Permeability: degree of fracturing

Coal seams have natural fractures, known as cleats. Cleats act as a major transport system for gas and water flow within a coal seam. There are two sets of cleats in coal, face and butt cleats. Face cleats are longer than butt cleats, hence directional anisotropy in coal permeability results from this phenomenon. Permeability of coal increases with cleat density and cleat width. The flow capacity of fractured media depends almost entirely on the number and width of fractures and their continuity (Dabbous et al. 1974). Lingard, Phillips and Doig (1984) showed that the dimensions of fractures influence coal permeability. The greater the fracture, the higher is the permeability of the coal. Flow through cleats is generally laminar flow.

Information relative to the cleat size and spacing in coal are useful in predicting permeability,and generally the larger the cleat size and cleat density, the higher the permeability. Secondary cleats also occur in coal as a result of induced stress and changes to coal geological structure or mining. These fractures normally cause permeability to increase, but sometimes they do the opposite and reduce the permeability, and such situations tend to occur in shear zones or near magmatic intrusions. According to Hayes (1982) permeability in the fractured and crushed zone ahead of the face side during mining is greater than permeability in the intact and solid coal area.