Strange Sample-Thickness Dependence of Multi-frequency Complex Dielectric Permittivity of Shales and Sandstones
Multifrequency complex dielectric permittivity measurements are widely used for material characterization. We used Keysight impedance analyzer E4990A and Keysight dielectric fixture 16451B to perform two-electrode complex dielectric permittivity measurements on 24%-porosity Berea sandstone and Shale samples from various shale plays in the frequency range of 100 Hz to 30MHz at ambient temperature and pressure conditions. The samples were studied in their dry state and also when fully saturated with deionized (DI) water and brine. All samples exhibit large frequency dispersion of complex permittivity and complex conductivity, which is attributed to the Maxwell-Wagner polarization mechanism. The dielectric constant of dry, DI-water-filled, and brine-filled samples vary smoothly from 3.5 to 15, 3.5 to 2000, and 20 to 105, respectively, for variation in frequency from 30 MHz to 1 kHz. The DI-water-filled samples and brine-filled samples exhibit peak dielectric loss factor within 1 kHz to 10 kHz and 10 kHz to 100 kHz, respectively. Conductivity estimates for the samples obtained between 100 Hz and 1 kHz are 50% lower than those obtained between 10 kHz and 1 MHz, which are 100% lower than those obtained above 10 MHz. The high-frequency permittivity measurements were compared against CRIM predictions based on AP-608 confined porosity measurements and fluid saturation estimates. In comparison to non-contact method, the contact method generates higher quality measurements. No significant change in permittivity estimates were observed upon cleaning the Berea and Shale samples with a mixture of toluene and methanol. The permittivity measurements decreased when the two transverse sample surfaces in contact with electrodes were polished to reduce the surface roughness. Permittivity measurements exhibit substantial low-frequency alteration below 20 kHz when the saturated samples were wrapped with parafilm to prevent water loss from the samples. Complex permittivity is a geometry independent property nonetheless, the measured complex permittivities decrease with the decrease in sample thickness, such that the complex permittivity variation with thickness increases at lower frequencies. For DI-water-filled and brine-filled samples, multifrequency complex conductivity obtained from the complex permittivity measurements exhibit large deviation from those obtained from a resistivity cell.
AAPG Datapages/Search and Discovery Article #90258 © 2016 AAPG Eastern Section Meeting, Lexington, Kentucky, September 25-27, 2016