In recent years, the reduction of CO2 emissions has become a joint effort throughout the world, and carbon capture and sequestration (CCS) is an effective approach to solving the problem of CO2 emissions. In the present study, the effects of adding CH4, Ar, and H2S to CO2 on the interfacial tension (IFT) and wettability (contact angle, CA) of the CO2/water/silica system have been investigated using molecular dynamics simulation methods at 20 MPa and 318 K when the molar concentration of impurity gas was fixed at 20%. For the conditions studied, (1) CH4 has no significant effect; (2) Ar leads to a higher IFT, a larger CA on silica surfaces with a high hydroxyl density, and a smaller CA on silica surfaces with a low hydroxyl density; and (3) H2S causes a decrease of the IFT and an increase of the CA. Capillary pressure and gas storage capacity were predicted using IFT and CA data, and the variation of IFT and CA were explained based on density profiles normal to the gas/water and gas/silica interfaces. These findings might be helpful for better understanding the effects of impurities on CCS.