An investigation into the stress-permeability relationship of coals and flow patterns around working longwall faces
This research aimed to establish a physical relationship between applied stress and permeabilities of different coals. Seven different coals, ranging from medium volatile to high volatile bituminous, were tested for stress-permeability relationship under simulated subsurface stress conditions. Prior to the experimental investigations, the stress conditions around a working longwall face were considered in order to achieve an accurate simulation of the stresses experienced underground. Laboratory stress-permeability experiments were carried out by passing nitrogen gas through a triaxially stressed cylindrical coal specimen. A slightly modified conventional triaxial testing apparatus was used for this purpose. The stress conditions employed simulated the stresses created in the front abutment zone, the crushing zone, the stress relief zone and the recompaction zone of a working longwall face. A number of specimens of the seven different coals were tested under such stress conditions and stress-permeability curves were obtained for each specimen. The effect of moisture and the direction of gas flow in relation to the direction of bedding planes and major fracture lines were also considered in laboratory investigations. A relationship between the stress-permeability behaviour and the rank of coals used was established. Combining the general pattern of stress-permeability behaviour obtained in this research together with the stress conditions created around a working longwall face a model was produced which presents the stress-permeability profiles of coal seams in the vicinity of the workings. From these profiles it was possible to suggest the flow patterns of gas around working longwall faces.