استخراج الکتریکی آبهای زیرزمینی

ELECTROKINETIC GROUNDWATER EXPLORATION:

A NEW GEOPHYSICAL TECHNIQUE

Sukhyoun Kim, Graham Heinson & John Joseph

CRC LEME, School of Earth and Environment Sciences, University of Adelaide, SA, 5005

ABSTRACT

Electrokinetic potentials, sometimes known as streaming potentials, are generated by fluid-flow, heat fluxes,pressure sources and diffusion potentials across boundaries between a fluid electrolyte and mineral grains orrock particles in fractured rocks and porous media. In the electrical double layer model (Stern model, Figure1), there is an immobile layer that is divided into two by the Stern plane. The layer nearest the mineral grainis immobile, and the surface of the mineral grain has a net charge so that it adsorbs cations from the fluid inan electrical double layer. Away from the grain boundaries, the mobile part of the fluid may have a surplus opositive or negative ions, depending on the electrostatic charge distribution at the electrical double layer. The excess density of ionic species can be transported with fluid flow producing an advective electric current. The advective flow of charge is balanced by a return electric current; this process is known as coupled flow. The electric potential in the Stern layer varies as a function of distance. The electric potential at the end of slipping plane (the end of the electrical double layer furthest from the mineral grains) is known as the zetapotential (ζ). To measure such electrokinetic potentials, a good understanding of petrophysical properties of the media, particularly the ζ-potential and electrokinetic coupling-coefficient are required. This research project aims to establish a new technology to measure fluid-flow and infer hydraulic conductivity from the electrokinetic potential in three approaches. Firstly, measurements of surface electrical potentials will be combined with hydraulic pumping tests to determine sub-surface fluid-flow. Measurements will be made using 36 non-polarized electrodes relative to a reference. This setup will allow monitoring of changes in electrokinetic potentials associated with draw-down. Data will be sampled at a rate of 1Hz with a resolution of 0.1 mV. The second step of this research will be the investigation of the relationship between porous media and water-flow in the laboratory. Experimental work will focus on the properties of clay and sand, which form most of the ground in the area along the River Murray. The key part of this experiment will be defining the ζ-potential and electrokinetic coupling coefficient of these porous media. The final aim of this research is to further develop numerical modelling of the electrokinetic potential data to quantitatively

interpret electrokinetic potential data.

INTRODUCTION

The electrokinetic potential occurs by fluid-flow in porous media and in fractured rocks. Interaction between fluid-flow and mineral grains produces an electrokinetic potential (Corwin & Hoover 1979). However, it is not easy to quantitatively interpret electrokinetic potentials because of the complex dependency on parameters such as temperature, pressure, pH and the heterogeneity of porous media in the ground. The theoretical basis of this method was originally described by Overbeek (1952) and Nourbehecht (1963). However, relatively few studies have been carried out to investigate the electrokinetic potential in applied geophysics, and to obtain quantitative interpretation of electrokinetic potentials with numerical modelling (Sill 1983, Wurmstich & Morgan 1994, Titov et al. 2002).

When fluid flows through pores and in fractures, it forms two layers which are separated by a slipping plane as immobile and mobile parts of fluid in the Stern model (Figure 1). The layer in the vicinity of the mineral grain boundary, which is immobile, is divided into two again. Next to the mineral grain boundary is the Stern layer, and the other layer is the diffusion layer. Together, these layers form the electrical double layer (EDL). The interaction between mineral grains and fluid generates the electrokinetic potentials. In the Stern layer, the negatively charged surface of the mineral grain adsorbs positive ions from the fluid in the immediate vicinity of the grain's surface. The diffusion layer consists of surplus positive ions which were attracted but not adsorbed. If the negative charge density is very high at the surface of the mineral grain, the voltage may rise above zero in the Stern layer and will then decrease toward zero as a function of distance in accordance with a Boltzmann distribution of the ions (Overbeek 1952) (Figure 1B). Normally, the surface of silicate mineral grains is negatively charged and attracts cations from the fluid  surrounding the mineral grain (Fitterman 1979). By this phenomenon, so-called electrostatic attraction, a distribution of ions is produced, and thus electrical potentials occur. In a system which consists of mineral grains and fluid, there must be an electrical equilibrium (Ishido & Mizutani 1981). This means the total charge of the system must not change in magnitude. In the electrical double layer, where the voltage approaches zero at the Stern layer, the electrical charge at the hydraulic slipping plane is the zeta-potential (ζ)

as shown in Figure1. Thus, the zeta-potential is directly related to the amount of transported electrical charge and is the magnitude of the potential change in the mobile part of the fluid. Values of zeta-potential and of the electrokinetic coupling coefficient are necessary to estimate the magnitude of electrokinetic phenomena (Ishido & Mizutani 1981).

The convection flow of pore fluid in the diffuse layer produces an advective electric current; such flow yields an electric field which produces a counter electric current (conduction current) through the interface. Assuming that the flow is laminar and the radius of curvature of interface between mineral grains is much bigger than the thickness of the double layer, the advective and conduction currents at equilibrium are equal in magnitude. The total current per unit area itotal in the system can, therefore, be expressed as:۰۰۰

 

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