Changes in Coal Microstructure and its Influence on Gas Production from Coal

This paper summarizes the results of an experimental study conducted to estimate the impact of physical changes in coal microstructure associated with flow of gas on the flow behavior of coal. The coal permeability and diffusion coefficients were found to change with continued flow. The permeability increased substantially, while the diffusion coefficient decreased. The variation is attributed to the shrinking of the coal matrix resulting from gas desorption and the associated increase in surface energy of coal. The compaction of coal matrix results in a change in pore sizes within solid coal, affecting its diffusion property. It also results in increased cleat aperture, and hence, in increased permeability. The increase in permeability results in increased gas production, a finding that is in agreement with field observations in several US basins. The diffusion property, on the other hand, affects gas production from gob wells as well as gas emission rates while coal is being transported after being mined, and after it is in stockpiles outside the mine.