Water adsoiption/desorption (sorption) isotherms, surface-area measurements, and electron and optical microscopy all demonstrate the presence of significant microporosity in low permeability gas sands as exemplified by cores taken during the course of the Multiwell Field Experiment (MWX). Pore space in these sandstones occurs primarily as solution pores that are often partially filled by fine-structured clay minerals such as illite and smectite. Quantitative information on pore size and volume of microporosity can be obtained from sorption isotherms. A hygroscopic index for rocks based on desorption behavior is proposed. To delineate changes in pore structure that can arise during drying, water desorption isotherms, absolute permeabilities to brine, and effective permeabilities to gas were compared for preserved (undried) and dried cores. Permeabilities of cores which had been allowed to dry tended to be higher than those of the fresh cores. Changes in effective permeabilities and sorption isotherms were consistent with observed decreases in absolute permeabilities. Low water content in preserved MWX cores demonstrated that the technique used to protect them from drying was sometimes inadequate.

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