Abstract

Protective seam mining is one of the most commonly used technique to realize stress relief and permeability enhancement of reservoirs, so as to improve gas drainage efficiency. However, for coal reservoir with a hard roof, such as the tight sandstone, magmatic rocks, the distribution and evolution of destressed zone and gas field have not been fully understood, which leads to an inefficient gas control in the field. In this work, taking the geological background of Pingmei 12th Coal Mine as an example, we numerically investigated the evolution and distribution of stress, permeability and gas field in the hard-roof stope during protective seam mining. The results show that in the zone with a distance greater than 3.7 m, stress in the hard-roof stope is significantly higher than that in the conventional stope and the permeability enhancement ratio in the hard-roof stope is significantly lower than that in the conventional stope. In the overlying strata, the existence of the hard roof inhibits the upward extension of the fractured zone, and the range of the fractured zone in the hard-roof stope is obviously smaller than that in the conventional stope. Therefore, in the hard-roof stope, the relieved gas accumulates in the gob, instead of entering the overlying fractured zone, which may put a threat to the mining safety. To solve this problem, the presplitting blasting technique was applied to cut the hard roof. After the collapse of the hard roof, the overlying fractured zone is fully developed, and the gas accumulated in the gob enters the fractured zone driven by the concentration difference and buoyancy. This can not only prevent gas overrun, but improve gas drainage efficiency of the upward borehole. This practice could be used as a valuable example for other coal mines with similar geological conditions.

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