Study on Roof High-Level Borehole Drainage Technology Based on Temporal and Spatial Evolution Law of Gas Migration

A full understanding of the gas migration law in the goaf overlying strata during high-level borehole drainage in the goaf is fundamental for improving the efficiency of gas drainage and preventing gas overrun in the working face. In this paper, by considering the 714 working face of the Qinan Coal Mine as the engineering background, we carried out an numerical simulation to study the gas distribution and the flow field development law when using different hole layout methods and extraction pressures for the roof high-level boreholes. The gas drainage effect of the high-level borehole was found to depend on the traction ability of pressure with respect to the high-concentration gas in the deep part of the goaf and near the residual coal. Furthermore, the distribution of double row holes in the “fracture zone” could effectively pull the high-concentration gas and parallelly improve the gas drainage concentration: An excessively small negative pressure of drainage would easily cause an insufficient drainage capacity of the boreholes, while a too large negative pressure of drainage would produce a “gas concentration gradient strip” (which would influence the gas drainage effect). The double row high-level drilling extraction test on the 714 working face of the Qinan Coal Mine indicated that the extraction effect of high-level drilling was considerably higher than that of the pipe buried in the goaf, in accord with the numerical simulation results. Overall, the results of this paper provide a scientific basis for determining the layout technology of high-level boreholes in the roof.