Comparison of the Oil Recovery and Pore Size Limits of Movable Tight Oil during N₂ and CO₂ Flooding

Tight oil has attracted much attention because of its huge reserves; therefore, it is necessary to find effective development methods to enhance the oil recovery. Among the common development methods, gas injection development has many advantages, such as low interfacial tension with tight oil, low miscible pressure, and strong diffusion effect. In this work, the effects of CO2 and N2 gas injection pressure, drawdown pressure, and fractures on tight oil recovery, minimum pore size, and T2 cutoff pore size of movable oil were studied. The results show that the development effect of CO2 flooding is better than that of N2 flooding, and the minimum pore size and T2 cutoff pore size of movable tight oil are smaller. With the increase of CO2 injection pressure, the drawdown pressure gradually decreases from 16 to 8 MPa, the tight oil recovery increases from 18 to 50.9%, and the minimum pore size of movable oil decreases from 0.021 to 0.005 μm; the T2 cutoff pore size of movable oil is reduced from 0.060 to 0.028 μm. The existence of fractures increases the contact area between CO2/N2 and the core, which is beneficial to the diffusion of the injected gas into the core matrix. Meanwhile, the contradiction between the low matrix permeability and the high fracture permeability is more prominent, the injected gas is more likely to form the dominant channel along the fracture, and the gas channeling time is greatly advanced. Therefore, it is necessary to adopt the refined gas injection method. This work can provide theoretical guidance for the gas injection development of tight oil.