Wettability of an oil/brine/rock system is a critical petrophysical parameter, which governs subsurface multiphase flow behavior, thus hydrocarbon recovery. While the mechanisms of CO2-assisted enhanced oil recovery (EOR) techniques have been extensively investigated in carbonate reservoirs, few have been done to identify the controlling factor of CO2-induced wettability alteration, and fewer have looked beyond the implications for CO2 assisted EOR We thus hypothesize that CO2-assisted EOR techniques cause a more hydrophilic system due to 1-1 adsorption on the interface of oil/brine and brine/carbonate as a result of CO2 dissolution. To test this hypothesis, we measured contact angles of two oils with different acid and base numbers in the presence of 1 M Na2SO4 at pH either 3 or 8. Moreover, we performed a geochemical study to identify the controlling factor of wettability alteration. Contact angle results support the hypothesis, showing that the oil/brine/rock system shifts toward more water-wet because of less bonds between oil/brine and brine/carbonate due to H+ adsorption on the interface of oil/brine and brine/carbonate. Our results also suggest that CO2-assisted EOR techniques likely shift relative permeability curves toward a lower residual oil saturation due to wettability alteration. We argue that geochemical reactions may need to be incorporated into a reservoir numerical model, thus better managing and predicting the performance of CO2-assisted EOR