Numerical Simulation Study On Effective Radius Of Degassing Borehole

Gas accident is the most serious disaster in the process of coal mining, and the most beneficial means to prevent it is gas extraction. The research on the variation law of effective extraction radius with the change of each controllable factor, the gas emission law and the interaction effect between boreholes in the process of gas extraction will be of significant importance in coal seam gas government, for which can provide theoretical foundation to the arrangement of gas extraction system.Starting from theoretical analysis, the mathematic control equation for gas extraction has been established based on the theory that the adsorbed gas move as diffusion in the coal matrix, while the free gas move as seepage flow in the fractures.The equation takes coal seam deformation, gas seepage and diffusion and the desorption expansion of adsorption gas process into account, and which would be close to actual gas emission process.By the use of COMSOL software, the thesis researched the change law of effective extraction radius with the change of extraction time, extraction pressure, borehole diameter and the initial permeability of coal seam and the law of gas emission in the process of extraction by single borehole. The simulation results show that, the bigger the distance from borehole is, the smaller the seepage velocity and the longer the time used for seepage velocity maximizing. At the beginning of extraction, the seepage velocity has already maximizing at the place nearby borehole. And the relationship between effective extraction radius and extraction time, borehole diameter and coal seam initial permeability conform to power function, while, the extraction pressure has little or no effect on effective radius.The geometric model of two boreholes extraction has been built to research the interaction effect between boreholes. In the process of simulation, one borehole position remains the same, while the other one position changes according to different borehole arrangement. The results show that, over the same extraction time frame, the smaller the space between boreholes is, the lower the gas pressure in control area of boreholes, and the faster the gas pressure drop velocity. The interaction of boreholes would has great influence on the time required to meet the extraction standard, and the smaller the borehole arrangement is,the shorter the time and the bigger the decrease extent. Theborehole gas flow quantity gets the maximum at the beginning of extraction, and would be decrease with time prolong, which tends to be constant. Meanwhile, the gas flow in one borehole is smaller on the condition of two boreholes extraction than the single extraction, and it will be decrease with borehole arrangement.