Naturally fractured carbonate reservoirs have very low oil recovery efficiency owing to their wettability and tightness of matrix.However,smart water can enhance oil recovery by changing the wettability of the carbonate rock surface from oil-wet to water-wet,and the addition of surfactants can also change surface wettability.In the present study,the effects of a solution of modified seawater with some surfactants,namely C12TAB,SDS,and TritonX-100 (TX-100),on the wettability of carbonate rock were investigated through contact angle measurements.Oil recovery was studied using spontaneous imbibition tests at 25,70,and 90 ℃,followed by thermal gravity analysis to measure the amount of adsorbed material on the carbonate surface.The results indicated that Ca2+,Mg2+,and SO42-ions may alter the carbonate rock wettability from oil-wet to water-wet,with further water wettability obtained at higher concentrations of the ions in modified seawater.Removal of NaCl from the imbibing fluid resulted in a reduced contact angle and significantly enhanced oil recovery.Low oil recoveries were obtained with modified seawater at 25 and 70 ℃,but once the temperature was increased to 90 ℃,the oil recovery in the spontaneous imbibition experiment increased dramatically.Application of smart water with C12TAB surfactant at 0.1 wt％ changed the contact angle from 161 ° to 52° and enhanced oil recovery to 72％,while the presence of the anionic surfactant SDS at 0.1 wt％ in the smart water increased oil recovery to 64.5％.The TGA analysis results indicated that the adsorbed materials on the carbonate surface were minimal for the solution containing seawater with C12TAB at 0.1 wt％ (SW + CTAB (0.1 wt％)).Based on the experimental results,a mechanism was proposed for wettability alteration of carbonate rocks using smart water with SDS and C12TAB surfactants.