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.
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.
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