Wettability alteration and oil recovery by spontaneous imbibition of low salinity brine into carbonates: Impact of Mg²⁺, SO₄ ²⁻ and cationic surfactant

A large amount of the discovered oil reserves are reserved in carbonate formations, which are mostly naturally fractured oil-wet. Wettability alteration towards more water-wet state reduces the capillary barrier, hence improving the oil recovery efficiency in such reservoirs. In this study, wettability alteration towards favorable wetting state was investigated by combining modified low salinity brine with surfactant during water flooding. The diluted brine was modified by increasing the concentration of Mg²⁺ and SO₄ ²⁻, individually as well as both ions in combination. Different brine formulations were tested experimentally through the observations of contact angle measurements and spontaneous imbibition tests for oil-wet limestone core samples both in the presence and absence of surfactant. Contact angle measurements revealed that both magnesium and sulfate ions can act as wettability modifying agents for the oil-wet calcite and all of the modified low salinity brine solutions changed the wettability of oil-wet surface towards more water-wet state. In the presence of cationic surfactant, utilizing both wettability influencing ions and surfactant molecules resulted in more water-wet surfaces. The spontaneous imbibition results showed that the combination of salting-in effect, mineral dissolution and electric double layer expansion, cause wettability alteration towards more water-wet state and result in critical increase in oil recovery efficiency during spontaneous imbibition of diluted brine into limestone core sample. Modified brine which contains a higher concentration of sulfate ions was more efficient in wettability alteration in comparison to the brine modified with magnesium ions. High oil recovery efficiency can be achieved by optimizing the brine composition without using costly surfactant solutions. However, the combination of the effect of wettability influencing ions (Mg²⁺ and SO₄ ^{2 −}) along with the cationic surfactant can result in a remarkable oil production during the spontaneous imbibition process.