For extra-thick coal seams, slice mining is a safer mining
method
than top coal mining, which can effectively reduce the strong mine
pressure behavior caused by mining. However, in the slice mining of
high-gas and extra-thick coal seams, the gas in the lower slice flows
into the goaf, which increases the gas control difficulty on the upper
slice working face. It is easy to cause the gas transfinite at the
upper corner in the upper slice and reduce the mining efficiency.
Therefore, it is of a great significance to carry out the research
on gas control technology in slice mining of the extra-thick coal
seam. There are some problems in the gas control of slice mining,
such as a single gas control method, low control efficiency, and unclear
gas migration law. Therefore, it is necessary to study the gas migration
law and propose a targeted prevention and control the technical scheme.
In order to improve the gas control efficiency of the extra-thick
coal seam, the evolution law of permeability of the lower slice is
obtained under mining through experimental research. The liquid–solid
coupling seepage-flow model for gas migration is established in the
lower slice. Comsol Multiphysics software is used to study the migration
law of pressure relief gas in the lower slice. Based on the gas migration
law, the gas extraction and cut flow technology for the lower slice
long borehole is proposed. Through this technology, the amount of
gas flowing into the upper slice goaf and the gas content of the lower
slice are reduced, and the drilling horizon is optimized. The research
results show that the determination of the optimal drilling horizon
of the lower slice needs to balance the amount of gas flowing into
the goaf and the total amount of gas extraction. The range of 3–7
m horizon in the lower slice is appropriate to the boreholes arranged.
When the borehole is located in the lower slice −3 m horizon,
the 360 day gas emission quantity of goaf can be reduced to 51.2%
of the nondrilled emission quantity, and the total extraction amount
is 1143 m
3
. When the borehole is located in the lower slice
−7 m horizon, the 360 day gas emission quantity of goaf can
be reduced to 95.31% of the nondrilled emission quantity, and the
total extraction amount is 1461 m
3
. Considering the gas
emission capacity of the upper slice and ensuring that the total extraction
volume of the lower slice is maximized and the boreholes in the lower
slice are not damaged, the boreholes are located in the −6
m horizon of the lower slice.