Geoscience Reference
In-Depth Information
Chapter 8
Electrokinetic Effect in Water-Saturated Rock
Abstract
Basics of electrokinetic and seismoelectric phenomena in water-saturated
rocks are discussed in this chapter. We study electrochemical and hydrodynamical
processes in the fluid flowing in pores space in order to derive the relationship
between the electrokinetic current and pore pressure gradient. Then we focus on
the electrokinetic effect in anisotropical and fractal media. The remainder of this
chapter covers seismoelectric effect caused by seismic waves.
Keywords
Electrokinetic effect Pore pressure gradient Porosity Seismo-
electric effect Streaming potential coefficient
8.1
Theory of Electrokinetic Effect
8.1.1
Basic Equations: Laboratory Study
A contact potential difference and electric charges are known to usually arise on
the interface area in multiphase heterogenous media. For example, double electrical
layers (DELs) are formed at the surfaces of pores, capillaries, and cracks in the
porous water-saturated rocks. The basic reason of this phenomenon is that the
groundwater is similar in content to the electrolytic solution which contains different
kinds of ions and dissociated molecules. The surfaces of pores, capillaries, and
cracks absorb the ions of specified sign from the water solution that results in charge
buildup at the interphase boundaries. The excess ions of opposite sign keep in the
groundwater so that the crack surfaces and the groundwater are charged oppositely
thereby forming DELs at the crack surfaces.
It is common knowledge that there are a few mechanisms of ion absorption
(Parks
1965
; Fuerstenau et al.
1970
; Wiese et al.
1971
; James and Healy
1972
).
One possibility is that the surface of solid phase gains the negative charges due to
acidic dissociation of surface hydroxyl groups (Parks
1965
):
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