A Practical Workflow for Reliable Water Saturation Quantification from Dielectric Permittivity Measurements

Summary Broadband relative dielectric dispersion measurements are a promising tool for assessing water saturation. While the widely used volumetric model, complex refractive index model (CRIM), is simple to implement, it does not account for the pore structure, leading to significant errors in water saturation assessment in certain situations. In this paper, we introduce and validate a dielectric permittivity model that preserves the algebraic simplicity of the CRIM equation while incorporating a physically meaningful parameter, the tortuosity-dependent parameter (ft,w), to honor the effects of pore geometry. This approach is expected to be suitable for practical well-log-scale applications. We use numerical simulations in 3D pore-scale images of sandstone and carbonate rocks at various water saturation levels to demonstrate the limitations of conventional CRIM and to validate our new workflow. A critical contribution of this work is that, for the first time, we quantitatively demonstrate that once primary volumetric effects are accounted for, a decrease in water saturation results in a corresponding enhancement in the effective dielectric response of the rock. This phenomenon, captured by the ft,w, is typically masked in bulk permittivity measurements by volumetric effects but is crucial for accurate physical modeling of dielectric properties of rocks. We found that while ft,w is a function of water saturation at low frequencies, it becomes effectively independent of water saturation at 1 GHz. This enables a practical workflow, where ft,w is determined from fully water-saturated samples and then reliably applied to interpret measurements in partially water-saturated zones. Estimated water saturations at 1 GHz yielded a mean absolute percentage error of 5.3%, a nearly threefold improvement over CRIM. The results demonstrate that our physics-based approach enables the reliable assessment of fluid saturation using dielectric permittivity measurements.