Influence of Salt Concentration and Type on Dielectric Permittivity of Rocks

Summary Ionic properties and concentration significantly influence the response of brine-saturated rocks to electromagnetic disturbance. However, the dielectric permittivity response of rocks under different ionic conditions is poorly described. This significantly limits the potential information that could be gained from dielectric permittivity measurements about the pore geometry and fluid content. The influence of salt concentration and type on broadband dielectric permittivity must be reliably quantified to enhance interpretation of dielectric permittivity measurements. The main objective of this paper is to quantify the influence of salt type and concentration on dielectric permittivity via experimental measurements and pore-scale simulations. We examine the impact of salt concentration and type on the dielectric permittivity of pore- and core-scale Berea sandstone (BS) samples. First, we perform frequency-domain dielectric permittivity simulations to quantify the response of the pore-scale models to electric field excitation. The frequency-domain dielectric permittivity simulator solves Maxwell’s equations under quasistatic conditions at discrete frequencies. We simulate the dielectric permittivity in the frequency range of 20 MHz to 3 GHz. We perform the simulations in samples saturated with NaCl, KCl, and MgCl2 brines. The salt concentration of the brine solutions ranges between 10 PPT and 100 PPT (parts per thousand). We fully saturate the samples with different brine solutions at varying salt concentrations for the core-scale analysis. In the core-scale domain, we use the brine solutions and salt concentrations assumed in the pore-scale analysis. The dielectric permittivity measurements were conducted using a network analyzer with a high-temperature coaxial probe setup in the frequency range of 300 MHz to 3 GHz. We observed that relative permittivity at 1 GHz increases with increasing salt concentration, irrespective of the brine type. However, the type of salt significantly controls the magnitude of the decrease in relative permittivity. After increasing the salt concentration from 10 PPT to 100 PPT, relative permittivity at 1 GHz increased by 11% and 7% when the samples were saturated with KCl and NaCl brine solutions, respectively; at 20 MHz, the same increase in salt concentration caused rock relative permittivity to increase by only 1% and 5% in the samples saturated with KCl and NaCl brines, respectively. The lower sensitivity of relative permittivity to salt concentration at 20 MHz compared to 1 GHz can be attributed to the combined influence of interfacial and orientational polarizations on rock dielectric permittivity. The impact of salt type on relative permittivity was negligible in samples saturated with 10 PPT brine solutions. Results demonstrated that taking the influence of salt concentration and type into account is critical for reliable interpretation of dielectric permittivity measurements. The novel contribution of this work is the documentation of how the saturating brine type influences the complex dielectric permittivity of the rocks. This work illuminates the extent to which the relative permittivity can be used for petrophysical analysis in cases where the formation brine salt concentration is uncertain. Additionally, the outcomes of this work will contribute to enhanced interpretation of dielectric permittivity measurements in formations with variable salt concentrations of formation water.