Broadband electrical properties of clays and shales: Comparative investigations of remolded and preserved samples

Abstract

The dielectric permittivity of mudrocks cannot be determined from the mixing ratios of the constituent minerals and brine and their individual dielectric response. The high-frequency dielectric permittivity is linked (R 2 ¼ 0.77 correlation) to water content, but the relationship is complicated by mineral type and the hydration state. The cation exchange capacity (CEC) and specific surface area (SSA) determine the establishment of a polarizable electrical double layer and give rise to the long range diffusion of ions leading to Maxwell-Wagner polarization. This ultimately determines the dielectric response below 50 MHz. Procedures for SSA analysis based on sorption of 2-ethoxyethanol and CEC analysis based on methylene blue titration, were developed for investigating swelling clays. These were combined with a procedure for preparing remolded paste samples from pulverized rock and drill cuttings to form the basis of this investigation. In a case study of 123 drill cutting samples retrieved from an undisclosed well, dielectric analysis of the remolded samples exhibits R 2 ¼ 0.93 correlation with SSA, R 2 ¼ 0.86 correlation with CEC, and R 2 ¼ 0.81 correlation with clay content. A pseudodielectric log of the remolded drill cuttings exhibited a weak correlation with downhole resistivity, gamma ray, and compressional slowness logs measured by a service company in the shaly sections of the well, but no correlation in the sandy section of the well. This attributed to better preservation of shaly cuttings due to finer microstructure and therefore less alteration during uphole transport and sample preparation.

Permeability Variations in Illite‐Bearing Sandstone: Effects of Temperature and NaCl Fluid Salinity

“…ν e ¼ 3 × 10 15 s −1 is the main electronic absorption frequency (Israelachvili, 2011). The dielectric permittivity of illite ε 1 is taken from Josh and Clennell (2015). The dielectric permittivity of quartz ε 2 is independent of temperature in the range between 20 and 175°C according to Stuart (1955).…”

The role of electrical conductivity in radar wave reflection from glacier beds

“…The relative permittivity considered in Fig. 1c spans that expected for common minerals and rocks in dry conditions at the low end to 100 % liquid water by volume at the high end (Midi et al, 2014;Josh and Clennell, 2015). For each of the considered frequencies, the range of electrical conductivities for which neither the low-loss nor the high-loss assumption is truly justified covers about 1 order of magnitude.…”

The role of electrical conductivity in radarwave reflection

“…100 Wm) for er = 5, typical for dry minerals and rocks (e.g, Josh and Clennell, 2015), but is an order of magnitude higher (s = 0.1 S m -1 and r = 10 Wm) for er = 55, which would be expected either for claypoor sediments with very high water content or saturated clay-rich sediments (Arcone et al, 2008;Josh and Clennell, 2015).…”

“…As a reminder, the angular frequency is related to the linear frequency by: w = 2pf. The relative permittivity considered in Figure 2 spans that expected for common minerals and rocks in dry conditions at the low end to 100% liquid water by volume at the high end (Midi et al, 2014;Josh and Clennell, 2015). For each of the considered frequencies, the range of electrical conductivities for which neither the low-loss, nor the high-loss, assumption is truly justified covers about one order of magnitude.…”