Characteristics and origins of coal cleat: A review

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Cleats are natural opening-modefractures in coal beds. They account for most of the permeability and much ofthe porosity of coalbed gas reservoirs and can have a significant effect on thesuccess of engineering procedures such as cavity stimulations. Becausepermeability and stimulation success are commonly limiting factors in gas wellperformance, knowledge of cleat characteristics and origins is essential forsuccessful exploration and production. Although the coal–cleat literature spansat least 160 years, mining issues have been the principal focus, and quantitativedata are almost exclusively limited to orientation and spacing information. Fewdata are available on apertures, heights, lengths, connectivity, and therelation of cleat formation to diagenesis, characteristics that are critical topermeability. Moreover, recent studies of cleat orientation patterns andfracture style suggest that new investigations of even these well-studied parameterscan yield insight into coal permeability. More effective predictions of cleatpatterns will come from advances in understanding cleat origins. Although cleatformation has been speculatively attributed to diagenetic andror tectonicprocesses, a viable mechanical process for creating cleats has yet to bedemonstrated. Progress in this area may come from recent developments infracture mechanics and in coal geochemistry.

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