Effect of depth on the sorption capacity of coals affected by outburst hazard

Abstract

The aim of the presented research was a quantitative evaluation of the burial depth impact on the methanesorption capacity of outburst coals inrelation to present-day temperatures and coalification degrees. Theavailable for sampling coal seams of Zofiówka mine in the Upper Silesian Coal Basin (USCB) were tested. Coal samples of R0 from 0.98% to 1.25% were taken from average burial depths between 678 and 863 [m] which covered a range of deposit temperatures from 33.0 to 38.4 [°C]. Methane sorption isotherms were performed for all the samples at temperature from 25 °C to 55 °C with interval of 10 °C. Sorption results up to apressure of 1.2 MPa were approximated with the Langmuir model. Overlapping effects with increasing depth were measured: (i) an increase in deposit temperature and (ii) an increase in degree of coalification. Within the considered depths, the sorption capacity of coal decreases together with depth increase. It was shown that degree of coalification havea dominant effect (about 89%) on the reduction of the sorption capacity and the effect of geothermal gradient is only slightly more than 10%. Reduction of the sorption capacity with depth was shown to be unfavourable from the point of view of the risk of outbursts. It contributes to an increase in the seam methane pressure and an increase in free methane content in total methane content (the value of w/TMC). Reduction of the sorption capacity with depth leads to the increase of desorbable methane content (DMC) and thus influences the scale of the potential outburst.