HR: 0800h
AN: T51B-1333    [Abstracts]
TI: Effect of the flow state on streaming current
AU: * Kuwano, O
EM: kuwano@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Nakatani, M
EM: nakatani@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Yoshida, S
EM: shingo@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AB: When fluid flows through a porous medium, charges in the electric double layer are transported, resulting in streaming current. This is the representative mechanism of self-potential widely observed in the field. In the classical Helmholtz-Smoluchowski relation, the streaming current (i_s) is represented as i_s=C_cgrad P_p, where C_c is streaming current coefficient and P_p is fluid pressure. C_c depends on hydraulic property of the rock and electrochemical property called zeta potential. In the previous study ( Kuwano et al., 2004, JEPS Joint Meeting) using crushed rock samples, we reported an apparent grain size dependence of zeta potential, which was unexpected from the nature of zeta potential. We set up two hypotheses to explain this effect. One is that new surfaces which have been created by sample crushing may affect the zeta potential. We thought if newly created surfaces have larger zeta potential, samples of smaller grain size would show a larger apparent zeta potential because they have more new surface area. The other is that the flow state may affect the zeta potential. The Helmholtz-Smoluchowski equation, which is used to infer the zeta potential, has been derived assuming laminar flow, which may not be the case for large Reynolds number (high flow rate or large grain size). In the present study, we examined the effect of the flow state on apparent zeta potential. In order to measure how the flow state affects the magnitude of streaming current, measurements need to be conducted for various flow rates and various sizes of particles with the same physico-chemical surface condition. So we chose various sizes of soda-lime glass beads as samples. Grain sizes of the beads are 0.177~0.250 mm (GB200), 0.350~0.500 mm (GB400), and 0.710~0.990 mm (GB800). These samples were soaked in acetone for 12 hours, washed with acetone several times, washed with distilled water and with KCl solution, which was used for background electrolyte in the experiment, and finally soaked in KCl solution for at least 24 hours before measurement. In the experiments, KCl solution was flowed through the sample column (20×15 mmΦ) inside a silica glass tube, while streaming current and flow rate were measured. The results were as follows. Apparent zeta potential was clearly affected by the flow state; it is constant for a small Reynolds number R_e<1~10 (grain diameter was adopted as the characteristic length), but it decreased with R_e when R_e>1~10. This is a mechanical effect (i.e. smaller apparent permeability for turbulent flow), not a chemical effect on the zeta potential. This effect of the flow state can quantitatively explain the apparent grain size dependency reported before. In addition, we have found that the decreasing rate of C_c with increasing R_e is much smaller than that of permeability. This means that turbulent flow induces more streaming current per unit flow rate.
DE: 3947 Surfaces and interfaces
DE: 5109 Magnetic and electrical properties (0925)
DE: 5114 Permeability and porosity
DE: 5139 Transport properties
SC: Tectonophysics [T]
MN: Fall Meeting 2005