Microcrack Permeability in Tight Gas Sandstone

Abstract

A model of stress-dependent permeability was developed on the basis of flow through cracks. The compliance of the cracks is controlled by elastic deformation of a Gaussian distribution of surface asperities, indented into the opposing crack face. The theory was applied to data from tight gas sand cores, with the following results. The theory explains the Jones and Owens permeability correlation1 and predicts a modification of it that yields a better fit. The slope of the permeability/stress curve on a log-log plot is predicted to be about 1.0, which agrees with the data. The Jones and Owens Klinkenberg correlation supports a slit-like flow model and indicates that flow occurs along almost every grain boundary. The size of the surface roughness was computed from the Klinkenberg factor and the slope of the permeability/stress curve. It is independent of stress, as predicted by the theory. The porosity of tight sand cores decreases linearly with the log of net confining stress, in accordance with the Walsh-Grosenbaugh model of crack compressibility. This detailed agreement supports the theory and indicates that the permeability of tight gas sand cores is due to microcracks.