| This paper is based on the geological data of 5211 working face in a coal mine in Shanxi.A two-dimensional model of a single borehole and a three-dimensional model of the pre-gas extraction scheme were established.The mathematical model was built by using Darcy’s law,Langmuir equation and mass conservation law by COMSOL Multi Physic software for numerical simulation.The following conclusions are drawn:1.There is a logarithmic function relationship between the effective extraction radius and the extraction time,and the longer the extraction time is,the greater the effective radius will be,but the growth range of the effective radius will be smaller and smaller,and eventually it will infinitely approach a fixed value.2.Within the drilling diameter range of 75mm to 130mm,the effective extraction radius increases with the increase of the drilling hole diameter,and the variation relationship is R=Ad~B.3.The correlation between the effective drainage radius and the change of negative pressure is not strong,but with the increase of drainage time,the increase of negative pressure can improve the gas extraction amount.4.The distribution law of gas flow field and pressure field in coal seam after 120days of drainage:During the drainage process,the gas flow field in coal seam gradually developed from the original chaotic state to the radial and unidirectional flow field state.5.By comparing the three gas pre-drainage schemes,the feasible gas pre-drainage schemes are as follows:hole radius 108mm,hole spacing 5m,intensive parallel hole arrangement 1.6m away from the bottom plate,and drilling depth 160m and 60m on both sides of the belt groove and track groove respectively.Then,the theoretical formula is used to calculate the gas extraction amount,coal seam residual gas pressure,and the amount of desorption gas under the scheme,and it is confirmed that the gas extraction effect of the pre-gas extraction scheme is up to standard and feasible. |
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