Effects of size and shape of specimens and gas slippage phenomena in the measurement of coal permeability to gas flow

The flow of fluids through porous materials is a subject of importance in the study of the gas emission in coal mines. Air is commonly used as the fluid medium in gas permeability tests. In this case, it is advisable to take into account the existence of gas slippage phenomena. The experimental results showed that the value of the observed permeability is always larger than the true permeability by 25 to 150 per cent.

The geometrical shape of the specimen has no effect on the permeability value. Comparisons were made for cylindrical specimens of one inch, two inch, and four inch diameters versus the corresponding dimensions of the cube specimens.

It has been found that the permeability value of coal increases with an increase in specimen size. The experimental results showed that specimen size of three inches in diameter can be considered as the representative size for coal permeability for the coal tested.