Dynamic behaviors and mechanisms of fluid-fluid interaction in low salinity waterflooding of carbonate reservoirs

Low salinity waterflooding of carbonate reservoirs has attracted wide attention and many mechanisms have been proposed. However, the dynamic behaviors and mechanisms of the fluid-fluid interaction in low salinity waterflooding have not yet been thoroughly classified. Regarding this, core flooding, contact angle & zeta potential, interfacial tension & interfacial dilatational rheology, total organic carbon, and cryogenic-scanning electron microscopy (cryo-SEM) experiments were combined in this study to investigate the role of the fluid-fluid interaction in low salinity waterflooding and reveal its in-depth mechanism. Core flooding & contact angle & zeta potential experiments results showed that the rock-fluid and fluid-fluid interactions act synergistically to enhance oil recovery. Thereof, as ionic strength decreases, the contribution of the fluid-fluid interaction for additional recovery first increases to the maximum 67.04 % in 20-times diluted formation waterflooding and then decreases to 29.99 % in 40-times diluted formation waterflooding. Moreover, Mg2+ and SO42− as the potential determining ions both showed positive effects on the fluid-fluid interaction in low salinity waterflooding. As Mg2+ concentration increases, the contribution of the fluid-fluid interaction for additional recovery increases to 82.63 % and then decreases. As SO42− concentration increases, the contribution of the fluid-fluid interaction continuously increases and reaches 84.56 %. Interfacial tension and interfacial dilatational rheology results illustrated that the interfacial viscoelasticity enhancement is the important mechanism of the fluid-fluid interaction in low salinity waterflooding which could suppress the snap-off and stabilize the crude oil. Moreover, the total organic carbon and cryo-SEM results showed the oil-in-water emulsions with strong interfacial viscoelasticity are stably dispersed in low saline water, which could block the pores and throats and increase the sweep efficiency. In conclusion, the enhancement of interfacial viscoelasticity and the formation and existence of oil-in-water emulsion are the two important mechanisms of the fluid-fluid interaction in low salinity waterflooding. Herein, these two mechanisms both first enhance and then weaken as ionic strength decreases; enhance and then weaken as Mg2+ concentration increases, continuously enhance as SO42− concentration increases.