Carbonate rocks tend to be oil-wet, leading to lower oil relative permeability and lower oil recovery in the life of a waterflood, which is about 3 pore volume (PV) injection. The goal of this work is to improve oil recovery during secondary waterflood by wettability alteration because of a surfactant addition. The effect of the surfactant is studied by contact angle measurement, spontaneous imbibition, and secondary water/surfactant flood in reservoir cores. The contact angle changes from oil-wet to intermediate-wet in the presence of 0.2 wt % of a surfactant. Brine (without surfactant) does not imbibe into the core spontaneously, but surfactant brine imbibes to the extent of 20% of the oil in place. Secondary waterflooding recovers about 62% of the oil in about 3 PV injection and 80% in about 16 PV. Secondary surfactant flooding recovers about 85% in about 3 PV and about 90% in 4 PV. Injection of 1 PV of surfactant solution followed by waterflood also recovers about 83% of the oil in a total injection of 3 PV. This increase in oil recovery from 62 to 85% by wettability alteration is very significant and needs to be evaluated at the field scale.