Fracture Evolution Of The Coal Subjected To Liquid Nitrogen Thermal Shocking

China is rich in coalbed methane resources,but the difficulty of mining is generally high.Compared with the traditional hydraulic fracturing technology,the anhydrous fracturing technology is more in line with the contemporary development concept.As a new type of anhydrous fracturing technology,liquid nitrogen fracturing is of great significance for both environmental protection and coalbed methane exploitation.In this paper,experimental test,theoretical analysis and numerical simulation methods are used to study the evolution characteristics of microcracks in coal and the key influencing factors of liquid nitrogen induced cracking.The physical and mechanical parameters of the coal samples used in the experiment were measured.After the liquid nitrogen impact cracking experiment,the high-resolution three-dimensional X-ray micro imaging system and its 3D reconstruction function were used to study the evolution law of the cracks in the coal samples before and after the liquid nitrogen impact and the growth rate of the pores,and the cracking effect of the liquid nitrogen impact on the coal samples was analyzed.The field emission electron microscope was used to analyze the liquid nitrogen impact cracking effect The change of surface structure and fractures of coal block before and after liquid nitrogen thermal shocking.Through a series of liquid nitrogen cracking experiments,the effects of three key factors,i.e.cycle impact times,water saturation conditions and initial crack size,on the effect of liquid nitrogen cracking on coal and rock were studied.Through CT scanning technology,the changes of the volume,thickness,shape and connectivity of coal micro cracks before and after the impact of liquid nitrogen were analyzed,and the evolution characteristics of coal micro cracks were obtained.Based on the liquid nitrogen cracking experiment,the evolution characteristics of coal micro cracks were discussed and the mechanism of coal fracturing subjected to liquid nitrogen were explored.Mechanical and fluid based on ANSYS Workbench simulation platform Flow(fluent)and other modules,established the thermal liquid solid coupling numerical model of high pressure liquid nitrogen impact on coal,simulated the impact cracking process of high pressure liquid nitrogen on coal,analyzed the pressure,velocity and turbulent kinetic energy in the fluid area,as well as the maximum principal stress,equivalent stress and deformation characteristics in the solid area,and analyzed the cracking effect under different impact pressure of liquid nitrogen,studied the impact cracking process of liquid nitrogen The effect of blow pressure on the cracking effect of liquid nitrogen.The paper has 79 pictures,9 tables,and 88 references.