Отправлено: 05.04.15 04:46. Заголовок: Electrokinetic experimental study on saturated rock samples: zeta potential and surface conductance

Electrokinetic experimental study on saturated rock samples: zeta potential and surface conductance
Jun Wang, Hengshan Hu, Wei Guan and Hui Li


SUMMARY
It is important to know the electrokinetic properties of crustal rocks for interpreting the conductivity
mechanisms and seismoelectric phenomena during earthquakes and seismoelectric
well logging. In this study, electrokinetic experiments are conducted using a special coreholder
by employing an AC lock-in technique. A series of experiments are conducted on
10 sandstone samples to measure the streaming potentials and streaming currents, and the
experiments on each sample are done at six different salinities. The streaming potential coeffi-
cient and streaming current coefficient are calculated from the measured streaming potentials
and streaming currents. The experimental results show that streaming potential coefficient
and streaming current coefficient decrease as the salinity increases. The dependence of these
two coefficients on permeability and pore radius are analysed and compared with previous
works. At low salinities, the streaming potential coefficient and streaming current coefficient
increase with the increasing permeability and pore radius. At high salinities, the streaming
potential coefficient (streaming current coefficient) almost share a same value for 10 different
samples. This conclusion indicates that the differences of rock parameters can only be well
recognized at lower salinities, and the electrokinetic signals are invalid at high salinities, which
offers a restrictive condition for using the amplitude of electrokinetic signals to estimate rock
parameters. The zeta-potential have also been estimated through combined measurements of
streaming potential and streaming current. The surface conductivity and its contribution to
electrokinetic effects are determined from a comparison of zeta-potentials by two different
methods, and then the validation of the Helmholz–Smoluchowski equation for a capillary tube
is tested in rocks. We also compare our date with theoretical and experimental works, and
set up an expression about the relationship between zeta potential and salinity, which fits the
experimental data well.
Key words: Electrokinetic properties; Rock and mineral magnetism; Permeability.