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.在水文地球物理研究中，电导率模型已被广泛用于估算土壤的含水率与岩石物理性质。然而，此类模型通常仅适用于具有非膨胀基质的土壤。含膨胀黏土的土壤，其基质会随水分得失发生胀缩现象。在本实验室研究中，我们证实：土壤基质变化会影响基于阿尔奇定律（Archie's Laws）的饱和度估算。阿尔奇定律最初是为纯净砂岩储集层岩石提出的，未考虑基质变形的影响。我们将一块膨胀黏土质土壤样品以氯化钾（KCl）溶液完全饱和后，置于室温环境下进行蒸发干燥。干燥过程中，我们每日测定土壤的电阻率，以及其因收缩产生的质量与体积变化。在为期11天的干燥周期内，该土壤样品的体积缩减了33%。在此期间，我们按时间序列测定了该土壤样品的孔隙度与饱和度。同步发生的蒸发与收缩现象，使得饱和度随含水率降低呈现非线性下降趋势。若未考虑因收缩导致的饱和度变化修正，应用阿尔奇第二定律会得到错误的电阻率预测结果。通过计算得到的体积缩减量对饱和度进行修正后，电阻率与饱和度之间呈现出幂律关系，其决定系数（R²）更高，且饱和度系数也更为合理。