Relation between electric properties and water saturation for hematitic sandstone with frequency

M M Gomaa M M Gomaa

doi:10.4401/ag-3015

Abstract

This paper focuses on the effect of water saturation on A. C. electrical conductivity and dielectric constant of fully and partially saturated hematitic sandstone sample (Aswan area, Egypt). The saturation of the sample was changed from partial to full saturation. Complex resistivity measurements at room temperature (~16°C), were performed in the frequency range from 0.1 Hz to 100 KHz. Experimental electrical spectra indicate, generally, that the electrical conductivity and dielectric constant vary strongly with water saturations and frequency. The low frequency electrical conductivity and dielectric constant are mainly controlled by surface conduction and polarization of the electrical double layer. The behaviour of the electrical conductivity and dielectric constant, with increasing water content, were argued to the orientational polarization of bound water for very low saturations, displacement of the excess surface charges for relatively low saturations, and free exchange of excess ions in double layer with the bulk electrolyte and generation of transient diffusion potentials which lag behind the applied field for high saturations.

Highlights

Electrical spectra of porous rocks in the low frequency range reflect numerous polarization processes resulting mainly from rock heterogeneity
The high frequency region has low values of the dielectric constant and nearly has no dispersion in dielectric constant for low saturations, while there is a slight dispersion for the dielectric constant with a very minute slope (≈ - 0.1) for high saturations
Effects of water saturation on the electrical conductivity and dielectric constant of humid, partially, and fully saturated hematitic sandstone sample is investigated in the frequency range from 0.1 Hz to 100 KHz

Summary

Introduction

Electrical spectra of porous rocks in the low frequency range reflect numerous polarization processes resulting mainly from rock heterogeneity. The first is the Maxwell-Wagner polarization (Mendelson and Cohen, 1982) due to differences in the electric bulk properties of rock components. Maxwell-Wagner theory allows the prediction of electrical spectra of mixtures from bulk partial volumes, the properties of their components and their microstructure. The problem is that the mixture theory does not take into account surface conductivity and polarization, as well as clustering of components in disordered mixtures. Surface conductivity has two contributions; one is associated with the Stern layer and one with the diffuse layer in which counter-ion density obeys a Boltzmann distribution. None of these contributions can be neglected but the Stern layer contribution dominates

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Conclusion