Hierarchical occurrence law of gas content in deep coal seams and its relationship with outburst prevention

The prediction indexes currently used in China for coal and gas outburst area include gas pressure and gas content, which exhibit varying sensitivity to outbursts in different mining areas. The formulation of rational prediction indexes for outburst risk is based on understanding the mechanism of coal and gas outbursts. According to the “internal gas controlled theory”, it is inferred that under deep conditions, the sensitivity of gas content is higher than that of gas pressure. To verify this inference, this study selects Pingmei No.8 Coal Mine as a typical research site due to its high temperature and high pressure conditions. These conditions result in a different occurrence pattern of gases in deep coal seams compared to shallow ones, where below a “critical depth”, the gas content exhibits negative growth leading to a step-like occurrence pattern, distinct from the linear increase observed in gas pressure. Using a combination of theoretical calculation, experimental analysis, numerical simulation, and on-site verification, the hierarchical occurrence law of gas content in Pingmei No. 8 Mine was first theoretically calculated, and a s solid-flow-heat three-field coupling model considering the competition effect of temperature and pressure was established to analyze the influencing factors of the reverse decrease of gas content with depth. Furthermore, the differential occurrence patterns of gas content and gas pressure during geological exploration/mining periods were compared, verifying the research results of theoretical research and numerical simulation. Additionally, the vertical distribution pattern of outburst energy within the study area was theoretically analyzed while verifying step-like distribution characteristics through positive lateral verification using data on emitted gases amounts and documented outburst accidents cases. Finally,the differences between sensitivities towards outbursts were compared and analyzed between deep conditions for bothgas pressureandgascontent,andthe underlying reasons behind these differences are clarified. The research results of this paper verify the correctness of “internal gas controlled theory” from a macro perspective, and have certain guiding significance for the prevention and control of deep coal gas dynamic disasters.