Quantitative Characterization And Change Mechanism Of Microscopic Pore Structure And Mechanical Properties Of Coal Freezing With Liquid Nitrogen

In view of the fact that the theory and technology of liquid nitrogen fracturing coal are still immature,and the changes in mechanical properties and microscopic pores and fissures of coal during the fracturing process and their change mechanisms are still unclear,laboratory experiment,numerical simulation and mathematical modeling analysis were used to study the above problems.The changes of microscopic pores and fissures,mechanical properties and permeability of coal pore structure before and after freezing were studied,the reasons for their changes were analyzed,and the mechanism of pore and fissure change was clarified.The research results have certain guiding significance for the theoretical and technical improvement of liquid nitrogen fracturing coal.The main research results are as follows:The surface structure changes of the coal samples before and after freezing with liquid nitrogen were observed by Scanning Electron Microscope(SEM),and it was found that the surface structure changes of the coal after freezing mainly included pore connectivity,crack expansion and coal chip splitting.According to the X-ray Computed Tomography(X-CT)experimental results of coal samples,the three-dimensionally spatial structure of coal samples was reconstructed based on the Feldkamp(FDK)algorithm,and the representative volume of coal sample was determined.Considering the pore variation law of each coal sample with the volume representative unit and its uniformity,the representative unit was finally determined to be 400 voxels.The meso-porosity of coal before and after freezing with liquid nitrogen was calculated based on the layer porosity.The average porosity of each coal sample increased after liquid nitrogen freezing.Among them,the porosity increase ratio of SJZ group was 30.49%,that of WY group was 20.96%,that of HY group was 17.76%,and that of SG group was 40.05%.The order of porosity increase ratio is: SG group>SJZ group>WY group>HY group,with the increase of the metamorphism degree of coal samples,the growth value of the porosity of the coal samples after freezing shows a downward trend.Small Angle X-ray Scattering(SAXS)was used to study the changes of nanopores in coal samples before and after freezing with liquid nitrogen,and it was found that the range of pores with increased pore volume fraction of coal samples is mainly concentrated in 2-4nm,while the range of pores with reduced volume fraction is 4-7nm,and there is no obvious change rule for other size pores.In addition,the fractal dimension of the nanopores of all coal samples increased after freezing,and the pore structure became more complex.The elastic modulus and hardness of the coal samples were tested by nanoindentation experiments,and the loading-unloading curves of the elastic modulus and hardness of the coal samples conformed to the typical loading-unloading curves.After freezing,the elastic moduli of coal samples SJZ,WY and SG increased,while the elastic moduli of HY decreased.The changes trend of hardness of the four coal samples were consistent with the elastic moduli.Permeability simulations were performed using COMSOL Multiphysics software based on the 3D reconstruction pore space model.The results showed that the average permeability of coal samples after freezing has been improved.Among them,the permeability change of SG coal sample group is the largest,and the permeability increases by 81.7%;the permeability change of HY coal sample group is the smallest,and the permeability increases by 55.8%.The coal samples are sorted in descending order according to the change of permeability: coking coal,lean coal and meager lean coal.With the increase of the metamorphism degree of coal samples,the growth value of the permeability of the coal samples after freezing shows a downward trend.The pore changes of coal samples freezing with liquid nitrogen mainly include pore expansion,new pore generation and pore connection,and the change forms of fissures include fissure expansion,new fissure generation and fissure connection.According to the mechanical constitutive relation of its change process,mathematical models were established,the conditions for the changes were deduced,and the mechanical change mechanism of the pores and fissures was clarified.There are 119 figures,17 tables,and 351 references in this dissertation.