Effect Of Deflagration Shock Of Supercritical CO₂ Energetic Fluid On Coal Micro-characteristics

Discussing the impact of impact on the microstructure of coal is of great significance to the study of permeability enhancement and gas drainage in low-permeability coal seams.The research content of this paper is centered on two aspects,one is the change of coal pore structure after impact,and the other is the change of coal surface chemical structure.This paper first conducts impact experiments to obtain cracked coal,use low-temperature N₂ adsorption experiments,low-temperature CO₂ adsorption experiments and FHH model theory to calculate fractal characteristics,discuss the changes in coal micro-pore structure;use X-ray diffraction spectroscopy(XRD)and X-ray photoelectron spectroscopy The test(XPS)discusses the changes in the chemical structure(crystal structure and functional groups)of the coal surface.A set of supercritical CO₂ energetic fluid deflagration impact test system was assembled,and coal samples from Taoyuan Coal Mine in Anhui Province with a low degree of metamorphism were selected,and the supercritical CO₂ energetic fluid deflagration impact test was performed after the preliminary treatment was completed.Cracked coal is used in the study of microstructure changes later in this article.Low-temperature N₂ adsorption and CO₂ adsorption experiments were used to analyze the changes in the micropore and full pore structure of cracked coal.The results of the experiment show that the pore structure of the coal sample is complex,including macropores,mesopores and micropores.The pore type is open type,mostly cylindrical pores and parallel plate pores.According to the classification of IUPAC,the adsorption curve is type III.The specific surface area of cracked coal decreases,the pore volume and the average pore diameter increase,and the deflagration impact produces pore increase and pore expansion.The median pore diameter of the micropores decreases,and new micropores may be produced.As the pore size increases,the impact of transient high pressure impact on the pore size increases,the ratio of mesopores to macropores increases,and the connectivity in the coal body increases.After the coal sample is impacted by the deflagration of supercritical CO₂ energetic fluid,the pore diameter is mainly concentrated in 1-10nm,and the micropores within 1nm are mainly concentrated in 0.5-0.65nm.The volume of micropores and specific surface area decreased,The transient high-pressure shock has the effect of hole enlargement and hole expansion,which is consistent with the results of low-temperature N₂ analysis.The calculation of the FHH model shows that the sample has good fractal characteristics.The fractal dimension D₁ of pores below 4nm is reduced,and the reduction of the number of surface micropores causes the surface of the cracked coal to become smoother and the complexity is reduced;the fractal dimension of pores above 4nm The increase in the number D₂is due to the increase in mesopores and macropores,and the different trends in the two stages are due to the different adsorption mechanisms of the two pressure sections.Using XRD and XPS to analyze the changes of crystal structure and surface oxygen-containing functional groups before and after impact.The crystallinity of the raw coal and the cracked coal is good,and the graphitization degree of the cracked coal is enhanced.The position characteristics of the 002 peak,the 100 peak and the?peak in the map are more obvious.After peak fitting and calculation,it can be seen that the layer spacing d₀₀₂ decreases,the stacking height Lc,the ductility La and the degree of coalification H increase,and the La/Lc increases.The content of oxygen-containing functional groups COO-,-OH,C=O,and-O-on the surface of cracked coal are all lower than that of raw coal.Combining with the results of XRD,the possible reason is that more surface of cracked coal is produced.A substance with an aromatic structure.The above results show that the microstructure of the coal surface has undergone a certain degree of change in the pore structure and chemical structure after the transient high pressure impact of the supercritical CO₂ energetic fluid,and the change of the coal microstructure is bound to affect the desorption and drainage of gas.