Pore-fracture Response Characteristics Of Structurally Heterogeneous Coal Under Impact Load

With the increase of mining depth and mining intensity,the conditions of gas extraction in coal seam are deteriorating,and most coal seams belong to low permeability coal seam.In the process of gas extraction in coal seam,blasting,CO₂phase transition cracking and other impact load disturbance methods can be used to cause the coal body to crack and expand to form cracks on the macro scale.Then,the impact on the pore structure of coal at the micro scale and the direct or indirect impact on the adsorption/desorption characteristics and permeability of coalbed methane are not clear.Therefore,it is necessary to carry out the study on the microscopic pore structure change,adsorption/desorption characteristics and permeability of coal rock under impact load.This is of great significance for improving the efficiency of coalbed gas drainage,optimizing the well network of surface coalbed methane drainage,the layout of underground gas drainage boreholes,and effectively preventing and controlling coalbed gas disasters.In order to reveal response characteristics of pore and cracks in structurally heterogeneous coal under impact load,firstly,the dynamic mechanical properties of structural anisotropic coal under low-medium-high strain rate are carried out by using horizontal/vertical Split Hopkinson pressure bar impact device,and the dynamic properties and damage characteristics of coal under low-medium high strain rate are systematically and completely clarified.Then,the changes of micro pore and macro fracture structure of anisotropic coal before and after impact are studied by low-temperature liquid nitrogen adsorption method,mercury injection method,adsorption test and permeability test respectively,so as to reveal the dual response mechanism of gas occurrence redistribution and gas migration channel network connection in coal seam impact reflection improvement.Finally,the field test method is used to carry out the field test research on the 2125 working face of No.21 coal seam in Hebi No.6 mine,Henan Province,which reveals the spatial coupling effect of pore evolution,fracture formation and development of coal body in the process of shock stress wave propagation.The main research contents of this paper are as follows:（1）The horizontal/vertical Split Hopkinson pressure bar impact device is used to carry out dynamic impact on structurally heterogeneous coal.The dynamic mechanical characteristics and damage characteristics of the coal under low,medium and high strain rate are analyzed.When the strain rate is constant,the peak stress of coal sample in perpendicular to the bedding direction is the largest.The peak stress,dynamic peak stress growth factor,dynamic elastic modulus and crushing energy consumption increase linearly with strain rate.With the increase of strain rate,the failure mode of coal sample changes from axial splitting tensile failure to compression failure.（2）The pore structure of coal samples before and after impact is measured by low-temperature liquid nitrogen adsorption test.At the same time,the pore surface fractal dimension is used to characterize the pore parameters of micro and small pores.The saturated adsorption capacity of raw coal sample is significantly greater than that of coal sample under dynamic load.Under the same impact load,the adsorption capacity and pore specific surface area of coal samples in perpendicular to the bedding direction are the largest.In the same bedding direction,the maximum adsorption capacity,specific surface area and fractal dimension D2 of specific surface area decrease exponentially with strain rate.（3）The pore structure of coal samples before and after impact is measured by mercury injection experiment,and the medium and large pore parameters are characterized by volume fractal dimension.Compared with the raw coal,the mercury injection lag ring of the coal sample after impact is wider,and the pores are mainly open pores.The impact load improves the pore structure of coal and increases the connectivity of pores.The maximum mercury input and mercury removal efficiency increase linearly and exponentially with strain rate,respectively.However,the volume fractal dimension D decreases linearly with strain rate.（4）The adsorption parameters of coal samples before and after impact are measured by adsorption test,and the internal relationship between gas adsorption performance and micro pore structure is discussed.The methane adsorption constant a of coal samples reduces exponentially with strain rate.The oblique 45^o to the bedding direction is the most sensitive to the ultimate adsorption capacity of coal samples.In the same bedding direction,the methane adsorption constant a of coal samples increases linearly and exponentially with adsorption capacity and specific surface area,respectively.The value of methane adsorption constant a is linearly and positively correlated with the fractal dimension D2 of pore specific surface area.（5）The discrete element software is used to establish the numerical model of the impact test of the Split Hopkinson pressure bar device.At the same time,the permeability of coal samples in different bedding directions under the vertical Split Hopkinson pressure bar impact device is studied,and the orthogonal anisotropy calculation model of coal permeability is established.The permeability distribution law of coal in different directions is analyzed by numerical calculation.In the oblique 45^o to bedding direction,the shear crack penetrates along some bedding planes,which greatly increases the connectivity of pores.Under the same impact load,the permeability of coal samples in parallel to the bedding direction is the largest and that in perpendicular to the bedding direction is the smallest.In the orthotropic model of coal permeability,the permeability distribution law of raw coal under different gas pressures is approximately oval,and the permeability distribution law of coal sample after impact is approximately double spindle.The greater the impact load,the greater the permeability.（6）Deep hole presplitting blasting is used in 2125 working face of No.21 coal seam in Hebi No.6 coal mine.Under the action of blasting impact force,the adsorption capacity,BET specific surface area and adsorption constant a of coal samples at different positions in the stress wave action area increase linearly with hole spacing,and the maximum mercury injection capacity of coal samples decreases linearly with spacing.At 1.0,1.5 and 2.0 m away from the blasting hole,the permeability of the coal seam after blasting is 2.07,1.84 and 1.52 times higher than that before blasting.Obviously,in the far area of the blasting hole,the change of the microstructure of the coal is small,and the impact of the explosion shock wave on the pore structure of the coal is weak.In the near area of the blasting hole,the explosion shock load greatly changes the micro pore and macro fracture structure of the coal.There are 79 figures,37 tables and 148 references in this paper.