Permeability: Effective stress

In situ stress fields and coal permeability are closely interrelated. In fractured reservoirs, such as coal-beds, the permeability of coal is sensitive to stress variation or pore pressure. The permeability of a particular coal was found to depend on the level of the confining stress, for example a permeability of 12 mD at 0.07 MPa confining pressure changed to 0.0035 mD at 20 MPa. Somerton, Soylemezoglu and Dudley (1975) found that increased applied stress in constrained coal samples caused a decrease in permeability of several orders of magnitude, up to the point where microfracturing occurred, and permeability increased again beyond this point.
Limited tests on measuring the permeability of coal have been undertaken on different Australian coals. The measured permeability for the Bulli coal seam under triaxial conditions varies from 0.001 mD (Lingard, Phillips and Doig, 1984) to 0.15 mD (Somers, 1993). Their results also showed that in all tests the permeability decreased with increasing stress. Xue and Thomas (1991) investigated the variation of Australian coal sample permeability with confining stress and changing gas mean pressure. They showed that the permeability of coal increased when the mean gas pressure was decreased. However, the coal samples were one to two orders of magnitude smaller in permeability as the confining stress increased from 1 MPa to 15 MPa.
Somers’s (1993) work on Australian coal permeability found that under 5 MPa confining pressure, Bulli coal samples had an average permeability of 30 mD, with Westcliff coal 21mD, Tower 45 mD, and Tahmoor 17 mD. Ulan coal permeability is around 16 mD. Gray (1995) (ACARP Report C 3079, page 331) also presented a relationship between effective stress and permeability for cores taken from Leichhardt Colliery in the Bowen Basin.