| Due to the long time of mining in China and the increase of coal demand,mines have started to expand deeper,resulting in a gradual decrease of permeability of soft coal seams and an increasing impact of gas disasters on coal mining and personnel safety.Gas extraction is recognized as one of the important measures to solve gas problems,among which,increasing the borehole diameter can improve the effect of gas extraction,however,in the soft coal seam,the borehole is very likely to be destabilized by the double effect of gas pressure and ground stress,the larger the borehole diameter,the higher the possibility of destabilization.In this case,increasing the borehole diameter will not improve the gas extraction effect,but also affect the construction progress.Jiahe Mine’s Pu Xi shaft has soft coal quality and low gas extraction efficiency.In order to determine the reasonable hole diameter of Pu Xi well,improve the gas extraction effect,and at the same time prevent the hole from destabilization,this paper takes Pu Xi well of Jiahe Mine as the research object,and uses theoretical analysis,numerical simulation,laboratory experiment and field measurement to analyze and study the reasonable hole diameter under the stable condition of soft coal seam,and the main results obtained are as follows.(1)The principle of instability occurring in the borehole was analyzed;and the distribution law of the plastic zone of the coal body around different borehole diameters was simulated using COMSOL Multiphysics software,comparing the simulation results with the calculation results,it was found that the radius of the plastic zone was a primary function of the radius of the borehole,and the modified formula of the radius of the plastic zone was obtained.(2)Simulation software was used to simulate the distribution of gas pressure around different boreholes.The simulation results show that the effective extraction radius of the borehole increases with the expansion of the borehole diameter,but the rate of increase increases first and then decreases;by analyzing the quantitative relationship between different extraction boreholes and the effective extraction radius,the reasonable borehole diameter of V coal seam in Pu Xi coal mine is determined.(3)An elastic cementing material for coal fracture consolidation was prepared with polydimethylsiloxane,hydrogen-containing silicone oil,foaming agent,straw fiber and polypropylene fiber as raw materials,and the effects of hydrogen content of hydrogen-containing silicone oil,polypropylene fiber and straw fiber addition on the material performance were investigated through experiments,so as to determine the optimal ratio of the material;it was proved through experiments that the elastic cementing material,after being injected into the loose coal body It can increase the mechanical strength of the coal body by bonding the broken coal body,and the consolidated coal body has good elasticity.(4)The critical hole size formula for the drill hole without spraying is derived;after injecting elastic cementing material,no instability of the drill hole occurred during the construction and extraction process,which proves that the elastic cementing material has a good effect on maintaining the stability of the drill hole;by analyzing the results of field tests and considering the safety,economy and difficulty of construction,it is determined that 94 mm is the suitable hole size for the Puxi well under the existing geological conditions.By analyzing the results of field tests and considering factors such as construction safety,economy and construction difficulty,it was determined that 94 mm is the suitable hole diameter for gas extraction borehole in Pu Xi well under existing geological conditions. |
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