The mechanism of wettability alteration through water injection, crude-oil effect (a) Rock is negatively charged in SW, 10SW, SW-4S, and 10SW-4S (ζ₁) and also the charge at the oil/brine interface is negative (ζ₂). Shifting to crude-B causes more negative surface charges at the oil/brine interface () while rock/brine interface () remains unchanged, therefore more electrostatic repulsion exists when crude-B is used and wettability shifts into more water-wet states. (b) Rock is positively charged in SW-4Ca, SW-4Mg, 10SW-4Ca, and 10SW-4Mg (ζ₁), and also the charge at the oil/water interface is negative (ζ₂). Shifting to crude-B causes more negative surface charges at the oil/brine interface () whilst the charge at the rock/brine interface () remains constant, therefore more electrostatic attraction exists when crude-B is used and wettability shifts into more oil-wet states.