 60.02%~80.11%), while low-permeability cores showed extensive utilization of sub-micropores and micropores (44.63%~94.68%), with nanopores contributing 18.52%. A strong positive correlation was observed between water flooding efficiency and both permeability (coefficient of determination, R²＞0.75) and average pore-throat radius (R²=0.85). In the miscible CO₂ flooding stage, the utilization rates across the full pore-throat size range in medium- and low-permeability cores (17.09%~31.49%) were significantly higher than those in high-permeability cores (14.57%~16.63%). The incremental oil recovery from CO₂ flooding showed a strong negative correlation with permeability, average pore-throat radius, and sorting coefficient (R²≥0.80).This study demonstrates that pore-throat structure is a key factor controlling oil displacement efficiency. It is recommended to prioritize the development of medium and high-permeability zones during water flooding and focusing on exploiting the potential of low-permeability zones during subsequent miscible CO₂ flooding. The research findings provide a scientific basis for enhancing oil recovery in the Jidong Oilfield and similar reservoirs.