Experimental Study Of Effects Of Liquid Nitrogen Freeze-thawing On Characteristics Of Mechanics Of Preheated Coal And Seepage Law

With the exploitation and consumption of shallow coal resources,China’s coal mining develops to deeper coal seams.At the same time of in-depth mining,the coal seams generally show the characteristics of high gas content but poor permeability,and the gas extraction is difficult and inefficient,which poses a serious threat to the safety of mine production.In this paper,the method of cracking and permeability enhancement by heating the coal sample,freezing and thawing of liquid nitrogen after heating and freezing and thawing of saturated liquid nitrogen after heating is used to study the mechanical properties and seepage law of preheated coal before and after freezing and thawing of liquid nitrogen.The main research results are as follows:(1)For the structural damage analysis of coal samples,it can be seen from the evolution of ultrasonic wave velocity and end-face damage dimension that the higher the heating temperature is,the more serious the structural damage is.At the same temperature,the damage of saturated liquid nitrogen freezing-thawing coal sample is greater than that of liquid nitrogen freezing-thawing coal sample,and the wave velocity decreases.The damage box dimension is positively correlated with the damage degree.(2)The coal sample was subjected to axial compression loading.The analysis of stress and strain showed that the coal sample with large axial compression had smaller strain degree,and the change law of stress and strain was consistent with the damage degree of coal sample structure.The mechanical strength of coal sample with serious structural damage was low,and the deformation degrees of axial strain,radial strain,volumetric strain and shear strain were large.(3)The confining pressure of the coal sample was unloaded,and the confining pressure and strain degree of the coal sample were analyzed.It can be seen that the confining pressure decreased with the increase of the treatment temperature of the coal sample itself.At the same temperature,the confining pressure of the saturated liquid nitrogen freezing and thawing coal sample was the smallest when it was crushed,and the decrease of the confining pressure of the coal sample with more serious structural damage was greater when it was unloaded.The radial strain,volumetric strain and shear strain deformation increase with the decrease of confining pressure.(4)The seepage flow of coal samples is closely related to their own structure.Heating and liquid nitrogen freezing and thawing lead to the increase of pores and cracks in coal samples,which is easy to break and form a fracture network.Taking axial pressure loaded coal samples as an example,the seepage flow is 1.23L/min at 50°C.With the increase of temperature,the seepage flow increases to 9.09L/min at 200°C.After heating at 200°C,liquid nitrogen freezing and thawing and saturated liquid nitrogen freezing and thawing are carried out,and the seepage flow is 70.05L/min and 299.7L/min,respectively.The seepage flow decreases firstly during axial compression loading,and then increases sharply due to coal sample crushing.The confining pressure unloading reduces the binding force of coal samples,and the pores and fissures closed due to extrusion are opened,and the seepage flow shows an upward trend.The coal sample with large seepage flow has good fracturing and permeability enhancement effect.The thesis has 50 pictures,8 tables and 73 references.