TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractOil recovery by water flooding in fractured formations is often dependent on spontaneous imbibition. However, spontaneous imbibition is usually insignificant in oil-wet, carbonate rocks. Sodium carbonate and anionic surfactant solutions are evaluated for enhancing oil recovery by spontaneous imbibition from oil-wet carbonate rocks. Crude oil samples must be free of surface-active contaminants to be representative of the reservoir. Calcite, which is normally positively charged, can be made negative with sodium carbonate. The ease of wettability alteration is a function of the aging time and temperature and the surfactant formulation.