IMPACT OF WATER SALINITY ON HIGH-FREQUENCY DIELECTRIC MEASUREMENTS IN ROCK-FLUID MIXTURES

摘要

Dielectric measurements have become attractivecandidates for assessment of water-filled porosity andhydrocarbon saturation in formations such as carbonatesand organic-rich mudrocks. The dielectric permittivitymeasurements are sensitive to the water-filled porosity,especially in the high frequency range (e.g., GHzfrequency range), because of the higher dielectricpermittivity of water compared to the other rock matrixcomponents. However, in the presence of saline water,the water molecules lose their orientation freedompartially due to hydration with ions. Thus, the salinity ofwater affects dielectric measurements. The impacts ofwater salinity on the real part (i.e., dielectric constant)and the imaginary part (i.e., dielectric loss which isproportional to conductivity) of dielectric permittivityare different and have not yet been quantitatively studiedin high-frequency measurements. To accurately estimatewater-filled porosity from dielectric permittivitymeasurements, the impact of water salinity on thesemeasurements needs to be quantified at high frequency.The objectives of this paper include (a) quantifying theimpact of water salinity on dielectric permittivitymeasurements and (b) investigating the impact of watersalinityand water-filled porosity on the criticalfrequency (> 1 GHz) at which the interfacial polarizationis completely negligible.We measured dielectric permittivity of brine atfrequencies ranging from 1 MHz to 3 GHz at roomtemperature and pressure conditions, where watersalinity varies from 0 Kppm to 160 Kppm. We alsomeasured the dielectric permittivity of dry and brinesaturatedrock samples. To investigate the impact ofwater salinity on dielectric permittivity of the fully andpartially brine-saturated rock samples, we first injectedbrine water to fully saturate the samples and thenmeasured the dielectric permittivity of the samples athigh frequencies (from 1 MHz to 3 GHz). Next, we usedthe centrifuge to partially saturate the rock samples, andcompared the water salinity impact on dielectricproperties of the rock-fluid mixture of different waterfilledporosity.We applied the described experimental method oncarbonate and sandstone rock samples with differentpore structures and quantified the sensitivity of dielectricpermittivity to water salinity, water-filled porosity, andfrequency. The results confirmed that there exists acritical frequency above which the water salinity doesnot affect the dielectric constant, and such criticalfrequency increases with the increase of water-filledporosity and water salinity. At high frequencies wheredielectric constant is independent of the frequency, thereexists a critical water-filled porosity below which watersalinity has negligible impact on the dielectric constant.However, in water-filled porosity of higher than thiscritical value, the dielectric constant slightly decreasesby increasing water salinity. Furthermore, we showedthat at frequencies above the critical frequency, there isa critical water salinity below which the dielectric lossincreases as the water salinity increases, while thedielectric loss decreases if the water salinity exceeds thecritical value. The quantitative results on the impact ofwater salinity on dielectric measurements can potentiallyimprove interpretation of dielectric permittivitymeasurements for reliable assessment of water-filledporosity.