Shrinkage of coal matrix with release of gas and its impact on permeability of coal

Adsorption/desorption isotherms were established for powdered coal samples. Gas pressure-permeability relationships for cylindrical specimens of coal, under triaxial stress conditions, were also determined. Gas pressure-volumetric strain relationships were established using strain gauges on the same specimens. The results indicate that the permeability of coal to methane increases with decreasing gas pressure, in spite of increased effective stress. The primary reason for this increase in permeability is the shrinking of the coal matrix, which is associated with desorption, thus enlarging the gas flow paths. The volume of the coal matrix shrinks by ≈0.4% when the gas pressure falls from 6.9 MPa to atmospheric pressure. The results suggest that higher flow rates can be expected as a consequence of the shrinkage, and the associated enhanced permeability, over the life of methane-producing wells in coalbeds.