It has been proposed that increased oil recovery in carbonates by modification of ionic composition or altering salinity occurs mainly at a temperature exceeding 70-80 °C. The argument was that elevated temperatures enhance adsorption of the potential determining ions which then modifies wettability to a less-oil-wetting state. According to this rationale, it becomes questionable if diluted brines or brines without these ions can be still applicable. Therefore, the aim of this paper is to investigate if the wettability alteration truly depends on temperature and if so how the trend with temperature can be explained. We followed a combined experimental and theoretical modeling approach. The effect of brine composition and temperature on carbonate wettability was probed by monitoring contact angle change of sessile oil droplets upon switching from high salinity to lower salinity brines. IFT measurements as a function of salinity and temperature along with extensive Ζ-potential measurements as a function of salinity, pH, temperature, and rock type were conducted. Interaction potentials between oil and carbonate surfaces were estimated based on DLVO theory, and its consistency with oil-droplet data was checked to draw conclusions on plausible mechanisms. Three carbonate rocks (two limestones and one dolomite) were used along with two reservoir crude oils, high salinity formation water (FW), seawater (SW), and 25 times diluted seawater (25dSW) as low salinity (LS) brine. It was observed that (i) wettability alteration to a less-oil-wetting state can occur at ambient temperature for specific rock types and brines, and (ii) there is no univocal increase in response to SW and LS brine at elevated temperature. The largest improvement in wettability was observed for dolomite, while, among the limestones, only one rock type showed noticeable wettability improvement at elevated temperature with SW. The difference in behavior between limestones and dolomite indicates that the response to brine composition change depends on rock type and mineralogy of the sample. These observations are consistent with the Ζ-potential trends with salinity at a given temperature. Dolomite generally shows more positive Ζ-potential than limestones. However, even the two limestones react differently to lowering salinity and exhibit different magnitude of Ζ-potential. Moreover, it is observed that, at a specific salinity, an increase in temperature leads to reduction of Ζ-potential magnitude on both rock/brine and oil/brine interfaces toward zero potential. This can affect positively or negatively the degree of wettability alteration (to a less-oil-wetting state) at elevated temperature depending on the sign of oil/brine and rock/brine Ζ-potential in SW/LS. The observed trends are reflected in the DLVO calculations which show consistency with contact angle trends with temperature and salinity. According to the DLVO calculation, the lack of response to SW/LS in some of the systems above can be explained by stronger electrostatic attractive forces under SW/LS than HS. This study concludes that a combined surface-charge change and double-layer expansion is a plausible mechanism for the wettability alteration in carbonate rocks.

Several studies conducted mainly on the laboratory scale indicate that in carbonate rocks oil displacement can be influenced by the ionic composition of the brine, providing an opportunity to improve recovery by optimizing the brine mixture used in secondary or tertiary recovery. In industry this topic has been termed "low salinity flooding (LSF) in carbonates" while the underlying mechanisms are not very well understood. The increased oil recovery has been attributed to wettability alteration to a more water-wet state. However, in some studies a positive low salinity effect (LSE) has been ascribed to dissolution of rock, which occurs on the laboratory scale but due to equilibration of brine with carbonate minerals on larger length scales this is not relevant for the reservoir scale. Therefore, the objective of this paper is to gain a better understanding of the underlying mechanism(s) and investigate whether calcite dissolution is the primary mechanism of the LSE. We used a model system where the contact angle of crude oil deposited on planar surfaces coated with crushed carbonate rock particles was monitored as a function of brine composition. The approach is similar to the one published in Mahani et al. (2014) for sandstone rock, but instead of clay minerals we used carbonate materials from natural limestone and Silurian dolomite rocks. Furthermore, the effective surface charge at the oil-water and water-rock interfaces was quantified via zeta-potential measurements at several salinity and pH levels in order to establish a link between changes in the intermolecular interactions at the solid-liquid interface and the contact angle at the brine-oil-rock contact line, which is an indicator for wettability change. The impact of mineral dissolution was addressed by comparing the response to brines that were fully equilibrated (and hence dissolution suppressed) and the response to those completely under-saturated with calcium carbonate (leading to dissolution). The investigation was accompanied by geochemical modeling using PHREEQC. It was observed that by switching from formation water (FW) to seawater (SW), diluted seawater (dSW) and diluted seawater equilibrated with calcite (dSWEQ), the limestone surface became less oil-wet reflected in contact angle decrease. The recession of the 3-phase contact line observed for both SW and dSWEQ, which are not impacted by dissolution, suggests that the LSE occurs even in the absence of mineral dissolution. The trends observed for the zeta-potential data on brine composition clearly support the surface-charge-change mechanism for limestone, where at lower salinities the charges at the limestone-brine interface become more negative, causing lower adhesion or even repulsion between oil and rock. Dolomite rock shows a different behavior. First, there is a much smaller response in terms of contact angle change. Also, the zeta-potential of dolomite shows generally more positive charges at higher salinities and less decrease at lower salinities, where in comparison to limestone the electrostatic interaction remains attractive or becomes only weakly repulsive. In summary we conclude that a positive LSE in carbonate rock exists without any dissolution and it is driven by the brine composition dependency of electrostatic interactions between crude oil and rock. However, the magnitude of the LSE is impacted by the mineralogy of carbonate material.