Abstract:

The Yanchang oilfield is characterized as a fractured, ultra-low permeability reservoir with low permeability, strong heterogeneity, and a complex network of natural micro-fractures and artificial fractures. It is also influenced by high salinity of formation water, high mass concentration of Ca²⁺, and weakly acidic formation water during the CO₂ flooding process, making conventional plugging systems unsuitable. In this work, the performance of a composite plugging system was evaluated, the injection parameters were optimized, and the slug size and combination method were also optimized. The adaptability of the system was verified through radial flow simulation of injection and production. The results showed that the foam composed of 0.2% AOS+0.2% MX-20+0.4% HPAM exhibited the best comprehensive performance, with a foam volume of 510 mL and a half-life of 3 940 seconds. The optimal gas-liquid ratio, injection rate, and injection volume were 3∶1, 0.3 mL/min, and 0.4 PV, respectively. The gel was compatible with CO₂, and its strength could meet the requirements for plugging fractures. The optimal slug size and combination method for the composite channeling plugging system were 0.1 PV gel+0.4 PV foam+0.1 PV gel. The validation experiment with radial flow model demonstrated that the composite system had good adaptability in fractured low-permeability oil reservoirs.

Key words: fractured ultra-low permeability reservoir, CO₂ flooding, composite channeling, CO₂ foam, polymer gel, Yanchang oilfield