In this study, the permeability tests of high gas pressure have been conducted in order to make clear the mechanism of irreversible change in gas permeability of coal. Practically, the opening behaviour of permeable cracks caused by action of pore-pressure on coal specimens under a constant hydrostatic pressure was monitored through the change in gas permeability and acoustic emission activity.
As these monitored results, the irreversible changes in the gas permeability and the opening rate of permeable cracks occurred according to the change of the amplitude-frequency distribution of acoustic emission as the effective pore-pressure became higher than the crack strength defined by the equation (4). After these irreversible changes, the relations between gas permeation rate and applied gas pressure changed to a straight line intersecting the origin and the opening of permeable cracks became invariable. From these opening behaviour of permeable cracks, it became clear that the irreversible change in gas permeability was caused by unstable crack propagation at a closing point of permeable cracks in which the permeative resistance acted selectively and effectively.
