From experimental results, it has been established that the permeability of coal to gas flow is highly dependent on confining stress. This relationship is complicated by the matrix shrinkage and swelling associated with the process of desorption and adsorption of the gas, usually methane in the field situation. In this paper the results of laboratory permeability tests are presented on several low rank coals, subjected to varying confining stresses and gas pressures. The tests were carried out in a modified conventional triaxial cell with the ability to vary the confining (lateral stress) independently of the axial stress. Three different gases were used in the experimentation, methane, carbon dioxide and nitrogen. In each test, various stress conditions were set and the permeability determined at different gas pressures. It was found at low confining stresses the gas pressure had a greater influence on the permeability than at high confining stresses. In some cases at low stress there was a marked increase in permeability as the gas pressure increased. It is possible that this is related to the increase in gas pressure opening the flow channels, thereby increasing permeability. At higher stresses some tests showed the reverse behaviour In that as the gas pressure was increased, the permeability reduced. This type of behaviour could be related the adsorption - swelling effect reducing the flow pathways. At the higher stress ranges there were some samples which exhibited mixed behaviour with an increase at low pressures and then the permeability reduced with further Increase in gas pressure. It is evident from the results that there is no clear relationship between the gas pressure, confining stress and permeability without taking into account effective stresses and swelling/shrinkage effects.
Coal is a complicated medium being composed of a matrix and fracture network. The matrix is a pore structure Containing both micropores >2 nm in size and macropores <50 nm and in its natural environment is under a confining stress. Gas, mostly methane and Water are adsorbed onto the internal surfaces of the coal. The fracture network (cleat) are the avenues for the fluids -gas, water and vapour to flow in coal. By reducing the fluid pressure through pumping or mining the adsorbed gas responds to the pressure and con- centration differential by migrating towards the low pressure/concentration regions along the main flow paths. The interaction between the shrinkage strains from desorption and the applied stress field are further complicated by the changes in the effective stress as determined by the desorbing gas pressure and stress Changes associated with the matrix shrinkage.
Previous research (Somerton et al. 1975, Durucan & St Wards 1986, Harpalani & McPherson 1985, hi George 1993) has clearly established a relationship between the confining stress and permeability. As the confining stress increases, the flow channels (cleat) close up and there is a reduction in permeability.