Study On The Damage Evolution Law Of Pore And Fracture Structure Of Coal Under Liquid Nitrogen Freeze-Thaw And The Mechanism Of Permeability Enhancing

Coal mine gas(Coalbed methane,CBM)is not just a major disaster,but an efficient and clean energy.CBM exploiation can control gas disaster accidents and optimize energy structure in China.For the complex coal geology condition in China,the coal reservoisr general have the characteristics of low porosity,low pressure,low permeability and high homogeneity,which restricts CBM exploitation.With the development technology of enhance permeability of coal seam,the waterless fracturing technology with liquid nitrogen(LN₂)as medium has attraccted much attention.In this paper,base on the engineering background of by fracturing and enhancing permeability technology with LN₂ freeze-thaw,the pore and fracture structure,physical and mechanical properties,permeability evolution characteristics of coal under different LN₂freezing times and freeze-thaw cycles were intervestigated through laboratory experiments and theoretical analyses,and the LN₂ freeze-thaw damage mechanism was analyzed by thermal stress and the effect of freeze-thaw.The follwing conclusions were drown:(1)LN₂ freezing and freeze-thaw can promote the development of pores of coal,the micropores of coal gradually expand,develop and connect to form mesopores and macropores,which enhance the pore connectivity of coal,the proportion of irreducible wate space of coal gradualy decrese,while the proportion of free water gradually space increase.With increase of freezing time and freeze-thaw cycles,the proportion of adsorption porees gradually decreases,while the proportion of seepage pores gradually increases.The total porosity,residual porosity and effective porosity are positively with the LN₂freezing times and freeze-thaw cyels.The nuclear magnetic resonance(NMR)experimental results show that damage degree of freeze-thaw to the pore structure of coal is greater than single freezing under the same freezing time.(2)The NMR fractal dimension of pores of coal samples can be divided into five types according to the state of fluid and pore size.The results show adsorption pores of coal samples did not have the fractual characteristics.The toatal fractual dimension D_w,irreducible fluid fractal dimension D_ir,free fluid fractal dimension D_fand the fractual dimension Ds of adsorption pores all decrease with increase of LN₂ freezing times and freeze-thaw cycles,which indicates that LN₂ freezing and freeze-thaw can reduce the complexity of pores of coal,and the distribution of pore was gradually uniform,which weaken the homogeneity of pores of coal.These properties favor the desorption,diffusion and seepage of CBM.(3)The total pore volume of coal increases after LN₂ freezing and freeze-thaw,and the change of pore volume of pore was maily concentrated in the mesopores and macropores.The specific surface area of coal samples increases with freezing times and freeze-thaw cycles.By comparing the changes in the ratio of the specific surface area of various types of pores,it is shown that the micropres gradually transform to the mesopores and macropores,which lead to the increase of proportion of mesopores and macropores.(4)In the process of LN₂ freezing and freeze-thaw,the damage of microscopic and macroscopic structure is an accumulative process.The indensity of pores increase with increase of freezing times and freeze-thaw cycles.The pores gradually connect and eventually form fractures,which lead to the increase of the length and width of initial fracture of coal samples,and the decrease of transverse wave velocity of coal samples.The initial fracture gradually expanded and connected and the secondary fractures were formed under the action of thermal stress and frost-heave force.The fracture structure of coal sample changed from single fracture to fracture network with main fracture and secondary fracture,and eventually formed through fracture,resulting in a large number of fractures,which provide space for migration of CBM.(5)After LN₂freezing and freeze-thaw,the effective load area of coal samples reduces,which lead to the decrease of mechanical strength and elastic modulus.The maximum acoustic ringing counts and cumulative ringing counts of coal samples decrease with increases of LN₂ freezing times and freeze-thaw cycles during the uniaxial compression,the acoustic emission amplitude of coal samples increased in compaction,elastic deformation and yield stage,but shows decrease trend during the stage of unsteady fracture.The total energy,esastic energy and dissipated energy decrease with increase of freezing times and freeze-thaw cycles during the during the uniaxial compression.(6)Under constant confining pressure and different gas pressure condition,the permeability,increment of permeability and relative change rate of permeability all increase with LN₂freezing times and freeze-thaw cycles.After 360 min freezing and 12freeze-thaw cycles,the permeability of coal samples increase by 125.7%and 228.2%over that of raw coal samples.Under constant gas pressure and different confining pressure condition,the permeability of coal samples after freezing and freeze-thaw are also significantly higer than that of raw coal sample.The improvement effect of LN₂freeze-thaw cycle on the permeability of coal samples is significantly greater than that of LN₂ freezing.In the practical application of the fracturing and enhancing permeability technology with LN₂ freeze-thaw,appropriate increase in the number of LN₂ freeze-thaw cycles can be better enhance the permeability of coal seam.