Analysis of water inrush from boreholes for drainage of confined aquifer by upward boreholes in underground coal mining face

Drilling holes into the roof aquifer of coal mining face to reduce groundwater in the aquifer is one of the main measures to control the roof water hazard in coal mines. The hydrogeological parameters of aquifer (water pressure,permeability,aquifer thickness,etc.) and the structural parameters of borehole (borehole inclination,depth,borehole diameter,etc.) are the main factors affecting the water inrush law of hydrophobic boreholes. According to the law that the dynamic change of borehole water inflow is in accord with exponential attenuation,and the transformation characteristics of groundwater flow pattern between aquifer and borehole,with COMSOL software as the simulation analysis platform,this paper presents the construction method of numerical analysis model of borehole water inrush in coal mining face,which generalizes the drained aquifer into infinite water-filled aquifer,uses finite element triangular tetrahedron division,generalizes borehole wall into Darcy seepage overflow boundary,and calculates borehole water inrush through real-time Darcy velocity integral in borehole wall area. Based on the single borehole drainage test of a coal mining face in Binchang Mining Area,Shaanxi Province,the simulation results show that when the borehole water inflow reaches a quasi-stable state,the groundwater head isosurface takes the borehole as the center and presents a"reverse funnel" state. The attenuation curve of borehole water inflow and the first-order single exponential attenuation mathematical model are fitted,the attenuation curve of the water inflow of the drainage borehole is basically consistent with the measured results,which are in accordance with the first-order single exponential attenuation mathematical model. In addition,the multi-scenario simulation studies the single borehole water inrush attenuation law under the conditions of different aquifer hydrogeological characteristics,and structural parameters. The results show that with the increase of initial water head and the permeability coefficient of aquifer,the peak water volume and seemingly stable water volume of boreholes increase linearly,but have little effect on the seemingly stable period of borehole water inflow. With the increase of elastic water supply coefficient of aquifer,the peak water volume of boreholes increases,and the seemingly stable water volume basically remains unchanged,but the seemingly stable period increases obviously,with the increase of borehole depth and borehole diameter,the peak value and quasi-steady water inflow of hydrophobic boreholes also increase. Moreover,the paper takes a computer workstation as hardware platform,and constructs a computer model of group boreholes interference drainage. The simulation results show that with the increase of the number of drainage boreholes and the coverage of the boreholes,the range of the water head is reduced obviously,and the water inrush capacity of the single hole and the drilling field decreases obviously,among which the water inrush capacity of the central hole in the drilling field is the mostly affected.