Ionic surface electrical conductivity in sandstone
- 10 November 1994
- journal article
- Published by American Geophysical Union (AGU) in Journal of Geophysical Research
- Vol. 99 (B11) , 21635-21650
- https://doi.org/10.1029/94jb01474
Abstract
Recent analyses of complex conductivity measurements have indicated that high‐frequency dispersions encountered in rocks saturated with low‐salinity fluids are due to ionic surface conduction and that the form of these dispersions may be dependent upon the nature of the pore and crack surfaces within the rock (Ruffet et al., 1991). Unfortunately, the mechanisms of surface conduction are not well understood, and no model based on rigorous physical principles exists. This paper is split into two parts: an experimental section followed by the development of a theoretical description of adsorption of ions onto mineral surfaces. We have made complex conductivity measurements upon samples of sandstone saturated with a range of different types and concentrations of aqueous solution with a frequency range of 20 Hz to 1 MHz. The frequency dependence of complex conductivity was analyzed using the empirical model of Cole and Cole (1941). The “fractal” surface models of Le Méhauté and Crépy (1983), Po Zen Wong (1987), and Ruffet et al. (1991) were used to calculate apparent fractal pore surface dimensions for samples saturated with different solution types and concentrations. These showed a pronounced decrease of apparent fractal surface dimension with decreasing electrolyte concentration and a decrease of apparent fractal dimension with increasing relative ionic radius of the dominant cation in solution. A model for ionic surface concentration (ISCOM I) has been developed as the first step in producing a rigorous physicochemical model of surface conduction in quartz‐dominated rocks. The results from ISCOM I show that quartz surfaces are overwhelmingly dominated by adsorbed Na+ when saturated with NaCl solutions of salinities and pH found in actual geological situations. ISCOM I also shows that the concentration threshold for dominance of surface conduction over bulk conduction is aided by depletion of ions from the bulk fluid as a result of their adsorption onto the mineral surfaces as well as by changes in the ionic mobility in the surface conduction double‐layer as the wetting solution becomes more dilute.Keywords
This publication has 13 references indexed in Scilit:
- Complex conductivity measurements and fractal nature of porosityGeophysics, 1991
- Sandstone fractal and Euclidean pore volume distributionsJournal of Geophysical Research, 1988
- The dielectric constant of sandstones, 60 kHz to 4 MHzGeophysics, 1987
- Complex resistivity measurements of confined rockJournal of Geophysical Research, 1985
- Fractal Sandstone Pores: Implications for Conductivity and Pore FormationPhysical Review Letters, 1985
- Surface and interfacial free energies of quartzJournal of Geophysical Research, 1984
- Anomalous low-frequency dispersion. Near direct current conductivity in disordered low-dimensional materialsJournal of the Chemical Society, Faraday Transactions 2: Molecular and Chemical Physics, 1984
- Introduction to transfer and motion in fractal media: The geometry of kineticsSolid State Ionics, 1983
- Parameter variation in low-quartz between 94 and 298KJournal of Physics and Chemistry of Solids, 1980
- Study of solid electrolyte polarization by a complex admittance methodJournal of Physics and Chemistry of Solids, 1969