Saneiyan et al. (2022) - On the accuracy of saturation estimation from electrical measurements of soils with high swelling clay content

Electrical conductivity models have been widely used to estimate water content and petrophysical properties of soils in hydrogeophysical studies. However, these models are typically only valid for soils with non-expandable matrices. Soils containing swelling clays are characterized by matrices that expand/contract upon gaining/losing water. In this laboratory study, we demonstrate that soil matrix changes affect the saturation estimation using Archie’s laws. Matrix deformation is not accounted for in Archie’s laws, which were originally introduced for clean sandstone reservoir rocks. A swelling clayey soil sample was fully saturated with a potassium chloride (KCl) solution, then left to dry evaporatively at room temperature. The resistivity of the soil, along with its weight and volumetric changes as a result of shrinkage, were measured daily during drying . Over a course of 11 days, the soil sample decreased in volume by 33%. During this time period, the porosity and saturation of the soil sample were determined as a function of time. The simultaneous evaporation and shrinkage caused a non-linear reduction in saturation with decreasing of water content over time. Application of Archie’s second law leads to erroneous predictions of resistivity if the correction for saturation changes due to shrinkage are not accounted for. Correcting for saturation using the calculated volume reduction results in a power-law relationship with higher R2 value between resistivity and saturation along with more reasonable saturation coefficients.