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量与岩石物理性质。然而，这类模型通常仅适用于具有非膨胀性基质的土壤。含膨胀性黏土的土壤，其基质会随水分的得失发生胀缩变化。本实验室研究表明，土壤基质的变化会影响基于阿尔奇（Archie）定律的饱和度估算。阿尔奇定律最初是针对纯净砂岩储层提出的，并未考虑基质变形的影响。研究人员将一块膨胀黏质土样品用氯化钾（KCl）溶液完全饱和，随后置于室温下进行蒸发干燥。干燥过程中，每日测量土壤的电阻率、重量以及因收缩产生的体积变化。为期11天的实验中，该土样体积缩减了33%。在此期间，研究人员按时间序列测定了土样的孔隙度与饱和度。同步发生的蒸发与收缩过程，导致饱和度随含水量的降低呈非线性下降趋势。若未考虑收缩引起的饱和度变化校正，直接应用阿尔奇第二定律会得到错误的电阻率预测结果。通过基于体积缩减量计算的饱和度校正方法，可使电阻率与饱和度间呈现幂律关系，此时二者的决定系数（R²）更高，且饱和度系数也更为合理。