Despite the broad range of interest and applications, controls on calcite surface charge in aqueous solution, especially at conditions relevant to natural systems, remain poorly understood. The primary data source to understand calcite surface charge comprises measurements of zeta potential. Here we collate and review previous measurements of zeta potential on natural and artificial calcite and carbonate as a resource for future studies, compare and contrast the results of these studies to determine key controls on zeta potential and where uncertainties remain, and report new measurements of zeta potential relevant to natural subsurface systems. The results show that the potential determining ions (PDIs) for the carbonate mineral surface are the lattice ions Ca, Mg and CO. The zeta potential is controlled by the concentration-dependent adsorption of these ions within the Stern layer, primarily at the Outer Helmholtz Plane (OHP). Given this, the Iso-Electric Point (IEP) at which the zeta potential is zero should be expressed as pCa (or pMg). It should not be reported as pH, similar to most metal oxides. The pH does not directly control the zeta potential. Varying the pH whilst holding pCa constant yields constant zeta potential. The pH affects the zeta potential only by moderating the equilibrium pCa for a given CO partial pressure (pCO). Experimental studies that appear to yield a systematic relationship between pH and zeta potential are most likely observing the relationship between pCa and zeta potential, with pCa responding to the change in pH. New data presented here show a consistent linear relationship between equilibrium pH and equilibrium pCa or pMg irrespective of sample used or solution ionic strength. The surface charge of calcite is weakly dependent on pH, through protonation and deprotonation reactions that occur within a hydrolysis layer immediately adjacent to the mineral surface. The Point of Zero Charge (PZC) at which the surface charge is zero could be expressed as pH, but surface complexation models suggest the surface is negatively charged over the pH range 5.5-11. Several studies have suggested that SO is also a PDI for the calcite surface, but new data presented here indicate that the value of pSO may affect zeta potential only by moderating the equilibrium pCa. Natural carbonate typically yields a more negative zeta potential than synthetic calcite, most likely due to the presence of impurities including clays, organic matter, apatite, anhydrite or quartz, that yield a more negative zeta potential than pure calcite. New data presented here show that apparently identical natural carbonates display differing zeta potential behaviour, most likely due to the presence of small volumes of these impurities. It is important to ensure equilibrium, defined in terms of the concentration of PDIs, has been reached prior to taking measurements. Inconsistent values of zeta potential obtained in some studies may reflect a lack of equilibration. The data collated and reported here have broad application in ...

The zeta potential is a measure of the electrical charge on mineral surfaces and is an important control on subsurface geophysical monitoring, adsorption of polar species in aquifers, and rock wettability. We report the first measurements of zeta potential in intact, water‐saturated, natural carbonate samples at temperatures up to 120°C. The zeta potential is negative and decreases in magnitude with increasing temperature at low ionic strength (0.01 M NaCl, comparable to potable water) but is independent of temperature at high ionic strength (0.5 M NaCl, comparable to seawater). The equilibrium calcium concentration resulting from carbonate dissolution also increases with increasing temperature at low ionic strength but is independent of temperature at high ionic strength. The temperature dependence of the zeta potential is correlated with the temperature dependence of the equilibrium calcium concentration and shows a Nernstian linear relationship. Our findings are applicable to many subsurface carbonate rocks at elevated temperature.

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Modifying the composition of injection brine can cause wettability alteration and lead to enhanced oil recovery (EOR) from carbonate reservoirs. The two main approaches are: (1) dilution of injected brine and (2) selective modification of the concentration of potential determining ions (PDIs) such as Ca2+, Mg2+ and SO42-. Each of these approaches will modify the surface charge of carbonate mineral expressed in terms of the zeta potential. Here we reports measurements of the zeta potential of intact carbonate samples obtained using the streaming potential method. This method allows us to directly link zeta potential and oil recovery in the same coreflooding experiments. We begin by using single-phase (brine only) experiments to investigate the effect on zeta potential of dilution and PDI modification using seawater as a reference injection brine composition. We then use multi-phase experiments (brine displacing oil) and carbonate samples at different initial wetting states to determine the correlation between injection brine composition, zeta potential, EOR and initial wetting state, using the same brine compositions as in the single-phase experiments. We find that typical formation brine yields a positive zeta potential, whilst seawater yields a negative zeta potential. Consequently, injection of seawater leads to a change in the polarity of the zeta potential and therefore affects fluid-rock interactions. This in turn affects oil recovery. Dilution of injected seawater and an increase in SO42- concentration both yield a more negative zeta potential compared to the seawater reference case. However, the change in zeta potential observed when adding SO42- is less than for bulk dilution. The magnitude of the zeta potential increases in the presence of an oil phase after aging and similar increase was observed on diluting the injection brine. In the experiments reported here, incremental oil production was not observed. We suggest that the increase in the zeta potential, and consequent increase in the electrostatic repulsion between the similarly charged mineral-brine and oil-brine interfaces, explains why most of the oil is produced during first brine injection. Other samples, which were not aged, yield no change in zeta potential compared to water-wet cases within experimental error.

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The impact of brine composition on rock wettability and oil recovery in carbonates has been an area of research in recent years. Many studies have reported contradictory results concerning the impact of water injection salinity and composition on oil recovery. The zeta potential, which is a measure of the electrical charge at the mineral surface, is highly variable in carbonates, depending on the ionic composition of the pore water. The zeta potential controls the magnitude and polarity of the electrostatic interactions between the mineral surface and polar species in the brine and oil; it also controls the magnitude and polarity of the streaming potential, an electrical potential which arises in response to pressure gradients across saturated rocks. Here we report the use of streaming potential measurements to characterize wettability and optimize injection brine composition during controlled salinity waterflooding (CSW) in carbonates. Crude oils, natural carbonate core samples and synthetic brines (equivalent to formation, seawater and modified seawater compositions) are used to evaluate wettability and CSW effect. We use the streaming potential measurements to determine the zeta potential, and correlate changes in zeta potential with changes in wettability and improved oil recovery. To predict the optimum brine composition for CSW requires knowledge of the zeta potential and how this responds to changes in brine composition. Such knowledge can be obtained using the streaming potential method reported here, which is much cheaper and quicker than conducting numerous multiphase coreflooding experiments and varying the brine composition on an ad-hoc basis.

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