The effect of clays, salinity and saturation on the high-frequency electrical properties of shaly sandstones
The effects of frequency, clay content, clay type, ionic concentration and saturation on the complex impedance of various shaly sandstone rocks have been investigated in a detailed and systematic manner. Synthetic samples have been used, so as to allow full control over clay content, clay type and clay distribution, hi addition, selected outcrop and reservoir samples with varying concentrations of clay were also used. Experimental measurements of complex impedance were made as a function of applied stress and saturation, for samples having differing amounts of clay, over a frequency range from 10 Hz to 2 MHz. In addition, the influence of ionic concentration (salinity) was also investigated. The modulus of impedance for the bulk sample response for fully saturated samples was frequency independent in the frequency range from approximately 10 kHz to 600 kHz. A strong linear correlation between the average complex impedance values and clay content was observed, with clay type influencing the magnitude of impedance. However, the reactance and dielectric constant were strongly frequency dependent. They both exhibited a power law dependency with frequency for the montmorillonite and illite synthetic shaly samples. It was possible to deduce correlations between the exponents from the reactance with clay content, and to a lesser extent clay type. The illite shaly samples usually showed the highest dielectric constant values, with the lowest belonging to the kaolinite samples. All samples showed strong relationships with salinity as the concentration was varied from 60,000 ppm to 15,000 ppm, with the modulus of impedance increasing as salinity decreased. The bulk sample response was frequency independent over all salinity concentrations. The dielectric constant showed increasing values as the salinity decreased, with a power law relationship best fitting the data. As the samples were desaturated, a frequency dependency developed for the modulus of impedance as a ftinction of saturation. As the samples were de-saturated the dielectric values decreased in magnitude. The concept of double layer thickness, and the variation in the cationic exchange capacity (CEC) with brine salinity and clay type, were used to explain the responses observed. Using this concept, the data was modelled using equivalent RC circuit models, and an impedance network model. It was found possible to model the data using combinations of capacitors and resistors.