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Volumetric Change
Volumetric strains also have a proportional relationship to the desorbed coal gas. Harpalani and Chen (1992) introduced the following equation:
In the conclusion to the results of their research they stated that: the permeability of coal was drastically increased by decreasing the gas pressure; by decreasing the fluid (gas) pressure the effective stress increases and tends to reduce the permeability, but coal matrix shrinkage influences the permeability and limits the decrease; permeability changes due to coal matrix shrinkage are dependent on the volumetric strain (upper figure).
Results of the research of Ceglarska-Stefanska and Holda (1994) on sorption of various gases by the coal matrix are shown (lower figure).
It is obvious that the volumetric changes to the coal matrix due to Helium gas were negligible, followed by Hydrogen gas which caused 0.05% swelling at 10 MPa, with nitrogen and Argon causing swelling amounts from 0.15% to 0.18%, respectively. The highest amount of swelling was related to Methane, which resulted in about 0.36% swelling. They concluded that by increasing the molecular weight of the gas the percentage of volumetric change would increase. However, one of the parameters, which was not mentioned, is the different affinity of those gases to the coal matrix, this is an important factor which should be taken into account. The figure opposite indicates that there is limited interaction between Helium and the coal matrix, however, coal adsorbs methane immensely.
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