Zeta Potential of Carbonates in Saline Brines as a Function of NaCl Salinity and Potential Determining Ions: Ca, Mg and SO4
Abstract:Measurements of zeta potential are typically obtained using crushed samples and commercial zetameters at ionic strength <1M. However, many natural brines have much higher salinity. This study reports zeta potential values interpreted from streaming potential measurements in two intact carbonate rock samples as a function of brine salinity (up to 5M) and potential determining ions (PDIs: Ca, Mg, SO4). The two samples appear to be identical pure carbonates in XRD analysis but differ in age and provenance.
For low salinity NaCl brines (<1M), in which the initial PDI concentration was established during pre-equilibration with the rock samples, the measured (negative) zeta potential decreased in magnitude with increasing NaCl concentration, consistent with published data and reflecting contraction of the electrical double layer. At higher NaCl salinity (>1M) the surface charge polarity was inverted for one rock sample, yet equilibrium Ca and Mg concentrations remained constant (c. 1.3x10-3 and 7x10-5M respectively) and similar for both samples, regardless of NaCl salinity. We suggest charge inversion was caused by differences in SO4 concentration (3x10-4 versus 1.2x10-3M) in the two samples after equilibration.
At higher and externally controlled concentrations of Ca (0.1 – 0.42M), the zeta potential was less affected by NaCl salinity and pCa was the dominant control, although the role of SO4 in distinguishing between rock samples remained valid. The iso-electric point was different for the two samples (pCa 1.7 versus 0.5) and lower than any previously published values. Our results suggest that small differences in PDI concentration may invert surface charge polarity at high NaCl concentration; moreover, trace amounts of anhydrite or other minerals that yield aqueous SO4 can significantly modify surface charge in carbonates that otherwise appear identical. These results are important when interpreting the streaming component of SP measurements in carbonates.