COAL MICROSTRUCTURE AND MICROPERMEABILITY AND THEIR EFFECTS ON NATURAL-GAS RECOVERY

Natural gas (methane) production from coal seams rather than from porous sandstone reservoirs is now recognized as a valuable and recoverable energy source in the USA, Australia and the UK. The Permo-Triassic Bowen Basin, Queensland, Australia possesses well defined coal seams which contain major methane resources. However, commercial gas production to date has been hampered by the low permeabilities of coal seams. Recovery of this valuable resource will be assisted by a fundamental understanding of the microstructures in coals and their influence on transmissibility of methane. This study examines coal microstructure (e.g. micrometre-sized fractures and cavities) which vary in width from 0.05 to 20 mum. These microstructures fall within the limits of meso-and macroporosity. Scanning electron microscope examination of the microstructure shows three porosity types: fracture porosity, phyteral porosity and matrix porosity. Fracture porosity is generally associated with bright coals although microfractures are present in maceral fragments from the dull coal layers. Characteristically the macro- and microfractures form a continuous structural fabric through the bright coal layers. In contrast, phyteral and matrix porosity is associated with the dull coal layers that are composed of plant fragments or a heterogeneous mixture of macerals. The continuity of the observed micrometre-sized fractures and cavities suggests that they make a significant contribution to overall permeability, and therefore play a major role in the transmissibility of methane at a level between diffusion at the micropore level and laminar flow at the cleat level. The effectiveness of gas drainage through the observed microstructures, however is likely to vary according to: (1) the type of microstructure present in each coal type; (2) microstructure density, orientation and continuity; (3) the amount of infilling in the voids; (4) the degree of coalification; and (5) the presence or absence of clay layers in the coal seam. This is important, as a knowledge of the microstructure system and its relationship to coal type and coal rank plays an important consideration in gas drainage modelling.