Study On The Influence Of Different LN₂ Injection Parameters On Coal Cracking

Safe and efficient improvement of coal seam permeability has always been the focus and difficulty of mine gas mining research.As a measure of anhydrous cracking and enhancing coal seam permeability by injecting liquid nitrogen into coal seam,its characteristic is to get rid of the dependence on water resources in the process of coal seam permeability enhancement.The low temperature characteristic of liquid nitrogen makes coal seam pores and fissures develop,changes the internal structure of coal body,so as to achieve high efficiency of coal seam permeability enhancement and enhancing coal seam permeability The purpose of cleft.At present,the research on the coupling relationship between the parameters of liquid nitrogen injection and the effect of coal cracking is insufficient in the experiments of liquid nitrogen cracking and coal permeability enhancement at home and abroad.At the same time,the research on the injection of liquid nitrogen to the coal sample under confining pressure environment is lacking,and the research on the influence of liquid nitrogen injection on the adsorption and desorption characteristics of coal seam is also lacking.In view of the above problems,this paper establishes an experimental system of liquid nitrogen injection in simulated confining pressure environment by combining theory with laboratory experiment.The system can simulate the stress state of coal body in different occurrence environment,and carry out liquid nitrogen experiment with different injection parameters on coal sample under the stress state,and explore the experiment of the influence of different injection parameters of liquid nitrogen on coal cracking and permeability enhancement under confining pressure The law and the adsorption,desorption and diffusion characteristics of coal body to mine gas after the action of liquid nitrogen.Liquid nitrogen circulates on coal body,resulting in the frost heaving force generated by water ice transformation,cyclic thermal stress and gas expansion force of liquid nitrogen vaporization.These stresses continue to destroy the pore fracture structure of coal body,continuously generate new pore fracture and expand the original pore fracture.The continuous process of liquid nitrogen injection circulation makes the coal body gradually form a porous structure,and the pores connect with each other to form a fracture network and finally change the internal structure of the coal body,making the pores of the coal body become larger and the fractures increase.The influence of different liquid nitrogen parameters on the internal pores of coal body is analyzed.The coupling relationship between the internal pores of coal body and different liquid nitrogen injection parameters is measured and summarized by means of nuclear magnetic resonance technology.Combined with T2 distribution curve,the change of total porosity and effective porosity of coal samples treated by different liquid nitrogen parameters in the process of liquid nitrogen injection is analyzed.The mechanical properties of coal under different liquid nitrogen injection parameters were studied.The changes of compressive strength and elastic modulus of coal were measured by uniaxial compression test,and the full field strain of coal surface was collected.The experimental results show that the modulus of elasticity and compressive strength of coal sample decrease,the elastic deformation increases,the rigidity decreases,the strain growth rate increases and the resistance to deformation weakens in the compaction stage,and the injection time of liquid nitrogen is positively related to the effect of liquid nitrogen on coal cracking.The influence of liquid nitrogen on the adsorption and desorption of methane was studied.Langmuir model and Fick model were used to analyze the isotherm adsorption and desorption diffusion characteristics of methane in coal with different liquid nitrogen injection parameters.The experimental results show that the ultimate adsorption capacity a value of coal body is negatively related to the effective porosity of coal body;the desorption capacity of coal body to methane increases with the increase of liquid nitrogen injection cycle times and liquid nitrogen injection pressure,and the more liquid nitrogen cycle times are,the greater liquid nitrogen injection pressure is,the more methane can desorb and diffuse in coal seam.