Advanced CO ₂ Injection in Tight Oil Reservoirs: Pore-Scale Mechanisms from High-Temperature, High-Pressure Visualization and NMR

The successful application of advanced water flooding techniques common in low-permeability reservoirs is often impeded in tight oil formations due to the predominance of microscale and nanoscale pores. CO2, owing to its superior injectivity and ability to achieve miscibility upon contact with crude oil, presents a promising enhanced oil recovery (EOR) method for such reservoirs. However, research on the pore-scale mobilization mechanisms during advanced CO2 injection in tight oil reservoirs remains extremely scarce. This study investigates Chang 8 tight oil cores from the Ordos Basin through comprehensive experiments conducted at reservoir temperature. Experiments included immiscible CO2 flooding, miscible CO2 flooding, and advanced CO2 injection, employing high-pressure microscopic visualization and nuclear magnetic resonance (NMR) long-core flooding. High-pressure microscopic visualization revealed that advanced CO2 injection achieved the highest recovery factor, followed by miscible CO2 flooding, with immiscible flooding exhibiting the lowest performance. During immiscible flooding, CO2 entered pores as discrete gas bubbles, coalescing into clusters that displaced oil primarily through squeezing, leaving behind continuous oil sheets and isolated droplets. Miscible CO2 flooding exhibited significant diffusion effects, extracting oil via swelling and extraction; residual oil manifested as minor continuous patches and thin films. Advanced CO2 injection further enhanced mass transfer through improved diffusion, effectively mobilizing film-like residual oil. NMR long-core flooding confirmed that advanced CO2 injection significantly improved the sweep efficiency, resulting in isolated residual oil distributions with minimal saturation differentials between the upstream and downstream core sections. Compared to miscible CO2 flooding, advanced CO2 injection increased the ultimate recovery by 10% and doubled the recovery from mesopores and micropores. Furthermore, the recovery factor for advanced CO2 injection demonstrated a positive correlation with injection pressure, achieving optimal results at 1.2 times the minimum miscibility pressure (MMP). These findings confirm the significant technical advantages of advanced CO2 injection for efficient tight oil development.