~ t Pergamon Solid State Communications, Vol. 90, No. 3, pp. 201-204, 1994 Elsevier Science Lid Printed in Great Britain. All rights reserved 0038-1098/94 $6.00 + .00 0038-1098(94)E0048-G ON THE LOW FREQUENCY PERMITTIVITY OF LIQUID-FILLED POROUS MEDIA B. Nettelblad and G.A. Niklasson Physics Department, Chalmers University of Technology, S-412 96 G6teborg, Sweden (Received 22 October 1993 by B. Lundqvist) We have performed dielectric measurements on natural and artificial rock samples in the range 5 Hz-13 MHz. For the lowest frequencies, we get very high permittivities, which do not depend significantly on sample thickness. We attribute these permittivities to diffusion effects at the solid-liquid interface. Calculations were made using the so called Grain Consolidation Model, which was generalized to include interface effects. The model gave reasonable agreement with experiments. SEVERAL measurements of the dielectric properties of porous, liquid-filled solids (such as rocks or ceramics) have yielded extremely high permittivities at low frequencies [1-3]. There may be two causes of such behaviour: electrode polarization effects or "bulk" polarization of the internal interfaces in the medium. High permittivities have also been measured on dispersions of solid particles in an electrolyte [4, 5]. In this case an electrochemical double-layer is present at the interfaces. The double-layer can be described as consisting of a thin layer of counterions at a fixed distance close to the charged surface of the solid and a diffuse layer of ions further out (the so called Gouy- Chapman-Stern model [6]). It has been shown that the high permittivities in these dispersions emanate from the electrochemical double-layers at the inter- faces, and several theories exist [7-10] that explain this behaviour. However, most theories assume that the dispersions are dilute, neglecting interactions between the charged particles, or treating the surrounding of one particle as an effective medium. Hence, they are not directly applicable to a porous medium. Yet, surface charges are predicted to be present on the main constituents of porous media, namely inorganic particles like silicon dioxide [11], when immersed in an electrolyte. In natural rocks there are also clay particles at the pore surfaces which are normally charged [12]. In this paper we investigate whether the high low- frequency permittivities in porous media can be understood in terms of the properties of the electrochemical double-layer at the interfaces. We present permittivity measurements on a natural sandstone as well as on artificial "rocks" composed of sand or glass beads that were glued together with epoxy. The results are analysed in terms of the so called "Grain Consolidation Model" (GCM) [13]. This simple structural model treats the solid phase as spherical grains, growing homogeneously until they touch each other. Once they have started to contact, they continue, to grow only where they do not intersect [see Fig. l(a)], until the appropriate porosity is attained. The original configuration of the spherical particles can either be stochastic, or deterministic, with the particles on a lattice. The latter model considerably simplifies the calculations. Shen et al. [14] gave a useful method for calculating the permittivity or the conductivity of such a lattice, using a Fourier expansion technique. The GCM has been generalized to incorporate a coating on the grain surfaces and to calculate the complex permittivity (which makes it possible to study the frequency dependence) [15]. We propose a model of the interfacial layer that simulates the properties of the electrochemical double-layer. Permittivity measurements were performed on natural sandstones from Lemunda in the Visings6 formation (a very clean sandstone, with low clay content and SiO2 as main constituent, that has been described in [16]), as well as on artificial "rock", composed either of Danish beach sand, or of glass beads (i.e., SiO2). The grains of the artificial rocks were in both cases glued with small amounts of epoxy, and subjected to pressures in the range 0.2- 10MPa before the epoxy was cured. Sample 201