Research On The Relationship Between The Diameter Of Drilling And The Effect Of Gas Drainage

Control gas disaster has been the focus of coal mine safety, and gas drainage is oneof the main measures in coal mine gas control technology. In recent years, gas drainagevolume increased steadily year by year in China, just less than the volume of UnitedStates. Despite this, comparing with the requirements of gas control governance, the gasdrainage rate is relatively low in China, and the mismatch of drainage system is one ofthe main factors restricting the ascension of gas drainage rate. With being aimed at thisproblem of Jiulishan Coal Mine and through the way of theoretical studies, numericalsimulation and on-the-spot investigations, this paper’s observation played a particularemphasis on the impact of different borehole diameter to the effect of pre-pumping gas.And it studied the quantitative relationship between the pre-pumping gas effect and theborehole diameter, explored a reasonable pre-pumping gas drilling diameter andprovided a scientific basis for the optimization of the drainage system.This paper first studied the form of gas storage in the coal body, adsorption theoryand the law of flow. It also analyzed the main factors of coal’s effect on the gasadsorption and the gas flow in the coal body. On the basis of theoretical studies, byconstructing gas migration equations and with the physical parameters of theexperimental site, this paper had simulated the changes of the gas pressure field aroundthe borehole under the condition of different borehole diameter through using theCOMSOL software. The results showed that the magnitude of the coal seam gaspressure’s reduction is getting smaller and smaller with increasing drainage time, andthe longer the drainage time, the smaller the difference of the magnitude of that arounddifferent borehole diameters. And with the increase of the borehole diameter, the rangeof gas pressure’s reduction around the borehole is getting wider, and it also increases themagnitude of surround coal seam gas pressure’s reduction.On the basis of the numerical simulation, the paper determined the relationshipbetween the borehole diameter and the drainage effect under the condition of differentdrainage time, and with the practical situation of Jiulishan Coal Mine, it selected asuitable drainage borehole diameter.