Abstract:

After being subjected to depletion production and water flooding, deep low-permeability oil reservoirs still retain about 60% of the original oil in place, which has become a major bottleneck restricting the efficient utilization of oil and gas resources. CO₂ miscible flooding, as an efficient enhanced oil recovery (EOR) technique, has attracted significant attention in recent years. However, during its practical application, the phenomenon of frontal breakthrough often occurs, leading to an uneven sweep efficiency and significantly reducing the overall oil displacement performance. This study employed nuclear magnetic resonance (NMR) and CT scanning techniques to systematically investigate the migration characteristics and microscopic mobilization mechanisms of CO₂ miscible fronts using deep low-permeability core samples with different permeability grades. The results indicated that core permeability significantly influenced the stability and migration behaviors of CO₂ miscible fronts. As permeability decreased, the pseudo-piston-like displacement pattern was disrupted earlier, and the degree of non-uniform frontal advancement intensified, specifically manifested as a shorter dimensionless migration distance and a notable decline in oil displacement efficiency in the middle and rear sections of the core. In terms of microscopic pore mobilization, CO₂ exhibited distinct sequential selectivity. It preferentially entered large pores, and gradually advanced into medium and small pores. With increasing core permeability, the mobilization of crude oil in medium pore throats significantly improved, reflecting more uniform displacement characteristics. Further correlation analysis showed a positive relationship between overall oil displacement efficiency and the dimensionless migration distance of the front, indicating that front stability was a key factor restricting the performance of oil displacement. This study reveals the migration patterns of CO₂ miscible fronts and pore-scale oil mobilization mechanisms in deep low-permeability reservoirs from a combined macro-micro perspective. The findings provide valuable insights for optimizing injection-production strategies and improving development performance, offering theoretical support and technical guidance for the efficient development of deep low-permeability reservoirs using CO₂ flooding.

Key words: deep low-permeability reservoir, CO₂ miscible flooding, front migration characteristics, microscopic mobilization mechanism, oil displacement efficiency