Frequency and water saturation dependency of dielectric properties of clay mineral

Abstract

An investigation of the frequency dependent dielectric polarizability of water molecules bound to clay mineral surfaces using molecular dynamic simulation is essential to improve understanding of the bulk electrical transport properties of shales in petrophysics and other clay science. The aim of the present study was to quantify the frequency-dependence of the complex permittivity of K+-smectite partially and fully saturated with water and to determine the relaxation frequency of the K+-smectite for pore size of ~1 nm and smectite matrix basal space of 1.66 nm with water saturations that varied from 20 to 100 wt% using molecular dynamics simulations. The simulation results of the dielectric spectra showed an unchanged polarization mechanism occurring within the K+-smectite with nano pore size, characterized by a water saturation dependent static permittivity. The characteristic frequency of the dielectric relaxation was found to be ~30 GHz for the system studied. The permittivity relaxation was found to be non-Debye type relaxation, with a circular arc locus providing a good representation of the simulated data. The deviation from pure Debye behavior reduced and the dielectric permittivity became much more significant as water saturation increases. The results presented here identified the dielectric relaxation as resulting from the orientational correlation of the water molecules near the K+-smectite surface and gave support to the idea that the relaxation frequency reflects the dimension of pores. This work constituted a novel contribution to the dielectric spectra of overall wet K+-smectite by extending the dielectric behavior of the clay mineral-water system to a microscopic level description.