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

Volumetric shrinkage associated with gas desorption from the coal matrix has a significant influence on the stress field. This is of particular practical importance around headings in underground coal mining, as it controls thr 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 sample from the South Island, New Zealand. The specimen was initially saturated with the gas to a pressure of 4 MPa. The volumetric strains were recorded during the adsorption and desorption processes. The overall shrinkage coefficient, C-m, of coal matrix was found to be 1.2 x 10(-3) MPa-1 for methane 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 elastic behaviour of the sample with some effective stress recovery associated with the incremental change in pressure. The elastic properties were determined from a uniaxial test on the sample. From this data and using Terzaghi's equation, a relationship was established between the effective stress and 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 to estimate permeability. (C) 2001 Elsevier Science B.V. All rights reserved.