Investigation of Uneven Gas Emission Mechanisms with Hard Roofs and Control Strategies by Ground Fracturing

The permeability of a coal seam is a crucial factor in coal seam gas extraction. Poor permeability of coal seams can lead to difficulties in over-pumping as well as high gas emissions after mining. This issue is particularly prominent when mining extra-thick coal seams with hard roofs, and it is the major problem that restricts the safe and efficient mining of coal seams. In the context of extra-thick coal seam mining in the Datong mine area, field investigation into the gas emission patterns of the working face reveals that the volume of gas emissions correlates closely with variations in working face pressure, demonstrating a high degree of consistency. The mechanism of irregular gas emission was analyzed, and the influence law of roof breakage on gas emission was obtained. It was found that roof breakage will aggravate gas emission. As a result, an integrated control technology involving “ground fracturing + gas extraction” was innovatively proposed. Based on the characteristics of ground fracture network, the mechanism of pressure relief and permeability enhancement of fractured wells and the characteristics of full time and space extraction were analyzed. Using the 8101 and 8204 working faces of the Tashan Coal Mine as a case study, the results demonstrated that vertical well fracturing of the 8101 working face enabled gas extraction 150 m ahead, with an accelerated increase in gas concentration within a 40 m range. Similarly, the horizontal well of the 8204 working face served as a drainage well after fracturing. Gas concentration at the mining position 50 m away from the horizontal well increased rapidly, and the gas extraction rate stabilized at approximately 30 m3/min. The approach effectively mitigated the problem of uneven gas emission caused by gas accumulation and roof fractures in the working face. Ground fracturing not only reduced the area and intensity of stress concentration in the advanced coal body but also enhanced gas emission. Furthermore, the fracturing well served as a gas drainage well, improving the control and achieving positive application results.