A laboratory investigation of the thermoelectric effect

One contribution to self-potential signals is the thermoelectric coupling associated with a temperature gradient in a porous material, which causes mobile ions to diffuse from hot to cold regions. We have developed 35 laboratory experiments to measure the value of the Seebeck coefficient in two clean (silica) sands and two clayey sandstones fully saturated by NaCl solutions to explore the influence of salinity upon the Seebeck coefficient over four orders of magnitude in salinity. The two sands are characterized by two distinct grain sizes (coarse and medium) and therefore two low values of surface conductivity. Portland sandstone, rich in illite and kaolinite, is characterized by a high surface conductivity, and the Berea sandstone is characterized by an intermediate surface conductivity. The Seebeck coefficient of the sands ranges from -0.4 (at intermediate salinities) to +0.5 mV°C^-1 at very low salinities, for which surface conductivity, associated with electromigration and electrodiffusion in the electrical double layer, dominates. For the Portland sandstone, the Seebeck coefficient is positive in the range +0.8 to +1.5 mV°C^-1 and decreases with the increase of the salinity. A similar trend is observed for the Berea sandstone with the Seebeck coefficient going from positive to negative values at high salinity. Our experimental data can be fairly reproduced by a simple model accounting for the effect of surface conductivity due to the electrical double layer coating the surface of the grains and the dependence on salinity of the partial entropies of the ions in the pore water.