Experimental Study On Damage And Anti-Reflection Of Fractured Water-Saturated Coal Caused By Liquid Nitrogen

With the increasing demand for coal resources,mining is gradually shifting from shallow coal seams to deep coal seams,and various problems are encountered in the mining process,among them,the deep coal seam has low permeability and high gas,gas extraction is difficult and inefficient,and gas as a clean and efficient energy source is gradually coming into view,So gas extraction has become a new trend for coal mines to manage gas disasters,improve production safety and provide clean energy.This paper adopts the experimental pretreatment method of freeze-thaw treatment of liquid nitrogen on dried water-saturated coal samples,and conducts research on the effect of coupling water immersion and liquid nitrogen freeze-thaw on the physical properties of coal and the percolation law,the main research results of this paper are as follows:(1)Liquid nitrogen freeze-thaw results in deterioration of mechanical properties of coal,increasing brittleness and weakening plasticity.The analysis of the compressive strength and stress-strain of the coal sample shows that the internal structure of the liquid nitrogen freezethawing coal sample is seriously damaged,and the degree of axial strain,radial strain,bulk strain and shear strain is less than that of the dry coal sample,and the degree of deformation at breaking time is small.The compressive strength and strain degree are positively correlated with the confining pressure.(2)Different loading methods have different cracking effects on coal.The strain of coal samples under different loading methods is analyzed.Under the same pretreatment and confining pressure conditions,the strain values of coal samples under confining pressure unloading experiment are all larger than those under axial pressure unloading experiment,which indicates that the internal structure damage of coal samples under confining pressure unloading is more serious and the overall deformation degree is large.The confining pressure unloading experiment method has better cracking effect on coal samples.(3)Cracks develop on the coal surface and the internal structure changes after liquid nitrogen freezing-thawing.The evolution of surface damage dimension of coal samples before and after liquid nitrogen freeze-thaw treatment and the actual state after crushing were analyzed.The structural damage of coal samples treated by liquid nitrogen freeze-thaw treatment was more serious,the degree of crack development was higher,and the cracking effect was more significant.(4)The gas permeability of the coal body changes after the liquid nitrogen freeze-thaw.The permeability is related to the internal structure of the coal sample,and the permeability is higher when the internal structure is seriously damaged.The gas permeability of coal samples before and after liquid nitrogen freeze-thaw is analyzed.The confining pressure is negatively correlated with the gas permeability.Under the same confining pressure,the gas permeability of liquid nitrogen freeze-thaw coal sample is greater than that of dry coal sample,and the internal structure damage of coal sample with high gas permeability is serious.Liquid nitrogen freeze-thaw has a good effect on cracking and reflection improvement of coal sample.The thesis has 38 pictures,6 tables and 66 references.