Earth Resources Laboratory :: Abstracts

Electrokinetics in the Earth

Philip M. Reppert

Submitted to the Department of Earth, Atmospheric, and Planetary Sciences in June 2000 in partial fulfillment of the requirements for the degree of Doctor of Philosophy

Abstract

The study of Streaming potentials has applications in the earth sciences, ranging from fluid flow monitoring, to permeability determination, to studying the surface chemistry of rocks and minerals. In this thesis study of frequency-dependent electrokinetics is presented with both theoretical development and experimental verification. The complex (real and imaginary) streaming potential coupling coefficient is explained. This is followed by the first experiments to measure the real and imaginary part of frequency-dependent streaming potential coupling coefficients. As part of this study an experimental apparatus and data acquisition system were constructed to measure the streaming potential coupling coefficients as a function of frequency. The purpose of the experiments was to measure, for the first time, the real and imaginary part of streaming potentials. In addition, the measured frequency range was extended beyond any previous measurements. Frequency-dependent streaming potential experiments were conducted on one glass capillary, two porous glass filters, and one rock. The sample pore diameters ranged from 34 micrometers to 1 millimeter. Without these experiments, the validity of modeling of the frequency dependence of the seismoelectric effect using existing models streaming potential models would be in question. Two frequency-dependent models (Packard and Pride) were compared to the data. Both PrideÍs and PackardÍs models have a good fit to the experimental data in the low and intermediate frequency regime where viscous terms dominate in the fluid. In the high frequency regime, where inertial terms start to dominate, the data fits the theory after being corrected for capacitance effects of the experimental setup. PrideÍs generalized model appears to have the ability to more accurately estimate pore sizes in the porous medium samples. PackardÍs model has one unknown model parameter whereas PrideÍs model has four unknown model parameters, two of which can be independently determined experimentally. PrideÍs additional parameters may allow for a determination of permeability.

As part of the study of frequency-dependent electrokinetics, this thesis presents the theory for frequency-dependent electroosmosis. It is shown that the electroosmosis frequency-dependent coupling coefficient is constant with increasing frequency until the critical frequency is reached, at which point the coupling coefficient starts to decrease with increasing frequency. The frequency response of the electroosmosis-coupling coefficient is dependent on the capillary radius. The smaller the capillary radius the higher the critical frequency. Data is presented for a 0.127-mm capillary.

In addition to studying frequency-dependent electrokinetics, this thesis examines the temperature-dependent behavior of streaming potential coupling coefficients. As part of this examination a review is made of the previous literature that discusses the temperature dependence of streaming potentials. The streaming potential coupling coefficient is determined using the permittivity, the conductivity, and the viscosity of the fluid. It has been determined that the temperature-dependent behavior of the permittivity, conductivity, and viscosity are well documented and do not alone account for temperature dependence of streaming potentials. The other quantity used in calculating the streaming potential coupling coefficient is the zeta potential. The temperature dependence of the zeta potential is not well understood at the present time. By examining the theory, it appears that the fluid concentration and the adsorption properties of the surface control the zeta potential temperature dependence.

After examining the theory, streaming potential experiments at elevated temperature and pressure were conducted on Fontainebleau Sandstone, Berea Sandstone, and Westerly Granite. The experiments were conducted in a pressure vessel with temperatures ranging from 23oC to 200oC at pore pressure ranging from 20 bar to 50 bar and confining pressures ranging from 200 bar to 250 bar. The zeta potential was found to increase with increasing temperature, and the coupling coefficient can increase or decrease depending on how the conductivity of the sample varies. This implies that in geothermal regions, streaming potentials can have large magnitudes depending on the conductivity of the geothermal fluid.