IONIC SURFACE ELECTRICAL-CONDUCTIVITY IN SANDSTONE

Recent analyses of complex conductivity measurements have indicated th at 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, t he mechanisms of surface conduction are not well understood, and no mo del 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 sands tone saturated with a range of different types and concentrations of a queous solution with a frequency range of 20 Hz to 1 MHz. The frequenc y dependence of complex conductivity was analyzed using the empirical model of Cole and Cole (1941). The ''fractal'' surface models of Le Me haute and Crepy (1983), Po Zen Wong (1987), and Ruffet et al. (1991) w ere used to calculate apparent fractal pore surface dimensions for sam ples saturated with different solution types and concentrations. These showed a pronounced decrease of apparent fractal surface dimension wi th decreasing electrolyte concentration and a decrease of apparent fra ctal dimension with increasing relative ionic radius of the dominant c ation in solution. A model for ionic surface concentration (ISCOM I) h as been developed as the first step in producing a rigorous physicoche mical model of surface conduction in quartz-dominated rocks. The resul ts 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 resu lt of their adsorption onto the mineral surfaces as well as by changes in the ionic mobility in the surface conduction double-layer as the w etting solution becomes more dilute.