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Abstract The effects of size effect，temperature and stress on the permeability characteristics are analyzed by the experiments. The results show that for homogeneous samples with the same diameter and different lengths，the error of testing value of permeability rate induced by the size effect is less than 10%，while for the heterogeneous coal core，the error is 4% to 40%，or even higher. The development degree of microcrack is the main factor that leads to permeability rate to change greatly. Under loading and unloading of confining pressure，the permeability rate of coal samples with small diameter is more sensitive than that of large-size coal samples；and the permeability rate is more sensitive to the stress than the porosity. When the coal body experiences higher confining pressures，plastic deformation occurs for the pore structure，the coal suffers permanent irreparable damage，and the porosity and permeability rate cannot restore to the initial values. After the coal experiences high temperature，big holes are compressed，but more micropores are developed. Meanwhile，the specific surface area of the coal increases，and the strength decreases. For the confining pressure，there is a critical point. When the confining pressure reaches the critical point，the permeability rate will be greatly reduced. The permeability rate change of raw coal and shaped coal under loading and unloading is consistent，which is an exponential function of the confining pressure.
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Received: 05 January 2012
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2012, Vol. 31 
