Measurements have been made of the dielectric constant (") of a tight gas sandstone as a function of water saturation (SW), in the frequency range of 10 kHz to 1 MHz. Regions can be recognized in the variation of with SW which we interpret in terms of monolayer and multilayer adsorption, capillary condensation, and the distribution of bulk water in the pore space. The level of water saturation in a vacuum dried sample was initially increased by leaving the sample in a high humidity environment to allow the adsorption of water vapour in the pore space. Through this procedure, the sandstone reached a level of 48% water saturated over a period of 24 days, " being monitored throughout. SW was increased to a level of 0.85 by immersing the sample in deionized water. The sample was then allowed to dry through evaporation of water from the pore space. There is a region at low saturation levels within which " measured during the saturation stage has the same value, at any value of SW, as " measured during the desaturation stage. We suggest that this region corresponds to the adsorption and resorption of one to three monolayer of water on the surface of the pore space. At higher saturations significant hysteresis exists in the kilohertz frequency range, with the dielectric data collected during saturation showing a large enhancement in the value of ". This we interpret in terms of differences in the distribution of water in the pore space during the saturation and desaturation stages. During saturation we suggest that the adsorption of water vapour leads to the development of thin gas pockets in the center of the partially saturated pore space. The large capacitance associated with these thin insulating gaps leads to high measured values of ". The drainage of pores during evaporation of water from the pore space does not favour the re-establishment of these thin gas pockets, thus leading to the large observed hysteresis.

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