Dielectric spectroscopy measured on dry, oil-, freshwater- and brine-wetted rocks using an insulating layer method and its relations to petrophysical parameters

Dielectric measurements, in the frequency range from 100 Hz to 15 MHz, were carried out using an insulating film method, on shaly and clean sandstone samples, dry, oil-impregnated, partially or fully saturated with fresh water or solutions of NaCl, at different concentrations. The results show that two kinds of polarization relating to interface dominate: the inner surface polarization (ISP) at the pore surface in rocks and the outer surface polarization (OSP) on the outer surface of the rock sample. Both ISP and OSP are of the Debye type and can be fitted very well using the Cole–Cole equation. The dielectric spectra of ISP reflect the length scale of pore size or throat size, whereas the relaxation strength depends heavily on the inner pore surface area. Clay contributes to dispersion in rocks, mainly through increasing the inner surface. When the effect of water content is minimized, the dielectric properties measured on brine-wetted rock samples show great salinity dependence. The increase in salinity shifts the relaxation spectra to the high frequency end; it also enhances the relaxation strength. In the frequency range studied, no dispersion in dry and oil-saturated samples has been observed, which underscores the key role played by water in dielectric polarization in rocks.