Microscale interactions are pronounced in shale nanopores, while the relevant mechanisms between multiphase fluids remain unclear at present. In this paper, the CO₂ displacement process in shale porous media with mixed wettability is simulated, with the aim to reveal the microscopic interphase mechanisms and assess CO₂ displacement efficiency and storage performance. The results indicate that in the initial state, under the predominant effect of van der Waals forces, oil molecules are present in adsorbed and free states, while water molecules exist as films and clusters in the three types of channels, driven by the combined action of van der Waals forces, Coulomb forces and hydrogen bonds. During the displacement process, CO₂ preferentially enters hydrophilic channels, followed by mixed-wetting channels, and finally lipophilic channels. Instantaneous dipole moments between non-polar molecules make van der Waals forces the dominant factor in mutual miscibility. The permanent dipole of polar molecules and the induced dipole of non-polar molecules synergistically enhance the contribution of Coulomb force during the competitive adsorption process. However, the presence of “water bridge” within mixed-wetting channels significantly inhibits CO₂ penetration and impairs oil stripping. The final displacement efficiency and storage efficiency are 43.08% and 5.99%, respectively, both significantly lower than those in hydrophilic and lipophilic channels. This study clarifies the microscale interaction mechanisms in shale reservoirs with mixed wettability, offering practical guidance for effective shale reservoir exploitation.