Laboratory studies have shown that wettability of carbonate rock can be altered to a less oil-wetting state by manipulation of brine composition and reduction of salinity. Our recent study (see Mahani et al. 2015b) suggests that a surface-charge change is likely to be the driving mechanism of the low salinity effect in carbonates. Various studies have already established the sensitivity of carbonate surface charge to brine salinity, pH and potential-determining ions in brines. However, it has been less investigated i) whether different types of carbonate reservoir rocks exhibit different electrokinetic properties, ii) how the rocks react to reservoir-relevant brine as well as successive brine dilution and iii) how the surface charge behavior at different salinities and pH can be explained.

This paper presents a comparative study aimed at gaining more insights into the electrokinetics of different types of carbonate rock. This is achieved by zeta-potential measurements on Iceland spar calcite and three reservoir-related rocks – middle-eastern limestone, Stevns Klint chalk and Silurian dolomite outcrop – over a wide range of salinity, brine composition and pH. With a view to arriving at a more tractable approach, a surface complexation model implemented in PHREEQC is developed to relate our understanding of the surface reactions to measured zeta-potentials.

The trends in the relationships between zeta-potentials on one hand and salinity and pH on the other were quite similar for different types of rock. For all cases, the surface-charge was found to be positive in high-salinity formation water, which should increase oil-wetting. The zeta-potential successively decreased towards negative values when the brine salinity was lowered to seawater level and diluted seawater. At all salinities, the zeta-potential showed a strong dependence on pH, with positive slope with pH which remained so even with excessive dilution. The sensitivity of the zeta-potential to pH-change was often higher at lower salinities.

The increase of zeta-potential with pH is consistent with the results of the surface complexation model, which indicate that formation of surface species, particularly >CaSO4- and to a lower extent >CO3Ca+ and >CO3Mg+, strongly influence the total surface charge. Increasing the pH turns the negatively charged moiety >CaSO4- into both negatively charged >CaCO3- and neutral >CaOHº entities. This substitution reduces the negative charge of the surface. The surface concentration of >CO3Ca+ and >CO3Mg+ moieties changes little with change of pH.

Besides these similarities, there exist notable differences even between carbonates that are mainly composed of calcite. Amongst all the samples, chalk particles exhibited the most negative surface charges, followed by limestone. In contrast to this, dolomite particles showed the most positive zeta-potential, followed by calcite crystal. Overall, chalk particles exhibited the highest surface reactivity to pH and salinity change, while dolomite particles showed the lowest.

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