An experimental and numerical investigation on the deformation of overlying coal seams above double-seam extraction for controlling coal mine methane emissions

Abstract

This study investigates the deformation and the resultant pressure relief of highly gassy coal seams where a double seam mining operation takes place at lower depths at the Wulan Coal Mine, China. In order to predict the depressurization effect in the overlying coal seams, we simulate the extraction process by constructing a scaled model in the laboratory using similar rock materials. Analyses of experimental results concluded that due to the mining-induced stress redistribution, the pressure within target coal seams, which were 109 m above the mined seams, could be fully relieved to attain the statutory approval for gas drainage. In addition to scaled-model studies, computational modeling studies were conducted using UDEC (Universal Distinct Element Code), which showed that the features of deformation resulting from the double-seam extraction were different from that of in the case of single-seam extraction. The results of the numerical studies revealed that in addition to the panel margin on the air return side, areas near the intake entry could also be considered as borehole drilling positions. Besides, it was found that the gob of the extracted seams below the gassy one provided a “buffering effect” for the would-mined protective coal seam and that the depressurization effect was largely weakened. The laboratorial findings are instructive to the field practice of methane drainage. During the mining operation, a “displacement comparison method” was adopted to measure the dilated amount of protected coal seams, and as expected, the maximum dilation percentage was much more than 0.2% — the critical value upon which the target coal seam is appropriately depressurized to allow gas drainage to be safely and effectively implemented.