Cleats & Joints
Fracturing is the basic element of tectonic disturbance. The process of folding gives rise to fracturing in beds. Cleats are natural fractures in coal. Methane and other coal seam gases will flow out of pores of coal if there is a pressure gradient acting as a driving force and the fractures are sufficiently open or permeable. When the pressure in the coal seam is reduced through mining, gas begins to desorb and migrate through the coal matrix and through natural fractures such as cleat. Natural fractures in coal can be divided into two main classes.
Micro and Macro cleat systems and Joints
Large Joints and shears
Cleats together with the bedding planes result in anisotropic behaviour of coal, both from the geotechnical and gas filtration points of view.
There are two mechanisms for the origin of cleat formation in coal.
Endogenetic cleat: This is formed during the process of physical changes in the properties of coal during the metamorphic process. Coal matter undergoes density changes and a decrease in its volume. These processes are associated with the changes in the internal stress system, compaction and desiccation, and the formation of cleat planes.
Exogenetic cleat: This is formed as a result of the external stresses acting on the coal seam. These include tectonic stresses, fluid pressure changes, folding and development of tensile stresses to which the coal seam is subjected during various time periods.