Quantifying the Impacts of Spatial Distribution and Petrophysical Properties of Clay Minerals on the Multifrequency Complex Dielectric Permittivity of Shaly Sandstones

Interpreting multifrequency dielectric permittivity in clay-rich formations is challenging compared to clay- free formations. While shaly sand analytical models are available, they are often heavily calibration-reliant and fail to honor the spatial distribution of clay minerals (e.g., grain-coating and laminated). Therefore, fundamental research is required to improve the interpretation of dielectric permittivity in clay-rich formations. The objectives of this paper are to quantify the impacts of (i) cation exchange capacity (CEC) and clay concentration on the dielectric permittivity of shale samples, (ii) CEC and lamination thickness on the dielectric permittivity of shale-laminated rocks, (iii) and spatial distribution on the complex dielectric permittivity of shaly sandstone formations. We develop 3-dimensional samples of synthetic shale, shale-laminated rocks, and rocks with grain-coating clay minerals. Next, we use the finite-volume method to solve Maxwell's equations in the frequency domain under quasi-electrostatic conditions. We calculate the effective permittivity of the clay network using a mechanistic model and Maxwell Garnett equation. The inputs to the numerical simulations include the pore-scale rock samples and complex dielectric permittivity of the components in the rock. The output is the electric potential, which is used to calculate the complex permittivity of the rock in the frequency range of 100 Hz to 2 GHz. The results showed a significant increase in relative permittivity of shale with an increase in CEC below 1 MHz. Moreover at 100 Hz, the dielectric constant increases 54% as the thickness of shale laminations increases 10 times at a clay CEC value of 9 meq/100g. Furthermore, it was found that the spatial distribution of clay minerals in the rocks have measurable impacts on the dielectric response, especially below 100 MHz. Results suggested that conventional shaly sand models for interpretation of dielectric permittivity measurements fail to account for the impact of clay concentration, shale lamination thickness, and spatial distribution, which significantly influence permittivity. The findings highlight the importance of incorporating clay properties into dielectric models for the reliable assessment of water saturation, salinity, and CEC.