**5.2. Electrical conductivity**

Clay particles are the porous materials. The pore fluid influences the electrical conductivity. The electrical conductivity (mS/m) of a porous material is the combination of electrical conductivities of the matrix material and the pore fluid [23].

Air, water, or saline water may be present in the pore. When the pore fluid is of low conductivity, for example, air or water, the bulk conductivity of clay mineral is contributed by the matrix material. Pore fluid having a higher electrical conductivity significantly enhances the total electrical conductivity of clay, for example, clay particles with a significant porosity level (40–50%), and saline water present in the pore; then the bulk conductivity is mainly the contribution of pore fluid. In this case, there would be negligible difference in the conductivities of sand and clay.

A higher content of clay particles with 2:1 structure present in montmorillonite sample produces an increased bulk electrical conductivity for non-saline soils [24]. This effect was attributed to the exchangeable cations or to proton transfer from the dissociation of interlayer water content. A reduced level of interlayer water contents in K-saturated clays resulted in the lowest electrical conductivity.

Since the clay content, pore fluid, clay type, saline water, and water saturation influence the soil conductivity, the assessment of electrical conductivity of reservoir rock may be used to estimate these factors [25]. However, the variation in the distribution of liquid and solid phases introduces the variation and complication in the electrical conductivity of heterogeneous porous medium.

The distribution of electrical ions around clay pores is called membrane polarization. In membrane polarization, the negative ions are oriented to one end of the pore under the influence of DC potential across the clay pore, and this polarization resists the current flow.

In the study of soil, the clay content in soil can be determined using electrical conductivity and membrane polarization as the function of clay content.
