Occurrence law of coal seam gas under influence of normal fault zone in coal measures

The coal measure fault zone generated by the superimposition of the polytropic movement and multiple structure has important effects on the coal seam gas occurrence. The method of combining the geologic analysis, COMSOL Multiphysics numerical simulation and field data monitoring was adopted to analyze the interaction relations between the stress distribution characteristics and faults of the normal fault zone in the coal measure, study the coal seam permeability variation characteristics influenced by the normal fault zone in the coal measure, and simulate the gas migration status and concentration distribution condition after the formation of the normal fault zone. The coal seam gas occurrence law was further analyzed and obtained based on the monitoring results of the gas content and the gas outflow. As indicated by the study result, the stress concentration of the coal measure fault zone was mainly distributed on the fault plane of the coal seam. The stress drop occurred at the intersections of rock strata, coal seams and fault planes. The coal seam permeability in the affected area of the coal measure fault zone was in the order of large to small: the fault plane、average value in the area、upper wall of the fault、lower wall of the fault. The coal seam gas concentration was decreased gradually with the simulation time. The gas concentration decrease rates of the internal graben, horst and step fault in the normal fault zone of the coal seam were slightly higher than those of the boundary fault blocks at the both sides, and the gas showed obvious accumulation characteristics in the faults and fault blocks and the boundary outside the normal fault zone. The gas content in the normal fault zone of the coal measure and the average volume fraction of the return gas during stoping were 2.592 1 m³/t and 0.224 0%. The average values at the both sides of the boundary outside the fault zone were 4.480 2 m³/t and 0.454 9%. Accordingly, the new gas-concentrated areas were formed at the both sides of the normal fault zone in the coal measure.