关键词: CO₂压驱, 混相程度, 注入速度, 基质, 驱替特征

Abstract: To clarify the effects of high-speed fracturing-flooding CO₂ injection on oil displacement characteristics, experiments were conducted to investigate the impact of injection speed under different miscible phases on CO₂ flooding in low-permeability cores. The experimental results indicate that increasing the injection speed effectively enhances the injection pressure. At the same injection speed, the pressure difference in immiscible CO₂ flooding is about twice that under near-miscible and miscible phases; moreover, a more pronounced increase can be seen in injection pressure during immiscible CO₂ flooding as the injection speed rises. On the other hand, the increasing injection speed leads to early gas breakthrough, thus reducing oil recovery rates, which are below 40 % under immiscible phases. However, under the enhanced miscible phase, recovery rates gradually rise to more than 70 % . Accordingly, the influence of injection speed on the final recovery rates is weakened, and the adverse effects of gas breakthrough are also mitigated at high injection speeds. In immiscible and near-miscible CO₂ flooding, the recovery of oil in large pores increases by 7.2 % and 24.6 % with the increasing injection speeds, respectively, while oil recovery in small and medium pores decreases by about 10 % . However, in miscible CO₂ flooding, injection speed has little effect on oil extraction characteristics in pores. Under the same miscible phase, injection speed does not significantly affect the lower limit of pore utilization; with the rising of miscible phase level, the lower limit for pore utilization gradually decreases from 0.03 μm to around 0.01 μm.

Key words: CO₂ fracturing-flooding, miscible phase level, injection speed, matrix, displacement characteristics