Mechanisms of CO2 nanobubble-enhanced oil recovery: Interfacial regulation and flow behavior control in porous media

Nanobubbles (NBs) have unique physicochemical properties, including a large specific surface area and high mass transfer efficiency. They can be produced using environmentally friendly, low-cost physical methods, making them a promising green agent for enhanced oil recovery (EOR). However, the mechanism by which NBs enhance oil recovery remains unclear. In this study, we systematically investigate the stability of NBs under varying conditions (e.g., salinity and temperature) using nanoparticle tracking analysis. The interfacial properties of NBs were evaluated through interfacial tension measurements, contact angle analysis, and oil film detachment experiments. Furthermore, multiscale flow experiments in porous media and oil displacement experiments were conducted to reveal the flow characteristics and oil displacement mechanisms of NBs. Results demonstrate that NBs exhibit excellent thermal stability, salt tolerance, shear resistance, and temporal stability. They can reduce oil–water interfacial tension while enhancing the solid interface's affinity for the aqueous phase and its oil-repellent properties. NBs improve oil recovery through multiple synergistic mechanisms (6%−9%) over conventional water flooding. NBs facilitate the penetration of the displacing fluid into microporous regions that are typically inaccessible to conventional water flooding. Simultaneously, Ostwald ripening—where smaller bubbles dissolve and larger ones grow via gas diffusion—in porous media leads to pore-throat blockage, increasing flow resistance and improving sweep efficiency. These synergistic mechanisms enhance the sweep efficiency of the displacing fluid by 27.72%. This study provides a theoretical foundation for the application of NBs in petroleum engineering and demonstrates significant potential for advancing efficient and environmentally sustainable oilfield development.