Influence of grain size and shape on dielectric permittivity and its implications for water saturation assessment

Dielectric permittivity mixture models often assume simplified rock geometries, limiting their accuracy in rocks with complex pore structures. Systematically evaluating the influence of pore geometry, grain shape, and grain size on model performance for water saturation assessment is experimentally challenging and thus largely untested. Frequency-domain dielectric permittivity simulations, however, provide a means to effectively model these geometric influences at the pore-scale. Therefore, this paper aims to (a) investigate the influence of grain geometry (size, shape, and alignment) on dielectric permittivity using synthetic samples and (b) evaluate mixture model performance in assessing water saturation in synthetic and actual rocks. We performed frequency-domain simulations in the frequency range of 10 Hz to 5 GHz. The dielectric permittivity dispersion significantly increased as grains flattened (i.e., aspect ratio increased). The frequency-domain simulations conducted over the range of 10 MHz to 5 GHz showed that grain size had a negligible impact on permittivity above 10 MHz. We observed that the relative permittivity in the z direction decreased with increased aspect ratio of grains. Simulations suggested that directional permittivity measurements can enhance grain shape characterization. The unique contribution of this paper is the comprehensive quantification of the impacts of grain size, shape, and alignment on dielectric permittivity. Conducting such an investigation is challenging and almost impossible in the core-scale domain.