Experimental Study On The Influence Of Coal Seam Permeability On Coal And Gas Outburst

Coal and gas outburst is a dynamic disaster that seriously threatens the safety of mine production,and it is also an extremely complex process.With the gradual normalization of deep resource mining and increasingly complex coal seams conditions,the number of high gas and outburst mines is increasing and the frequency of outburst disasters is more frequent,resulting in immeasurable economic and personnel damage.The permeability of coal seam plays a decisive role in the stability of the coupled coalgas system.Therefore,this thesis used the coal and gas outburst physical simulation device to carry out the outburst test under different permeability conditions.The temperature and pressure changes of coal seam and roadway,acoustic emission signals of coal seam,migration of two-phase flow,outburst strength and particle size distribution were compared and analyzed to explore the influence mechanism of coal seam permeability on the outburst.The main conclusions of this thesis are as follows:(1)The coalbed methane pressure decreases rapidly after outburst.The closer to the outburst mouth,the earlier the pressure is released.With the increase of permeability,the pressure relief time of coal seam decreases obviously,indicating that the duration of outburst dynamic phenomenon becomes shorter.The change in coal seam temperature is lagging behind that of air pressure.The lower the permeability,the greater the decrease of coal seam temperature and the longer the decrease time in the process of outburst.After the outburst dynamic phenomenon,the coal seam is still conducting gas desorption.(2)The acoustic emission signal is generated again within 0.5s before the outburst occurs,which plays a certain early warning role.The AE count and AE energy reach the amplitude when the outburst occurs,and the acoustic emission signal will be continuously collected for a while after the coalbed methane pressure is released to atmospheric pressure.This indicates that fractures are still generated inside the coal seam.Comparing the acoustic emission signals of the four groups of tests,it is found that the fragmentation degree inside the coal seam is greater than that near the outburst mouth,which indicates that the outburst is a dynamic process from the outburst mouth to the inside of the coal seam.Analysis of the acoustic emission signals reveals that the degree of fragmentation inside the coal seam was greater than that near the outburst mouth,which indicates that the outburst is a dynamic process developing from the outburst mouth to the interior of the coal seam.The AE count and AE energy collected at each position of the permeability 1.38 m D test are the largest.The lower the permeability,the higher the degree of coal seam fracture.(3)The shock wave starts to be generated in the roadway when the outburst occurs and propagates with alternating positive and negative pressure.The lower the permeability,the longer the time of shock wave overpressure in the negative pressure zone.In the four groups of tests,the peak overpressure and wavefront velocity reached6.20,4.34,4.02 and 5.15 MPa,342.6-349,342.3-346.2,340.9-345.8 and 342-347.4m/s at 2-6m from the outburst mouth,respectively,indicating that this area was the key protection area.The roadway temperature rises slightly in a short time after the outburst,then decreases rapidly,and finally slowly rises to the ambient temperature.The roadway temperature is still decreasing for a period of time when the outburst phenomenon ends,indicating that the coal seam is still desorbing gas.During the outburst process,the overall migration process of the two-phase flow can be divided into an acceleration stage and an acceleration and deceleration cycle stage.However,the overall flow rate is still declining after the acceleration stage.In the three groups of experiments(permeability of 1.38 m D,6.69 m D and 9.10 m D),the two-phase flow migrated 1.713,3.43 and 6.34 m in 0.067,1.67 and 4s,respectively,and the velocity reached the peak value of 31.98,26.19 and 20.10m/s.(4)The quality of outburst coal in the dust bag is much larger than that of roadway settlement.The large particle pulverized coal in the roadway mainly settles in the front,and the small particle pulverized coal mainly settles in the middle and back end.The pressurized gas will crush the outburst coal powder,and the outburst coal powder has the highest crushing rate at the front of the roadway.In the 4 groups of tests,the coal quality is 15.126,8.93,7.602 and 2.193 kg,the relative outburst strength is 75.6%,44.6%,38% and 11%,respectively.The trends of coal quality and relative outburst strength are opposite to those of permeability,but the trends of crushing rate are consistent with those of permeability.At the same time,the fitting found that the relative outburst intensity and coal seam permeability has a negative exponential relationship,and the higher the coal seam permeability,the smaller the relative outburst intensity.This indicates that the outburst may no longer occur by continuing to increase the coal seam permeability under this test condition.This indicates that if the seam permeability continues to increase,the outburst may not occur again.(5)Combined with the experimental results and previous experience,the mechanism of coal seam permeability affecting outburst is studied and analyzed.It is considered that the outburst risk of coal seam with low permeability is much higher than that of high permeability coal seam.In the case of outburst,the relative outburst strength of low permeability coal seam is also greater.It shows that coal seam permeability can be used as a means of on-site outburst prediction and early warning.In the actual outburst of the mine,the coal seam permeability can be increased by using measures such as mining the protective layer and hydraulic permeability enhancement to optimize the mine safety production environment and prevent the occurrence of outburst.There are 37 figures,13 tables and 93 references in this thesis.