Dielectric Polarization Studies in Partially Saturated Shale Cores

Dielectric measurements of reservoir rocks are used to estimate important petro-physical properties such as water-filled porosity and pore surface textures. However, complex dielectric polarization processes that occur in rocks are strongly dependent on frequency, making physically meaningful interpretation of broadband dielectric data difficult. At high frequency (> 10 MHz) dielectric permittivity primarily relates to the volume fractions of constituents (i.e., saturation, minerals), while at lower frequency (< 10 MHz), interpretation is complicated by interfacial polarization, electro-diffusion phenomena, and ohmic conduction. The ability to de-convolve these electrical processes is critical for interpreting petro-physical properties from broadband dielectric data. Here we demonstrate the application of Tikhonov regularization methods to compute dielectric relaxation time distributions from broadband (40 Hz to 110 MHz) dielectric data for ten shale core samples at varying partial saturation. Furthermore, via the Kramers-Kronig relation, the contribution from in-phase conduction currents to the imaginary component of the dielectric response was quantified. The evolution of dielectric polarization processes with increasing moisture content was analyzed directly from changes in relaxation time distributions. It was found that the dominant polarization mechanism, up to a critical partial saturation, occurred as surface polarization within the electrical double layer. Above this critical partial saturation, electro-diffusion mechanisms acting between the Stern and diffuse layers resulted in a large low frequency response. This work provides valuable insight into dielectric polarization mechanisms in shales and demonstrates such measurements are sensitive to electrical double layer properties and electro-diffusion length scales that are potentially relevant to characterizing pore-scale properties in shales.