We have developed the first experimental acoustic properties measurement during gas-hydrate formation and dissociation in crushed bituminous coal samples. We also compare the results with acoustic properties measurements during freezing and thawing water in the same samples. The results show a more complicated behavior that differs from similar experiments with sand. For the samples with adsorbed water, it does not freeze at anticipated temperatures, but acoustic velocities gradually increase with the decreasing negative temperatures. It is caused by complicated pore surface structures causing partial formation of ice/hydrate from bound water at different temperatures. We also observe that, for the same samples, the acoustic properties change significantly, becoming stronger during gas-hydrate formation than during freezing. We explain it by competitive sorption of methane and water in the coal pore space; methane under pressure replaces part of the adsorbed water from micropores so that this water can easily form hydrate. The difference in the acoustic properties of frozen and hydrate-bearing coal samples is important for developing seismic methods for geophysical characterization of coal seams.

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