Effect of compressive stresses on permeability of coal: Experimental and modeling estimates

Research findings on permeability of fractured coking fat coal from Tikhov Mine are presented. It is shown that rock pressure has considerable effect on coal permeability under various gas pressure gradients. In particular it is found that an increase in the triaxial compression from 1 to 8 MPa reduces coal permeability by 8-1 times if the gas pressure gradient is 0.67-1.34 MP/m and by 6.1-7 times if the higher gradient is equal to 2-3.3 MPa/m. Parameters of cleats and sizes of joints (blocks) in coal matrix are studied before and after loading of coal specimens. It is obtained that the size of the coal matrix blocks in the unloaded state is 530 mu m at the width (opening) of cleats of 12.7 mu m, which decreases by 1.8 times under the triaxial compression equal to 8 MPa. The measured parameters of cleats and coal matrix blocks are used to assess permeability of coal using theoretical models relating coal permeability with porosity or effective stress (compression) of coal. The comparison of the theoretical estimates and experimental studies of fractured coal shows that the measured permeability of coal under compression from 1 to 5 MPa agrees with the estimates by the permeability-porosity model. If the compression is higher than 5 MPa the estimates by different models coincide. It is found that the highest disagreement between the measured and calculated permeabilities takes place at the low gradient of gas pressure which is probably connected with the Kinkenberg effect.