Full-Scale Experimental Study on the Effect of CO₂ Flooding on Storage-Seepage Capacity of Tight Sandstone Reservoirs

In the CO₂ flooding stage of tight sandstone reservoirs, the CO₂-crude oil-rock interaction causes asphaltene deposition and mineral dissolution effect, which would affect the reservoir performance and reservoir physical property. To reveal the variation law of reservoir performance and percolation capacity in the CO₂ flooding stage, the core samples of typical tight sandstone reservoirs with different permeability levels were selected. An indoor physical flow simulation experiment of CO₂ flooding was then performed. The change in reservoir performance characteristics in the CO₂ flooding stage of the tight sandstone reservoir was quantitatively evaluated, and the evolution law of reservoir macroscopic seepage ability during the CO₂ flooding process was revealed. The experimental results show that the damage degree of CO₂-crude oil-rock interaction on reservoir foundation performance is negatively correlated to the initial permeability of the reservoir. The lower the initial permeability of core samples, the greater would be the damage degree of the experimental process on the effective porosity and permeability of rock samples. However, in reservoirs with relatively high permeability (0.5-0.6 mD), the physical properties of rock samples improved after CO₂ flooding with an average increase in effective porosity and permeability by 4.54 and 10.23%, respectively. The damage degree of reservoir effective porosity is slightly affected by CO₂ injection pressure, while the damage degree of permeability has a significant negative correlation with injection pressure. In the high-pressure miscible flooding stage of 30 MPa, the organic matter deposition caused by the interaction of CO₂-crude oil-rock causes the highest degree of damage to the permeability of rock samples (up to 33.33%). Moreover, the flooding efficiency in the CO₂ flooding stage of a tight sandstone reservoir is positively correlated with the reservoir permeability and CO₂ injection pressure. In the near-miscible flooding stage to miscible flooding stage, the flooding efficiency increases rapidly. After entering the miscible flooding stage, the growth rate in flooding efficiency decreases gradually with the increase in injection pressure.