Effect of Al3+ on low-salinity water flooding in carbonate reservoirs: a comparative study with Ca2+, Mg2+, and SO42−

In low salinity water flooding (LSWF), oil–brine and rock–brine interactions are strongly influenced by potential determining ions (PDIs), which have been predominantly studied for divalent ions such as Mg 2+ , Ca 2+ , and SO 4 2− . However, the performance of trivalent ions remains poorly understood. This study investigates whether Al 3+ is more effective than conventional PDIs under the tested conditions and elucidates the underlying enhanced oil recovery (EOR) mechanisms using carbonate rock samples. Interfacial tension (IFT), contact angle, zeta potential, and calcite-coated micromodel flooding experiments were conducted using individual ions in distilled water and low concentrations in low salinity water (LSW). Al³⁺ exhibited the strongest IFT reduction (Ca 2+ < Mg 2+ < SO 4 2− < Al 3+ ) and altered wettability to strongly water-wet conditions, achieving a minimum contact angle of 18.4°, whereas divalent ions resulted only in neutral wettability. At 100 ppm in LSW, Al 3+ reversed both oil–brine and rock–brine zeta potentials to positive values, while Mg 2+ , Ca 2+ , and SO 4 2− remained ineffective. Wettability alteration was primarily driven by calcite dissolution and the high charge density and small ionic radius of Al 3+ . Micromodel flooding confirmed that Al 3+ increased oil recovery from 50.6% to 57.7%. These findings suggest that Al 3+ , at the tested concentration of 100 ppm in LSW, is a promising low-concentration additive for improving LSWF performance in carbonate reservoirs. Future studies can use this research to improve ion-tuned water flooding and develop new models for IFT and wettability alteration, ultimately enhancing oil recovery.