Abstract
We describe in this paper the theoretical background for the electrokinetics in rocks and in porous media, to be included in the special issue “Electrokinetics in Earth Sciences” of International Journal of Geophysics. We describe the methodology used for self-potential (SP) and for seismoelectromagnetic measurements, for both field and laboratory experiments and for modelling. We give a large bibliography on the studies performed in hydrology to detect at distance the water flow, to deduce the thickness of the aquifer and to predict the hydraulic conductivity. The observation of SP has also been proposed to detect fractures in boreholes, to follow the hydraulic fracturing, and to predict the earthquakes. Moreover, we detail the studies on geothermal applications.
Highlights
The electrokinetic phenomena are induced by the relative motion between the fluid and the rock matrix
The electric current density, linked to the ions within the fluid, is coupled to the fluid flow [1] so that the streaming potentials are generated by fluids moving through porous media [2]
Another interpretation was proposed on the basis of laboratory observations showing that the electrokinetic coupling of a rock was enhanced by fracturing [32, 33], when stresses rise to over 75% of the yield stress that ruptures the seismic zone
Summary
The electrokinetic phenomena are induced by the relative motion between the fluid and the rock matrix. The observed SP decay after shut-in was interpreted as related to large fluid-flow persisting after the end of stimulation and correlated to the microseismic activity [45] Another field experiment was performed with periodic pumping tests (injection/production) and showed that the attenuation of SP amplitude with distance was roughly similar to the pressure attenuation [46], leading to the conclusion that the hydraulic diffusivity could be inferred from SP observations. Time-varying fluid flow has been identified through SP measurements, showing clearly both rainfall and evaporation events [47] Modelling of such observations confirmed that SP measurements allow effectively to estimate the direction of water flux at the scale of the electrode separation (usually several decimetres), that is, at a much larger scale than tensiometric measurements [41]. We specially point out the use of self-potential for geothermal applications
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