Complex Permittivity of Shale and Brine: Core Analysis Results and Interpretation

Determination of in situ water saturation is difficult in unconventional reservoir (e.g., shale) formations. Traditional resistivity-based measurements, normally applied to conventional reservoirs, cannot be easily applied to organic rich shales (ORS). Therefore, core analysis, with its related expense and elapsed time for measurement, is still the primary method of water saturation determination in unconventional shale reservoirs. Dielectric logging provides an alternative method to determine in situ water saturation from well-logs. At high frequency (100's MHz), the complex permittivity measurement (i.e., dielectric constant and conductivity) is dominated by the volume and salinity of brine in the pore space, whereas at lower frequencies the measurements are increasingly sensitive to how the water is distributed within the porous rock, i.e., the rock texture. In wet shale, there are two main processes of dielectric polarization. Firstly, Maxwell-Wagner space- charge polarization in the pore space. Secondly, polarization of ions (bound water) in the electrical double layer. In shale, this can dominate over the Maxwell-Wagner effect, especially at frequencies below 30 MHz. In this study, we present results from a unique dataset of broadband complex permittivity measurements on shale core samples at different hydration states, from dry to fully brine saturated. The hydration state has a very large effect on shale dielectric properties and dispersion behavior. The dispersion behavior can be described by the Cole-Cole model. Increased water saturation increases the magnitude of the dispersion and relaxation frequency. Both the dielectric constant and conductivity increase with increasing water volume. This rate of increase is greatest at the low frequency (kHz) for dielectric constant, and high frequency for conductivity (MHz). This change in complex permittivity with water saturation is linear and the regression coefficients depend on the pore volume and clay properties (i.e., the amount of bound water). This is expected given the dominant polarization mechanisms. Dielectric logging is essentially a measure of water content in a shale, or indeed any rock. Estimation of in situ water saturation is done via a mixing model with user defined complex permittivity for each of the mineral and fluid components – with greatest sensitivity on brine properties. The complex permittivity of brine varies as a function of state (pressure and temperature), composition (salinity), and frequency of measurement. In addition to the core plug measurements, we also present results for brine samples over a wide range of salinities and measurement frequencies at ambient conditions. These dielectric laboratory experiments are then used to validate several popular brine permittivity models from the open literature. In general, there is good agreement between these models at low salinity (<50 kppm), but at higher salinities the models can diverge significantly.