A novel in-seam borehole hydraulic flushing gas extraction technology in the heading face: Enhanced permeability mechanism, gas flow characteristics, and application
The heading face in soft and low-permeability coal seams is one of the most high-risk areas for Chinese coal and gas miners because of the number of outburst-related accidents. To prevent and control these disasters, the in-seam borehole gas extraction technology is widely used because of its low construction cost. Unfortunately, few studies consider its gas extraction efficiency.
Thus, a new in-seam borehole hydraulic flushing (ISBHF) gas extraction technology is proposed in this paper, where the hydraulic flushing technology is used to enhance the gas extraction of in-seam boreholes. Based on the engineering background in the Xinjing coal mine, a detailed study of this new technology is conducted. First, we model the permeability-increasing mechanism of this technology and the gas flow characteristics of the hydraulic flushing boreholes using a finite-element method. During this process, the strain-softening behavior of the coal mass and the effect of the coal mass failure on the permeability evolution are considered. Second, a reasonable gas extraction scheme is designed, which considers the divergent characteristics of boreholes in the heading face and the interactions among multiple boreholes in the gas extraction process. Finally, the gas extraction scheme is implemented at the S5 intake airflow roadway to verify the simulation results and to evaluate the efficiency of this technology.
The field test results are consistent with the simulation results, which validates our model. Moreover, using this new technology, the number of gas extraction boreholes in the heading face decreased by 50%. Conversely, the gas extraction-based concentration and flow increased 10- and 6-fold, respectively, which results in a 65% decrease in gas extraction time and a 100% increase in monthly excavation length of the coal mine roadway. These results indicate that this new technology is a promising method to achieve efficient gas extraction and rapid excavation in the heading face of soft and low-permeability coal seams.