CO2 water-alternating-gas (WAG) injection has been proven as a promising and effective technology to significantly enhance the recovery of the residual oil. However, the detailed displacing process and exquisite interaction mechanism still remain ambiguous. By using Nonequilibrium Molecular Dynamic Simulation (NEMD), three comparative displacement processes of adsorbed oil layer in nanoslit using water, supercritical carbon dioxide (scCO(2)), and CO2 WAG injection were conducted to study the enhanced oil recovery (EOR) mechanism of CO2 WAG injection. Simulation results indicate that the scCO(2) and water play different roles in CO2 WAG: the scCO(2) slug could selectively dissolve apolar oil components, and the water slug provides stable water/scCO(2) promoting interface, which compels the miscible oil/scCO(2) slug out of the nanoslit. Furthermore, the synergistic effects of scCO(2) slug and water slug were found to account for a decreased injecting pressure and an improved dissolution capability of scCO(2) slug, which endows the CO2 WAG higher oil recovery efficiency than that of both scCO(2) injection and water injection. Our work here provides a comprehensive interpretation of EOR mechanism of CO2 WAG, and the results could provide insights into the optimization of the engineering process. (C) 2016 Elsevier Ltd. All rights reserved.