Evolution Law And Application Research Of Unloading Induced Permeability Of Underlying Protected Coal Seams At Different Distances

Through unloading effect of protective coal seam mining,the fractures of the protected coal seam are opened,and unloading damage or even destruction occurs in the protected coal seam,which makes its permeability increase dramatically,and finally realizes the efficient drainage of pressure relief gas and eliminates the risk of coal and gas outburst.Finding out the permeability evolution law and distribution characteristic of the protected coal seam is the key to rationally arrange gas drainage boreholes and improve the timeliness of pressure relief gas drainage.In recent years,a lot of achievements have been made in the theoretical research on the protective coal seam mining,but most of the researches are carried out for the protected coal seam or protective coal seam with a specific occurrence condition,and there is no systematic study on the change law of the permeability of the protected coal seams with different distances.In this paper,the theoretical analysis,laboratory tests,numerical simulations,and field measurements are used to systematically study the mining stress evolution law of the protected coal seam at different distances and the mechanical characteristics,acoustic emission characteristics,failure characteristics and gas seepage law of gas-bearing coal under corresponding loading and unloading paths.A permeability subsection model of the mining-disturbed coal body is established and the permeability evolution law and distribution characteristic of the underlying protected coal seam are obtained.The research provides a theoretical basis for the reasonable arrangement of pressure relief gas drainage boreholes in the protected coal seam.Based on theoretical analysis,the mining stress evolution law of underlying protected coal seams at different distances is obtained.After mining the protective coal seam,a stress concentration area is formed in front of the working face,and a pressure relief area is formed behind the working face.In the process of the protected coal seam entering the stress concentration area and the pressure relief area,it experienced the loading process and three-dimensional stress unloading process in turn,and the change range of the vertical stress was significantly larger than that of horizontal stress.With the increase of the distance of the underlying protected coal seam,the ratio of the vertical stress unloading speed to that of horizontal stress increases during the stress unloading process.According to the calculation results of mining stress of the underlying protected coal seam,the corresponding stress path is extracted and the mechanical-permeability test of unloading coal body is carried out.The process of the protected coal seam entering the stress concentration area from the original stress area is simplified as that the specimen is gradually compressed to the yield state.When the protected coal seam enters into the pressure relief area from the stress concentration area,the confining pressure and axial pressure of the test system are respectively used to simulate the unloading process of vertical stress and horizontal stress.The different unloading velocity ratio N（hereinafter referred to as unloading velocity ratio N）between confining pressure and axial pressure is used to simulate the stress unloading process of underlying protected coal seams at different distances.The test results show that the stress-strain curve of the specimen goes through the initial compaction stage,elastic stage and yield stage before unloading.The AE signal increases with the increase of axial strain,and the permeability decreases with the increase of axial strain.The test results are similar to those of conventional triaxial compression test.After unloading,the AE signal increases rapidly and reaches the maximum near the peak point,and tends to moderate gradually after the specimen is damaged.Permeability also increases rapidly with the unloading of stress.With the increase of unloading velocity ratio N,the peak differential stress,the absolute value of the slope of the differential stress-radial strain curve before failure,the AE count and AE energy near the peak point,and the accumulated count and energy after failure all increase.When the unloading velocity ratio N is small,the specimen shows the mixed characteristics of splitting failure and shear failure.The main failure surface runs through both ends of the specimen,and the permeability increases in quadratic polynomial relationship with the volumetric strain.When the unloading velocity ratio N is large,the specimen shows shear failure characteristics.The main shear section does not effectively penetrate both ends of the specimen,and the permeability increases linearly with the volumetric strain.Based on the theory of porous media seepage and considering the influence of stress path on the characteristics of coal fracture propagation and permeability evolution,a permeability segmentation model is established in elastic stage,expansion-yield stage and yield-failure stage.The coefficients （?）₁,（?）₂ and （?）₃are introduced to represent the ratio of volumetric strain caused by effective pore change of each stage to the total volumetric strain of that stage.The model is fitted with the measured permeability data,and the correctness of the model is verified.With the increase of unloading velocity ratio N,the ratio of volumetric strain caused by effective pore change to total volumetric strain in this stage decreases,and the sensitivity of permeability to volumetric strain decreases too.Both confining pressure and stress path will affect the propagation characteristics of fractures in coal,and then affect the change trend of permeability.Based on the mining of the upper protective coal seam in Zhongxing coal mine,the distribution characteristics and evolution law of mining stress and strain of the underlying protected coal seam are obtained by numerical simulation.After the protective coal seam is mined,the stress state of the protected coal seam can be divided into original stress area,stress concentration area,pressure relief area and stress recovery area.The range beyond 60m in front of the working face is the original stress area.The stress concentration area is the range of 5m to 60m in front of the working face.The pressure relief area is the range of 5m in front of the working face to 30m behind the working face.The stress recovery area is the range from 30m behind the working face to the central area of the goaf.With the mining of the protective coal seam,the protected coal seam along the mining direction experienced the loading process and unloading process of three-dimensional stress at the same time,which is consistent with the theoretical calculation results of the mining stress of the protected coal seam.The x-direction（inclination direction）strain of the protected coal seam is mainly distributed near the coal pillars on both sides and inside the goaf.The y-direction（strike direction）strain is mainly distributed near the working face,the open-off cut and inside the goaf.The z-direction（vertical direction）strain is distributed near the working face,the open-off cut,the coal pillars on both sides and inside the goaf.The change trend of strain in x direction and y direction is opposite to that of vertical stress,and the change trend of strain in Z direction is consistent with that of vertical stress.The volumetric strain distribution characteristics and changing trend are basically consistent with that of z-direction strain,which fully illustrates the dominant role of vertical stress on z-direction strain and volumetric strain.Combining the volume strain of the protected coal seam obtained by simulation with the permeability subsection model established,the permeability evolution law and distribution characteristics of the protected coal seam are obtained by using Python programming method.After the protective coal seam is mined,the protected coal seam can be divided into original permeability area,permeability reduced area and permeability increased area.The permeability of the protected coal seam decreases gradually from 100m in front of the working face,and decreases to the minimum value at 15m in front of the working face.The permeability decreases slowly in the range of100～60m,and rapidly in the range of 60～15m.The permeability began to pick up at15m in front of the working face,and increased to the maximum value after entering the goaf and kept stable in a certain period.The variation law of the gas drainage data of the underlying protected coal seam with the advance of the working face is obtained by field measurement in Zhongxing coal mine.At the initial stage,the mixed gas drainage volume is relatively high,and decreases in a fluctuating manner with the advance of the working face.From about15m in front of the working face,the mixed gas drainage volume,the pure gas drainage volume and gas concentration began to increase rapidly,reached the maximum value after entering the goaf and remained stable in a certain period of time.The variation law of the gas drainage data of the protected coal seam is consistent with the theoretical results of permeability evolution law and distribution characteristics.