The method of gas permeability measurement in triaxial state of stress by means of triaxial cell KTK I00 was developed the Institute of Geonics. The series of measurement were made to find the parameters influencing the gas permeability. The changes of permeability of coal samples during deforrnation process even after post-failure part were determined. The specimens were tested either in increasing confining pressures (from 5 to 50 MPa) or with constant confining pressure and increasing axial stress.
The knowledge of gas permeability of coal in the stress conditions in massif is of great importance for all works related to rock massif as an environment in which filtration of gases takes place (Durucan & Edwards 1986, Harpalani & McPherson 1985, Somerton et al. 1975). Result of this investigation is very important especially for the judgement of the exploitation possibility of the coalbed methane from coal seams. The methhod of gas permeability measurement in triaxial state of stress by means of triaxial cell KTKI00 was developed in the the Institute of Geonics in 1994 (Konecny & Kozusnikova 1996). The series of measurement was made to find the parameters influencing the gas permeability. It was confirmed, that the increase of confining pressure decreases the gas permeability. However, the mineral composition, diameter of grains, volume of communicative pores or degree of diagenesis influence the gas permeability, too (Konecny 1998, Konecny et al. 1999). In this paper the gas permeability of coal samples in different stress regimes is analysed.
The triaxial cell KTK 100 was used for the laboratory tests for simulation of stress conditions in rock massif. Confining pressure in the triaxial cell is applied by the hydraulic pressure of oil, range of confining pressure was 5 to 50MPa. Confining pressure in triaxial cell was constant during the test. The compressed nitrogen was applied as a reference gas. Upstream pressure of the gas was 3 MPa and it was constant during the measurement. The volume rate of the gas was calculated as the volume of liquid squeezed out by passed through gas (Konecny&Kozusnikova 1996) or was measured by the flowmeters in the range from 5 to 600 cm-min. The schema of the measurement apparatus is shown in Figure I. The cylindrical samples were prepared from coal with diameter 48 mm and high 96 mm.The samples were dried before experiment and the density was calculated. The test specimen were protected by a special rubber which acted against penetration of oil. The set of 27 bituminous coal samples from the Czech part of The Upper Silesian basin (Upper Carboniferous) was analysed (Konecny &Kozusnikova 1999). From the macropetrographic point of view the majority of samples were dull banded coal. The degree of coalification represented by the reflectance of Vitrinite R0 was in range 0,8–1,6 %. The detailed micropetrographic analysis of samples was performed. The most of samples was trimacerite with content of inertinite macerals in range 8.4–49.0%.