ABSTRACT

Laboratory measurements have been made of the dielectric constant (K") of a tight gas sandstone as a function of water saturation (Sw) in the frequency range of 10 kHz to 1 MHz. Sw was varied through adsorption of water vapor, imbibition, and drying. We have found that K" of a partially saturated sandstone depends strongly on the geometrical distribution of water and gas in the pore space. Our interpretation of the observed change in K" with S, is based on the modified Maxwell-Wagner model proposed by Sen (1980), in which platy insulating grains surrounded by conducting pore fluid act as capacitors, contributing to the total measured K ". In the region of Sw < 0.03, there is little change in K" with Sw; we interpret this region as corresponding to the presence of a monolayer of water on the surface of the pore space. In the region of 0.03 < Sw < 0.12, there is a rapid increase in K" with increasing Sw; we interpret this region as the wetting of the pore surfaces by two to three monolayers of water, creating both water-grain and water-gas capacitors. In the region of Sw > 0.12 dielectric hysteresis occurs; we interpret this region as corresponding to the filling and emptying of the central volume of the pore space with water, the hysteresis being a result of the changing geometry of the liquid and gas phases. It has been shown that in the interpretation of in situ measurements of K" a geometrical parameter is needed to account for the geometry of the grains. Our results suggest that the distribution and resulting geometries of the liquid and gas in the pore space must also be accounted for. Alternatively, measurement of K" could be used to obtain information about the microgeometry of a sandstone and the contained fluid phases.

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