Insights into the Mechanism of Wettability Alteration by Low-Salinity Flooding (LSF) in Carbonates

The low-salinity effect (LSE) in carbonate rock has been less explored in comparison to sandstone rock. Laboratory experiments have shown that brine composition and (somewhat reduced) salinity can have a positive impact on oil recovery in carbonates. However, the mechanism leading to improved oil recovery in carbonate rock is not well understood. Several studies showed that a positive low-salinity flooding (LSF) effect might be associated with dissolution of rock; however, because of equilibration, dissolution may not contribute at reservoir scale, which would make LSF for carbonate rock less attractive for field applications. This raises now the question whether calcite dissolution is the primary mechanism of the LSF effect. In this paper, we aim to first demonstrate the positive response of carbonate rock to low salinity and then to gain insight into the underlying mechanism(s) specific to carbonate rock. We followed a similar methodology as in sandstone rock [Mahani, H.; Berg, S.; Ilic, D.; Bartels, W.-B.; Joekar-Niasar, V.Kinetics of low-salinity-flooding effect. SPE J. 2015, 20 (1), 8-20, DOI: 10.1021/ef5023847] using a model system comprised of carbonate surfaces obtained from crushed carbonate rocks. Wettability alteration upon exposure to low-salinity brine was examined by continuous monitoring of the contact angle. Furthermore, the effective surface charge at oil-water and water-rock interfaces was quantified via zeta-potential measurements. Mineral dissolution was addressed both experimentally and with geochemical modeling using PHREEQC. Two carbonate rocks with different mineralogy were investigated: limestone and Silurian dolomite. Four types of brines were used: high-salinity formation water (FW), seawater (SW), 25x diluted seawater (25dSW), and 25x diluted seawater equilibrated with calcite (25dSWEQ). It was observed that, by switching from FW to SW, 25dSW, and 25dSWEQ, the limestone surface became less oil-wet. The results with SW and 25dSWEQ suggest that the LSE occurs even in the absence of mineral dissolution, because no dissolution is expected in SW and none in 25dSWEQ. The wettability alteration to a less oil-wetting state by low salinity is consistent with the zeta-potential data of limestone, indicating that, at lower salinities, the charges at the limestone-brine interface become more negative, indicative of a weaker electrostatic adhesion between the oil-brine and rock-brine interfaces, thus recession of the three-phase contact line. In comparison to limestone, a smaller contact angle reduction was observed with dolomite. This is again consistent with the zeta-potential of dolomite, generally showing more positive charges at higher salinities and less decrease at lower salinities. This implies that oil detachment from the dolomite surface requires a larger reduction of adhesion forces at the contact line than limestone. Our study concludes that surface charge change is likely to be the primary mechanism, which means that there is a positive LSE in carbonates without mineral dissolution.