Assessment of the permeability and pore size of quartz-based consolidated sedimentary rocks by dielectric spectroscopy in the frequency range of 1 kHz-500 MHz

The spectral-induced polarization (SIP) response of porous materials at frequencies below 1 kHz can be used to determine the pore size and rock permeability. Although the results of certain studies are encouraging, in recent years it has been argued that there cannot be a universally applicable approach to predict permeability using the SIP response. This article proposes a broadband dielectric spectroscopy method in the frequency range from 100 Hz to 500 MHz for assessing the permeability and average pore size. An original experimental setup has been developed that makes it possible to measure the dielectric spectrum of quartz-based consolidated sedimentary samples of porous rocks in this frequency range. The rock samples with permeability from 0.17 to 206 mD and an average pore size from 6.3 to 21.1 mu m have been studied during the experiments. The dielectric spectra of four sandstone samples and two siltstone samples moistened with distilled water and a NaCl solution with a con-centration of 1.5 and 4 g/L were measured. The volume fraction of the solution in the samples varied from complete saturation to a moisture of 0.04-0.05 m3/m3. The dielectric spectra were modeled by three relaxation processes, the parameters of which were determined using the Debye and Cole-Cole formulas. The strength of two low-frequency processes with the relaxation times of about 1.2 ms and 0.15 ms are well correlated with the low-frequency conductivity of the samples; however, no stochastic dependence on the pore size and permeability was found. The parameters of the high-frequency relaxation process with the relaxation times of tens and hundreds of nanoseconds depend both on the solution ratio in the sample and its concentration, as well as on the pore size and permeability. The strength of this process is better related to the pore size and permeability when moistened with distilled water, and the relaxation time correlates best with them when using the solution with a concentration of 4 g/L. No significant differences in the process parameters in the samples of sandstone and siltstone were found.

Automated summary

This research proposes a broadband dielectric spectroscopy method for assessing the permeability and average pore size of porous rocks. The study finds that the parameters of relaxation processes are well correlated with the low-frequency conductivity of the samples, but show no stochastic dependence on pore size and permeability. The parameters of high-frequency relaxation processes, however, depend on the solution ratio, concentration, pore size, and permeability.