Effect of Water Salinity and Water–Filled Pore Volume on High–Frequency Dielectric Measurements in Porous Media

Abstract

Summary High-frequency dielectric measurements have been attractive candidates for assessment of water-filled porosity in porous media. In the presence of saline water, the water molecules lose their orientation freedom partially because of hydration with ions and make these measurements sensitive to water salinity, which makes the interpretation of the dielectric-permittivity measurements challenging. The effects of water salinity on the real and the imaginary parts of dielectric permittivity have not yet been quantitatively studied in high-frequency (e.g., greater than 1 GHz) measurements. We measured dielectric permittivity of brine at frequencies ranging from 1 MHz to 3 GHz at room temperature and pressure conditions, where water salinity varies between 0 and 160 kiloparts per million (kppm). We also measured the dielectric permittivity of rock samples with different brine saturations. Our experimental results confirmed that there exists a critical frequency above which water salinity does not affect the real part of the dielectric constant, and such critical frequency increases as water-filled porosity and water salinity increase. At high frequencies where the real part of the dielectric constant is independent of the frequency, there exists a critical water-filled porosity below which water salinity has negligible effect on the real part of the dielectric constant. However, when water-filled porosity is higher than this critical value, the real part of the dielectric constant slightly decreases by increasing water salinity. Further, the results showed that at frequencies greater than the critical frequency, there is a critical water salinity below which the imaginary part of the dielectric constant increases as the water salinity increases, whereas the imaginary part of the dielectric constant decreases if the water salinity exceeds the critical value. The quantitative results on the effect of water salinity on dielectric measurements can potentially improve interpretation of dielectric-permittivity measurements for reliable assessment of water-filled porosity and hydrocarbon saturation.