Year

2005

Degree Name

Doctor of Philosophy

Department

School of Civil, Mining and Environmental Engineering - Faculty of Engineering

Abstract

In recent decades, the subject of coal and gas outburst in underground coal mines has been a focus of interest in Australia and worldwide. Much of this interest has been the result of the alarming increase in outburst related incidents and associated fatalities world wide, particularly in China, Russia, Ukraine and other major coal producing countries. Australia, on the other hand, has seen a relative decline in outburst related incidents, for the last three years no outburst related incident has been reported. Effective gas drainage programmes, better management of outburst prone zones, tougher regulations and a continuing programme of dedicated vigilance and research have collectively contributed to improvements in outburst prevention. Still, however, difficult problems remain to be addressed. Geological disturbances such as dykes, shear planes and increased mineralisation can influence coal permeability and porosity. These disturbances must be fully understood in order to develop an effective on gas drainage programme and reduce outburst risks. Accordingly, a programme of laboratory studies was undertaken to investigate the relationship between coal composition, coal volumetric change and coal permeability. Coal samples for this study were obtained from four different coalmines in Australia (Tahmoor, Metropolitan, Dartbrook and North Goonyella) and Tabas coalmine in Iran. Coal samples were tested in different types of gases and under different gas pressures and stress conditions. Coal permeability tests were conducted in the gas pressure chamber of the multi-function outburst research rig (MFORR), and the volumetric change tests were carried out in a modified pressure bomb. Microscopic studies provided a better correlation between coal composition permeability and shrinkage characteristics. A numerical model was developed to simulate single gas flow through a coal sample. The simulation further supported the experimental studies. The petrographical tests showed that most of the Australian coals tested were inertinite rich coals. The mineral matter in the Australian coal samples were mostly carbonate (calcite) and clay, but the Iranian Tabas coal had pyrite as the dominant mineral matter. Tabas coal has the highest vitrinite concentration (70%) and lowest proportion of inertinite elements (8.18%), the lowest vitrinite content was obtained from North Goonyella coal. There was a definite correlation between coal composition, coal volumetric change and coal permeability. Volumetric strain changes during the adsorption stage in all gas environments were greater than the volumetric strains in the desorption stage. The level of coal shrinkage was affected by the type of gas desorbed. Carbon dioxide appears to have the greatest influence on the matrix and nitrogen the least. The permeability of coal was also influenced by the gas type and pressure. Greater gas permeability was obtained in N2 gas, and the lowest permeability was obtained in a CO2 environment. The sorption characteristics of CO2 are a major factor. The degree of coal permeability is reduced exponentially by increasing the applied stress and also by increasing the confining gas pressure, irrespective of the gas type. The permeability tests showed that with an increased inertinite content the permeability of coal increased, except in the case of Tahmoor Colliery 900 Panel which showed a decrease. Comparison of the Tahmoor coals from 800 and 900 Panels showed that the permeability of coal was influenced by the mineral content and the carbonates, as well as the cavities. In particular; there was a reduction in coal permeability with increasing mineral content and carbonate content of the coal. With an increase in the percentage of inertinite, the permeability of coal increased. The numerical modelling provided an opportunity to quantify the flow mechanism in coal. It was possible to simulate the flow duration across the coal samples as a function of time with different gases and coal types. It was recommended that the study be extended to include more coal deposits and coals with different geological variations, so that an effective data bank can be established for Australian coals.

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