| China coal seams are characterized by the complicated geological structures,high gas content and adsorption,low porosity and permeability.As a result,which severely restrict coal safe development and gas extraction efficiency.With the development of coal seam fracturing technology,research on the extraction of gas by liquid nitrogen cold soaking fracturing coal has attracted widespread attention.The pore,crack and chemical structure inside the coal change under the combined effects of thermal stress and phase change frost heave force,which causes the adsorption and desorption characteristics of coal to change.Studying the evolution law of coal's pore,crack structure and surface functional groups and the mechanism of adsorption deformation under the liquid nitrogen cold soaking has important theoretical significance and application value for safe and efficient coal mining.This paper focuses on the coal microstructure characteristics and its influence on adsorption deformation under the liquid nitrogen cold soaking.Starting from the physical and chemical structure of coal,this paper uses laboratory experiments,theoretical analysis and physical modeling to study the evolution characteristics of pores,fissures and chemical structures of different rank coal samples under liquid nitrogen cold soaking.Gas adsorption capacity of coal and the law of swelling deformation under the different times of liquid nitrogen cold soaking were analyzed.The influence mechanism of coal adsorption deformation under the liquid nitrogen cold soaking was comprehensively discussed.The adsorption strain model of coal under the different times of liquid nitrogen cold soaking was established,and the correctness of the theoretical model was verified by combining the experimental results.The main conclusions obtained in this paper are as follows:(1)The industrial micro-CT experimental system was used to study the internal micro-fracture structure characteristics of different rank coal under the liquid nitrogen cold soaking.It was found that the liquid nitrogen cold soaking promoted the formation of new cracks,and the expansion of the original cracks to form a crack network in lignite,bituminous coal and anthracite.Matlab programming and VG Studio MAX image analysis software were used to extract and calculate the cracks in the CT scan results of coal samples.Statistics found that the quantified values of cracks in coal increased after liquid nitrogen cold soaking,and the increasing order of cracks in different rank coal samples was:lignite>bituminous coal>anthracite.(2)The temperature distribution of coal during the liquid nitrogen cold soaking process was monitored in real time.The influence of different times in liquid nitrogen cold soaking,coal rank and the shape of coal sample on the temperature field of coal were analyzed.It was found that the temperature of coal first accelerates and decreases then decelerates,and finally reaches a low temperature equilibrium state with the increase of liquid nitrogen cold soaking time.During the liquid nitrogen cold soaking process,the temperature change rate of anthracite is the smallest,followed by bituminous coal.The temperature change rate of lignite is the largest.The time required for different coal samples to reach low temperature equilibrium:anthracite>bituminous coal>lignite.Under the combined effects of thermal stress and phase change frost heave force,the creak structure of coal becomes more developed,which in turn accelerated the speed of temperature transfer.And the time required for coal to reach low temperature equilibrium was gradually shortened as the number of liquid nitrogen cold soaking in increased.(3)The nuclear magnetic resonance test of different rank coal samples was carried out under different liquid nitrogen cold soaking times,and the influence of liquid nitrogen cold soaking on the pore structure characteristics of coal was obtained.It was found that the liquid nitrogen cold soaking promoted T2 spectrum peak of coal shifted to the right,and made the anthracite develop from the interval T2 spectrum to the continuous T2 spectrum.It showed that liquid nitrogen cold soaking caused the internal pores of coal to transform to large size,the pore connectivity became better,and the pore structure was more developed.As the number of liquid nitrogen cold soaking increased,the internal pore size and number of pores in coal gradually increased,and the order of increase is:anthracite>bituminous coal>lignite.(4)The pore structure characteristics of coal under the liquid nitrogen cold soaking were studied experimentally.The BET specific surface area,total pore volume of BJH and adsorption pore specific surface area of different rank coal samples were obtained with the number of liquid nitrogen cold soaking.It was found that as the number of liquid nitrogen cold soaking increased,the total pore volume of BJH and seepage pore volume of coal gradually increased,while the adsorption pore specific surface area and BET specific surface area gradually decreased.Based on the experimental results of nitrogen adsorption,the pore structure fractal dimension D1 and the surface fractal dimension D2 of coal are calculated using the fractal theory.It is found that D1 increased linearly with the increase of the number of liquid nitrogen cold soaking,while D2decreased gradually.It indicated that the effect of liquid nitrogen cold soaking complicated the pore structure of coal with different rank coal,and promoted the surface of the coal to be smoother.(5)An elemental analyzer,X-ray diffractometer and Fourier infrared spectrometer(FTIR)were used to analyze the chemical microstructure of coal samples.The results showed that as the coal rank rises,the content of C,N and S elements gradually increased,and the content of H and O elements gradually decreased.The Fourier infrared spectrum of each coal sample could be divided into 4absorption bands:aromatic structure,oxygen-containing functional groups,aliphatic and hydroxyl absorption bands.The peak intensity area of hydroxyl and oxygen-containing functional groups in the coal after liquid nitrogen cold soaking increased,and the value of A(C=O)/A(C-O)increased.The decrease of A(CH3)/A(CH2)was mainly due to the gradual decrease of methyl side chains and the increase of methylene straight chains in coal by liquid nitrogen cold soaking.(6)The adsorption isotherm deformation experiment system was used to study the methane adsorption capacity and the law of expansion and deformation of different rank coal under the different liquid nitrogen cold soaking times.By comparison,it was found that increasing the adsorption temperature could inhibit the adsorption expansion and deformation ability of coal to methane.After liquid nitrogen cold soaking,the methane adsorption amount and volume strain of different rank coal samples all decreased.And coal methane adsorption capacity and expansion deformation gradually weakened with the increase of the liquid nitrogen cold soaking times.The isothermal adsorption equation of coal under the equilibrium pressure and temperature was constructed,which can well describe the adsorption characteristics of liquid nitrogen cold soaking coal under various temperature and pressure conditions.(7)The effect of coal pore structure and chemical microstructure on the deformation of adsorption isotherm was explored under the liquid nitrogen cold soaking.The was found that the saturated adsorption capacity of coal under liquid nitrogen cold soaking had a good linear relationship with the adsorption pore T2 spectrum area,BET specific surface area,and total pore volume of BJH.After liquid nitrogen cold soaking,the internal pore size of the coal developed towards the direction of large pores,the specific surface area of the pores was reduced,and the adsorption space of methane was reduced,thereby inhibiting the adsorption of methane by coal.After liquid nitrogen cold soaking,the content of hydroxyl and carboxyl groups on the surface of coal increased,which reduced the interaction energy between coal and methane,thereby reducing the coal's ability to adsorb methane.Based on the principle of thermodynamics,the adsorption deformation model of coal under different liquid nitrogen cold soaking times with clear parameters and not limited by the pressure range was established,and its feasibility was verified. |
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