The dielectric constant of sandstones, 60 kHz to 4 MHz

Complex impedance data were collected for eight sandstones at various levels of water saturation (S,) in the frequency range of 5 Hz to 4 MHz. The measurements were made using a two-electrode technique with platinum electrodes sputtered onto the flat faces of diskshaped samples. Presentation of the data in the complex impedance plane shows clear separation of the response due to polarization at the sample-electrode interface from the bulk sample response. Electrode polarization effects were limited to frequencies of less than 60 kHz, allowing us to study the dielectric constant K’ of the sandstones in the frequency range crf6O~kwrtrr4 Mii2; K’ of all samples at all levels of saturation shows a clear power-law dependence upon frequency. Comparing the data from the eight sandstones at S, = 0.36, the magnitude of the frequency dependence was found to be proportional to the surface area-to-volume ratio of the pore space of the sandstones. The surface area-tovolume ratio of the pore space of each sandstone was determined using a nitrogen gas adsorption technique and helium porosimetry. K’ also exhibits a strong dependence on S,. K’ increases rapidly with S, at low saturations, up to some critical saturation above which K’ increases more gradually and linearly with S,. Using the surface area-tovolume ratios of the sandstones, the critical saturation in the dielectric response was found to correspond to water coverage of approximately 2 nm on the surface of the pore space. Our interpretation of the observed dependence of K’ on both frequency and S, is that it is the ratio of surface water to bulk water in the pore space of a sandstone that controls the dielectric response through a Maxwell-Wagner type of mechanism.