Will greenhouse gas emissions increase with mining depth in coal mines? An analysis of gas occurrence under varying in-situ stress conditions

The rapid development of the economy leads to the high demand for deep coal resources, which further poses the potential problem of deep gas (or methane) emissions. The clarification of deep gas occurrence law for coal mines provides theoretical and data support for methane emission predictions, and assists industrial and mining enterprises in planning targeted emission reduction measures. This study defined and verified the existence of a critical depth for the deep gas occurrence in coal mines based on a multiple-scale case study of how the gas occurrence is associated with depth and stress status changes in the Pingdingshan No.8 Coal Mine. In addition, 882 sets of gas content data from 7 major mining areas in China were collected and their gas content distributions among various depths were statistically analyzed to prove the universal existence of critical depth. The results show that the critical depth of Pingdingshan No.8 Coal Mine is 509 m, and the critical depth of other Chinese areas is about 400 to 1000 m. Significant differences were observed in the pore space, surface, and gas desorption characteristics for coal samples with different depths and stress states. The pore structure in the critical depth area is relatively developed, and gas is easily accumulated. The gas occurrence of both normal and abnormal gas gradually increases with the depth's increase in areas above the critical depth, whereas the gas occurrence gradually decreases for areas below the critical depth, showing that the existence of critical depth lead to significant deviations in gas emission predictions. The results provide a fundamental reference for gas emission prediction, greenhouse effect assessment, and carbon emission factor calculation and indicate that using the traditional linear method may be misleading for evaluating deep gas occurrence and emission.