Wetting Solution and Electrical Double Layer Contributions to Bulk Electrical Conductivity of Sand–Clay Mixtures

Abstract

This study investigated the contributions of the wetting liquid and electrical double layers (EDLs) to the bulk electrical conductivity (σ) of sand–clay mixtures. The conductivity is small at small water content due to few conductive pathways and low ionic mobility. The fully expanded state of the EDLs provided the largest σ in distilled water but not in salt solutions. At small water content, σ increases following a logarithmic function, but changes linearly at large water content after reaching a transitional large value. This conductivity increases with increasing electrical conductivity of the wetting solution (σw) exponentially at small water content, but linearly at large water content. Existing bulk electrical conductivity models described the measured σ values well and resulted in stable values of the electrical formation factor (F) and dimensionless parameter. Due to considerable leach cations in the clay, the cations in the bulk solution exerted negligible influence on the surface conductivity (σs). Although F is independent of σw, the confounding effect of shrink–swell in the samples with changing σw shows an apparent dependency of F on σw The surface mobility of the counterions decreased nonlinearly with increasing water‐filled porosity (ϕ), and also with increasing σw The cementation exponent (m) decreased monotonically from −4.4 at ϕ = 0.22 to −94.1 at ϕ = 0.93 in the samples wetted with distilled water. With increasing σw, m also increased. The value of m for three samples wetted with salt solutions of σw = 0.878, 1.273, and 1.572 S m−1 ranged from 2.1 to 2.5 at ϕ = 0.22 and 4.1 to 5.3 at ϕ = 0.93.

Pore-scale modeling of electrical and fluid transport in Berea sandstone

“…Here, m is grain density, is the porosity and ␤ S is the surface mobility of the counterions, which is independent of electrolyte conductivity ͑at least above 10 ‫3מ‬ mol L ‫1מ‬ ͒ and clay mineralogy ͑Shu-bin et al, 1996;Mojid and Cho, 2008͒. The CEC indicates the maximum number of exchange counterions per unit mass of the rock.…”

Estimating properties of unconsolidated sand-clay from spectral-induced polarization

“…The EDLs started dissociating from each other as the water content increased further with a consequent decrease in electrical conductivity of the samples (Mojid and Cho, 2006;. As demonstrated by Mojid and Cho (2008;their Figure 2(ab)) for sand+30% clay, a gradual increase in conductive paths with decreasing water content of low salt concentration caused an eventual increase in the real part of electrical conductivity of sand+20% clay sample ( Figure 5) that was responsible for the decrease in phase shift. Although the phase shift increased for sand and decreased for sand+20% clay with increasing porewater content, for the two other sand-clay mixtures (sand+5% clay and sand+10% clay),  first increased and then decreased with the increase in pore-water content.…”

“…Actually, the real part of electrical conductivity was very small in the dry state of the sample, but it increased to a maximum value at some large water content, after which the real part of electrical conductivity decreased. At low salt concentration of pore water (e.g., C1), the EDLs expanded considerably (Mojid and Cho, 2008; their Equation 1) and a large fraction of the current flow occurred through the EDLs, giving rise to a large magnitude of electrical conductivity (Waxman and Smits, 1968). At very small water content, however, the EDLs were very thin and only a few clay particles remained in electrical contact with each other, resulting in small electrical conductivity of the samples (Nye, 1979).…”

Measuring and modeling water-related soil–vegetation feedbacks in a fallow plot

“…In fact, it is not uncommon to observe that intense precipitation events have the effect of reducing soil bulk electrical conductivity by displacing the in situ pore water whose solutes have had the time to reach an equilibrium with the soil components, or that had different temperature (see e.g., Cassiani et al, 2006a). The incoming precipitated water pushes down the existing pore water in a sort of piston-like effect (see also Winship et al, 2006), thus causing a decrease in electrical conductivity in the upper part of the profile (Mojid and Cho, 2008) and an increase in the lower part, totally analogous of our observations here. Indeed, this hypothesis is confirmed by the evidence from the TDR probes permanently installed in the fallow plot (Fig.…”