Shrinkage With Desorption

Shrinkage/Swelling of Coal due to Desorption/Adsorption

This laboratory test provides information on how coal changes dimensionally with adsorption/desorption of gas. This information is required as a component of the process of determining the stress changes that a coal seam will undergo during drainage. The stress path is a key to determining how coal permeability may vary.

The process of the test involves examining the core from a coal seam following desorption by Sigra. Samples are then selected which represent the various plies within the coal seam. As a general rule intact coal core is not available and fragments must be chosen. These are then strain gauged using two rosettes placed on the cleat faces and placed in a pressure vessel which is then evacuated. Gas is introduced into this pressure vessel and held at a number of pressure steps up to reservoir pressure. A single gas type is used and if the gas composition is a mixture of more than 7% of any particular gas, then these gas types should be used individually and the results subsequently combined. Inbetween each step the core is allowed to reach dimensional equilibrium. When the maximum pressure is reached the core is allowed to desorb slowly in a fewer number of pressure steps to obtain confirmation of the coal’s dimensional state.

The process is slow and a single gas adsorption/desorption cycle may take several months to complete.


Example of the dimensional change of coal samples from a particular formation tested with methane and with carbon dioxide.

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