Complex electrical properties of shale as a function of frequency and water content

Abstract

An experimental research program has been initiated to investigate the electrical properties of swelling shales (shales that have been exposed to water and are therefore expanding) across a wide frequency range, 5 Hz to 1.3 GHz. This range spans the spectrum of the commonly used downhole logging measurements, from the deep laterologs to the microwave dielectric tools. Three different methods of varying the sample's water content have been used: desiccator, electro-osmosis and air exposure methods. Two distinct measurement techniques have been used to span the frequency range: four-electrode setup for the low frequencies (5 Hz -13 MHz), and open-ended coaxial probe with network analyzer at the high end (20 MHz - 1,3 GHz), The probe technique is simple to use, potentially enabling field measurements of complex permittivity to be taken, although some accuracy is sacrificed. The effects of swelling in shale are most pronounced at the lowest frequencies. This investigation discovered a phenomenon of shale; shale will generate a direct electrical current under stress that has potential for a wellbore diagnostic tool. Also, the best fluid for shale preservation was found to be Isopar M™ (a mineral oil), saturated with deionized water.