Permeability characteristics of remolded tectonically deformed coal and its influence factors

The characterisations of Tectonically deformed coals (TDCs) are different from those of undeformed coals. Permeability characteristics of TDC and its influence factors should be studied deeply. In this paper, five samples of TDCs in various degrees of deformation and one undeformed coal were selected. The microstructure was observed by using scanning electron microscope. The pore structure characteristics were analyzed through a mercury injection experiment. Permeability was tested by steady flow. In TDCs, the crumpled and cracked microstructures are well developed and the mesopores and macropores account for large proportions. The permeability of TDC is mainly controlled by structural deformation degree, effective stress (or effective confining pressure), adsorption swelling effect and slippage effect. On the premise of only effective stress increasing, the internal space of deformed coal is compressed and the slippage effect is weakened. The both factors contribute to the permeability reducing in an exponential form. On the premise of only pore pressure increasing, the gas seepage channels are narrowed and obstructed because of the adsorption swelling effect and the slippage effect is also weakened. In this case, the permeability will also be reduced. In TDCs, the pores and fractures are strongly developed, the gas seepage channels are markedly unobstructed and the slippage effect is enhanced. Thus, the permeability of TDC is much larger than that of undeformed coal. Generally, the permeability change rate of TDCs is larger than that of undeformed coal and shows a negative correlation with effective confining pressure. The stress sensitivity coefficient of permeability also shows a negative correlation to the deformation degree of coals.