Physicochemical interactions during surfactant flooding of fractured carbonate reservoirs

Recovery from fractured, oil-wet/mixed-wet, carbonate reservoirs by waterflooding is poor. Dilute surfactant methods are being developed to improve oil recovery from fractured carbonates. This paper investigates the interactions of dilute alkaline anionic surfactant solutions with crude oil on carbonate mineral surfaces. Wettability, phase behavior, interfacial tension, and adsorption experiments have been conducted. Anionic surfactants have been identified that can change the wettability of the calcite surface to intermediate/water-wet condition as well or better than the cationic surfactant dodecyl trimethyl ammonium bromide (DTAB) with a west Texas crude oil. All the carbonate surfaces (lithographic limestone, marble, dolomite, and calcite) show similar behavior with respect to wettability alteration with an anionic surfactant. Anionic surfactants, which lower the interfacial tension with the crude oil to very low values (< 10(-2) mN/m), have also been identified. The adsorption of the sulphonate surfactants can be suppressed significantly by the addition of Na2CO3, because the addition of carbonate can chance the zeta potential of calcite to a negative value. Greater than 50% OOIP can be recovered from oil-wet carbonate cores by spontaneous imbibition of 0.05 wt% anionic surfactant solutions in the laboratory scale.