Importance of Fabric and Composition on the  Stress Sensitivity of Permeability in Some Coals, Northern Sydney Basin, Australia: Relevance to  Coalbed Methane Exploitation

Abstract

Fabric and composition of a series of Upper Permian high-volatile to low-volatile bituminous coals of the Sydney basin have a marked effect on stress sensitivity of permeability, and thus on the reservoir characteristics of the coal. The coals vary in composition from end members of predominantly bright-banded coal comprised mainly of the microlithotype vitrite and the maceral vitrinite, to dull coal composed of significant amounts of ash, inertinite group macerals, and the microlithotype inertite. The brighter coals are more extensively fractured with one or, more commonly, two or three regularly spaced fracture sets (cleats) spaced at 5-20 µm. Fusinite and semifusinite, common macerals in the dull coals, are characterized by phyteral porosity (mainly cell lumens) and fabric- selective intergranular porosity. The permeability of tested samples varies significantly with composition and effective stress. The fabric of the samples is the most important factor in determining permeability and stress sensitivity of permeability. Coals with the highest permeability are those with at least one well-developed, throughgoing fracture set; these samples generally include abundant vitrite bands. The lowest permeability samples are nonbanded, with an attrital fabric and significant authigenic mineralization. At ©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.1Manuscript received April 1, 1996; revised manuscript received December 4, 1996; final acceptance June 16, 1997. 2Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, BC V6T 1Z4, Canada. This study was financially supported by a grant from CSIRO (Syndal) and by The Earth Resources Foundation, University of Sydney. Samples and some data were provided by Pacific Power Ltd., Sydney. I thank Paul Gamson, formerly of CSIRO, for overseeing the permeability analyses and facilitating some of the study. I thank K. Wright of the Earth Resources Foundation, University of Sydney; J. Enever of CSIRO; J. C. Close of Meridian Oil; and Chris Clarkson of University of British Columbia for their comments on an earlier draft of this paper. Reviewers Andrew Scott, Roger Taylor, and W. Ayers are thanked for their thoughtful comments.