The change in effective stress associated with shrinkage from gas desorption in coal

Volumetric shrinkage associated with gas desorption from the coal matrix ha s a significant influence on the stress field. This is of particular practi cal importance around headings in underground coal mining, as it controls t hr permeability to gases and will affect the strength of the strata. In this paper, the volumetric changes of the coal matrix were monitored for four different gases (methane, nitrogen, carbon dioxide and helium) on a s ample from the South Island, New Zealand. The specimen was initially satura ted with the gas to a pressure of 4 MPa. The volumetric strains were record ed during the adsorption and desorption processes. The overall shrinkage co efficient, C-m, of coal matrix was found to be 1.2 x 10(-3) MPa-1 for metha ne and 5.2 x 10(-3) MPa-1 for carbon dioxide desorption. A series of loading/unloading cycles applied by gas pressure were performed on the specimen using a non-adsorbable gas (helium). The results showed el astic behaviour of the sample with some effective stress recovery associate d with the incremental change in pressure. The elastic properties were dete rmined from a uniaxial test on the sample. From this data and using Terzagh i's equation, a relationship was established between the effective stress a nd gas pressure. It was also found that the slope of valumetric strain rate and gas pressure is a function of coal permeability, which could be used t o estimate permeability. (C) 2001 Elsevier Science B.V. All rights reserved .