Evolution of methane permeability and fracture structure of coal: implications for methane extraction in deep coal seams.

A traditional view is that gas in the intact coal is easier to be extracted compared with tectonic coal after stress relief. With the increase in buried depth, however, the structure of coal and the permeability evolution law may change. Thus, the variation of permeability characteristics and the efficiency of gas extraction method at a deeper coal seam need further study. In this paper, the intact coal and tectonic coal in the Qinan coal mine are taken as the research objects. The permeabilities of both coal samples were tested under hydrostatic pressure, and the effective stress was loaded to a high level and then unloaded to the initial value. The differences of fracture structure between the intact coal and tectonic coal were also analyzed. The results show that, after loading and unloading, the permeability recovery rate of the intact coal is lower than that of the tectonic coal, and the fracture loss degree is higher than that of the tectonic coal. The reason is that the contact area between the matrixes of intact coal is relatively small, and the high stress leads to the plastic deformation or fracture of the matrix rock bridge. The contact area between the matrixes of tectonic coal is much larger and has a stronger resistance to compression deformation. The fracture structure of intact coal makes it more difficult to extract gas under high stress conditions. It is necessary to combine various stress relief methods such as protective layer mining and hydraulic punching to expand the fracture aperture, and enhance the permeability and the gas extraction efficiency.