Hysteresis In The Dielectric Response Of Partially Saturated Sandstones: The Importance Of Microscopic Fluid Distribution

Microscopic fluid distribution can have a significant effect on the dielectric properties of a partially saturated rock. Evidence of this effect is found in laboratory data presented by Knight and Nur (1987) in which different methods for controlling the level of water saturation produced very different results for the dependence of the dielectric response on water saturation. This effect can be theoretically assessed by calculating the dielectric constant (?") of a partially saturated rock model for various geometrical distributions of water and gas within the pore space. It is observed that both the pore geometry in which saturation conditions are varying and the gas-water geometry within a given pore space are critical factors in determining the effective dielectric behavior of a partially saturated material. In particular, the saturation hysteresis observed in the laboratory measurements on a tight gas sandstone by Knight and Nur (1987) is analyzed. On a microscopic scale, the existence of this hysteresis in ?" of the partially saturated rock can be attributed to the initial development of thin gas pockets within the central volume of the pore space during the imbibition process. At a critical value of overall water saturation, the gas pockets within the pore throats collapse. These features are not recreated during the drainage process which favors the segregation of pore fluid such that each pore tends to be saturated with a single fluid phase. By using a simplified representation of the microscopic fluid distributions which occur during imbibition and drainage, good agreement is obtained between the numerical model and experimental data. The effects of salinity, measurement frequencies, and pore geometry parameters are inferred by using these models for the microscopic fluid distribution.