Study On Fracture Field And Gas Flow Field In "Three Soft" Coal Seam

With the promotion and application of mechanized mining technology,the production is improved and the forward speed is promoted by the technology,meanwhile,mine gas emission seriously restricts the high production and rapid coal mining,which restricts the development of China's coal industry to a great extent.In order to solve the problem of gas emission in coal mine under the background of "Three Soft" coal seam,the study on the mining fracture field and gas flow field law of the "Three Soft'" coal seam are the theoretical basis for the pressure relief gas control.In this paper,the gas parameters of the main coal seam are measured,and the attenuation coefficient of the gas flow in the coal seam is P = 0.2269296,which is much larger than the critical value of 0.05(0.003-.05 d-1).The 5#coal seam in the experimental coal mine belongs to the difficult gas drainage coal seam;The initial velocity of gas emission in the working face is between 7.05—11.0,and the critical value of 10.0 is within this range;The porosity of coal is 11.63;the strength coefficient of coal seam is 0.5411;the average gas content of coal seam is 3.0678 m3/t,the relative gas emission quantity of the working face is predicted to be 2.288 m3/t by using the method of sub source prediction,and the absolute gas emission quantity of the working face is 11.122 m3/min according to the daily output of 7000 t/d.Based on the physical similarity simulation experiment and JDEC numerical simulation,the law of overburden fracture?the development of fissure?the evolution law of spatiotemporal evolution and the distribution form and the pressure relief range and characteristics are analyzed.The results of the rocking and the collapse of the strata show that the average step distance is 8m in the working period,and after the seventh cycle of the working face,the main key stratum is broken,the fractures are closed and the surface subsidence forms a basin,mined-out area is gradually fully compacted,goaf overlying in the mining process basically tend to be stable.The fractures of the seams of 17?9m from the bottom of the coal seam are more developed,and the fractures are below 110m.In the range of 95m?110m,the overlying strata will only bend and sink,and no fracture failure.The maximum height of caving zone in working face is 8?16m,the height of the fissure zone is 45?65m,and in the caving zone,the rock mass is fully broken and the fracture is developed.According to engineering survey,combined with the physical similarity simulation experiment and the UDEC numerical simulation results,a three-dimensional trapezoidal ANSYS FLUENT physical model based on overlying rock migration law is established on the basis of the model hypothesis.By setting the model boundary condition,numerical calculation is carried out when the stope does not take any measures,maximum gas concentration reached 53.10%.The maximum gas concentration and the diameter of the borehole can be seen by comparing the diameter of the upper drilling with different diameter of 0.073m,0.089m,0.094m,0.113m and 0.153m respectively,the concentration was negatively correlated with the borehole diameter,and the correlation coefficient was 0.9115.When the diameter of drilling hole is 0.153 m,the numerical simulation of gas migration in the stope is carried out by the negative pressure of the high level borehole extraction is 10kpa,15kpa,20kpa,25kpa and 30 kpa,when the negative pressure is 30 kpa,the maximum gas concentration in the stope is 14.06%.It can be seen from the cloud atlas that the extraction effect is very significant and the gas at the upper comer is obviously under control.There are 12 high-level drilling sites and 72 high-level drilling holes in the working face,the cumulative drilling length is 4860m,the gas concentration in the pipeline is 2%?42%,and the gas content is 1.29?9m3/min,the total amount of pressure relief gas extraction is 458200 m3,the gas concentration in the working face is effectively reduced,which indicates that the parameters of the high pressure drilling and demolition are reasonable,achieving the expected results,it provides a guarantee for the safe production of the working face,at the same time,it verifies the scientificalness of the research method and the correctness of the conclusion.