Laboratory studies on coal permeability with particular reference to mixtures of seam gases

Gas flow in coal was studied in the laboratory to investigate thevariation of coal permeability with changes in seam gases andconfining stress with the aim of better understanding the properties ofgas flow in coal seams.In addition to some modifications of the experimental apparatus, anextensive gas flow measuring and monitoring program in coal sampleswas carried out in a five-cell semi-automated triaxial stress test rigdeveloped jointly by the Department of Mining Engineering and theDepartment of Industrial Chemistry at the University of New SouthWales.Three gases, namely, nitrogen, methane and carbon dioxide, were usedto investigate the effect of gas adsorption on permeability of coal fromthe Bulli seam. It was found that this coal has the highestpermeability to nitrogen, methane has an intermediate value andcarbon dioxide has the lowest value. This sequence is inverselyproportional to the adsorption capacity of coal to each gas. Thisconclusion can also be applied to the samples which were subjected totriaxial stresses.The experimental results showed that the value of coal permeabilityranged from 0.0029 mD to 10.68 mD without confining stress and thatthe Klinkenberg effect applied well to coal samples in a range of meangas pressures from 0 to 20 atm. The effect of triaxial stress on coalpermeability is quite obvious from the experimental results. Most ofthe coal samples had one to two orders of magnitude decrease inpermeability as the confining stress was increased from 10 atm to 150atm.The relationship between the permeability of coal to a mixture ofmethane and carbon dioxide, and its permeability to methane andcarbon dioxide separately was established theoretically. It was foundthat the difference between theoretical calculations and experimentalresults was less than 15%