Microscopic characteristics of microfractures in coals: an investigation into permeability of coal

This study aims at identifying the microfractures characteristics and their significances to gas permeability in coals. The microscopic measurements of 189 polished coal surfaces show that there are commonly two kinds of microfractures: the exomicrofracture formed by tectonic stress and the endo-microfracture formed within certain macerals as the result of peat formation and coalification. The exo-microfractures are commonly densely developed, with typical textures of irregular dendritic, filamentous, turtleback, and typical frequency of hundreds per 9 cm2. Comparably, the endo-microfractures commonly have the frequency < 100 per 9 cm2. It has been confirmed that endo-microfractures are generally developed with regular texture, good connectivity and orientation, and thus are favorable for gas permeability. Some overdeveloped exo-microfractures (>200 per 9 cm2) with filamentous texture are mostly formed by intensive tectonic stress, and are indicators for “outburst-prone coals”. The highly microfractured coals have poor connectivity and orientation, and thus are unfavorable for permeability and gas production.