Abstract:

Ultra-low permeability reservoirs typically exhibit significant heterogeneity and complex fracture networks, making them highly susceptible to gas channeling during CO₂ flooding, thereby limiting the improvement of oil recovery efficiency. To explore the oil displacement effect of CO₂ flooding in ultra-low permeability reservoirs and develop a channeling plugging system with excellent performance, in this work, the minimum miscibility pressure experiment in Yanchang oilfield was carried out through slim tube experiments. Through core plugging and oil displacement experiments, the differences in plugging performance and oil displacement performance were compared among gel, foam and foam-gel composite channeling plugging system; the combinations of these channeling plugging systems in multiple rounds were evaluated according to oil displacement performance. The results indicated that the minimum miscibility pressure in this region was 31.1 MPa, at which injecting CO₂ could achieve complete miscibility with crude oil. The injection performance of foam was the best, and the injection performance of gel was the worst. The maximum injection pressure difference was 0.557 MPa, and the maximum plugging rate was 94.5%. The oil recovery of foam-gel composite channeling plugging system was superior to that of single foam system or gel system, and the maximum oil recovery could reach 77.5%. The oil displacement test for multi-round combined channeling plugging systems showed that, after two rounds of profile control and flooding, the oil recovery of foam-gel and then foam reached 86.8%, which was higher than the 78.2% for foam-gel and then gel. The maximum injection pressure was not more than 1 MPa, which was safe and convenient for construction.

Key words: ultra-low permeability reservoir, CO₂ flooding, minimum miscible pressure, foam-gel, plugging system