Effect of changing water salinity on complex conductivity spectra of sandstones

We analyzed the influence of pore fluid composition on the complex electrical conductivity of three sandstones with differing porosity and permeability. The fluid electrical conductivity (sigma(w)) of sodium and calcium chloride solutions was gradually increased from 25 mS/m to 2300 mS/m. The expected linear relation between sigma(w) and the real component of electrical conductivity (sigma') of the saturated samples was observed. The imaginary component (sigma '') exhibits a steeper increase at lower salinities that flattens at higher salinities. For a glauconitic sandstone and a high porosity Bunter sandstone, sigma '' approaches an asymptotic value at high salinities. Sodium cations result in larger values of sigma '' than calcium cations in solutions of equal concentration. Debye decomposition was used to determine normalized chargeability (m(n)) and average relaxation time (tau) from spectral data. The behavior of sigma '' is comparable to m(n) as both parameters measure the polarizability. At lower salinity, the relation between m(n) and sigma(w) approximates a power law with an exponent of similar to 0.5. The average relaxation time shows only a weak dependence on sigma(w). The normalized chargeability of sandstone samples can be described by the product of the pore space related internal surface and a quantity characterizing the polarizability of the mineral-fluid interface that depends on fluid chemistry. We introduce a new parameter, the specific polarizability, describing this dependence. We propose relations between polarizability and fluid chemistry that could be used to estimate pore space internal surface across samples of varying sigma(w). We observe a consistent maximum polarizability for quartz dominated siliceous material.