Laboratory Measurements of Zeta Potential in Fractured Lewisian Gneiss: Implications for the Characterization of Flow in Fractured Crystalline Bedrock

Abstract

Despite the broad range of interest and possible applications, the controls on the electric surface charge and the zeta potential of gneiss at conditions relevant to naturally fractured systems remain unreported. There are no published zeta potential measurements conducted in such systems at equilibrium, hence, the effects of composition, concentration and pressure remain unknown. This study reports zeta potential values for the first time measured in a fractured Lewisian gneiss sample saturated with NaCl solutions of various concentrations, artificial seawater and artificial groundwater solutions under equilibrium conditions at confining pressures of 4 MPa and 7 MPa. The constituent minerals of the sample were identified using X-ray diffraction and linked to the concentration and composition dependence of the zeta potential. The results reported in this study demonstrate that the zeta potential remained negative for all tested solutions and concentrations. However, the values of the zeta potential of our Lewisian gneiss sample were found to be unique and dissimilar to pure minerals such as quartz, calcite, mica or feldspar. Moreover, the measured zeta potentials were smaller in magnitude in the experiments with artificial complex solutions compared with those measured with NaCl, thus suggesting that divalent ions (Ca2+, Mg2+ and SO42−) acted as potential determining ions. The zeta potential was also found to be independent of salinity in the NaCl experiments, which is unusual for most reported data. We also investigated the impact of fracture aperture on the electrokinetic response and found that surface electrical conductivity remained negligibly small across the range of the tested confining pressures. Our novel results are an essential first step for interpreting field self-potential (SP) signals and facilitate a way forward for characterization of water flow through fractured basement aquifers.

A unified model for the permeability, electrical conductivity and streaming potential coupling coefficient in variably saturated fractured media

“…This finding is in good agreement with the result of Vinogradov, Hidayat, Kumar et al. (2022), where ζ was reported to be −10.52 and −10.69 mV, respectively.…”

“…The obtained values of the ζ are -11.8 mV and -12.0 mV for the confining pressures of 4 MPa and 7 MPa, respectively. This finding is in good agreement with the result of Vinogradov et al (2022a) where ζ was reported to be -10.52 mV and -10.69 mV, respectively.…”

“…To the best of our knowledge, there have been only few publications on SPCC measurements of fractured media under saturated conditions (e.g., Moore & Glaser, 2007; Vinogradov, Hidayat, Kumar et al., 2022) and no publications under unsaturated conditions. For example, Fagerlund and Heinson (2003) measured the zeta potential of fractured rocks by crushing rocks and packing obtained crushed material into a tube.…”

“…Vinogradov, Hidayat, Kumar et al. (2022) measured the SPCC for a fractured Lewisian gneiss sample which was assumed to have a single fracture at different values of confining pressure and ionic strengths. For example, the measured values for the SPCC at two different values of confining pressures (4 and 7 MPa) for the ionic strength of NaCl of 0.7 M, which is denoted by $$C_f$$, were reported to be −1.21 and −1.23 mV/MPa, respectively.…”

A Fractal Model for Effective Excess Charge Density in Variably Saturated Fractured Rocks

“…In this section the saturated effective excess charge density model $${\hat{Q}}_{v}^{REV,sat}$$ is confronted to experimental data. Note that only few studies reporting electrokinetic measurements on fractured material have been published in the literature (e.g., Moore & Glaser, 2007; Vinogradov et al., 2022). Therefore, we compare the prediction of our model with the coupling coefficient C c data obtained by Moore and Glaser (2007) for microcracked Sierra granite samples.…”