Deformation And Metamorphism Characteristics Of Deformed Coal And Its Effect On Methane Adsorption

Influenced by multi-stage tectonic movements in Paleozoic coal-forming basins in China,different types of deformed coal are widely developed in coal seams.Exploring the deformation and metamorphic characteristics of deformed coal and its influence on gas adsorption have important research significance for coal and gas outburst prediction and coalbed methane mining.Deformed coal with different deformation degrees of low-middle rank in Shoushan No.1 and middle-high rank in Yi'an mines were collected.Based on the experiments of low temperature liquid nitrogen adsorption(2-50nm)and carbon dioxide adsorption(<2nm),and the multi-fractal theory was introduced to explore the influence of tectonic deformation on nanometer pore structure characteristics.The evolution law of molecular structure of low-middle and middle-high rank bituminous coals in the process of dynamic metamorphism was discussed by means of spectral test.Based on HRTEM technology,the influence of tectonic deformation on the aromatic microcrystalline of coal was analyzed.Combined with the experimental of gas adsorption,spectrum test,molecular simulation technology and methane isothermal adsorption,the effects of molecular structure,pore structure and defects on gas adsorption during tectonic deformation were analyzed.The main results are as follows:(1)Based on the low temperature nitrogen adsorption,carbon dioxide adsorption experiment and multifractal theory,it is found that the tectonic deformation increases the complexity of micropore-mesopore of bituminous coal.The spectrum width of generalized fractal dimension D10--Di10+and singularity exponent ?0 increase with the increase of deformation degree,while the mesopore decreases first and then increases.In the weak deformation stage,the information dimension D1 and Hurst exponent H of micropore gradually decrease,the multifractal singular spectrum width ?? gradually increases,and the mesopore D1 and H gradually increase,while ??gradually decreases.During the strong deformation stage,no significant changes are observed in micropore D1,H and ??,while the mesopore D1 and H decrease and ?? increases significantly.By analyzing the variation of multifractal structure parameters of deformed coal in different deformation stages,the relationship between the nanoscale pore structure characteristics of deformed coal and destroy strength is clarified.(2)Based on the molecular structure characterization experiment,the control mechanism of dynamic metamorphism on the molecular structure of low-middle and middle-high rank bituminous coals with different deformation degrees is revealed.In the weak deformation stage of low-middle rank bituminous coal,La and Lc gradually decrease,the dynamic metamorphism has no obvious ability to improve the metamorphism degree of coal.While in the strong deformation stage,IG/ID1 and d002 gradually decrease,DOC,La and Lc gradually increase,indicating that the dynamic metamorphism effect increases rapidly,and promotes the metamorphism degree of coal.For middle-high rank deformed coal,the change intensity of the spectral structural parameters in the weak deformation stage is lower than that of the low-middle rank deformed coal.This is because the metamorphism causes the stress sensitivity of the coal organic matter to decrease.The dynamic metamorphism in the strong deformation stage accelerates the coal polycondensation,which shows that the aromatic structure content increases rapidly and the size enlarges.(3)The distribution characteristics of microcrystalline structure of low-middle rank bituminous coals under the influence of tectonic deformation are quantitatively characterized by HRTEM experiment.In the early stage of weak deformation,aromatic clusters only grow in a small size range(<3×3),and the evolution of coal structure is mainly stress degradation.In the late stage of weak'deformation and the stage of strong deformation,aromatic clusters have developed in a large size direction(3×3?8×8),and the effect of stress polycondensation is increasing gradually.The influence of tectonic deformation on the distribution of relative molecular weight of coal is analyzed.It is found that the aromatic cluster(40-449 Da)with lower molecular weight is more obviously affected by the weak deformation,while the aromatic cluster(450-3099 Da)with higher molecular weight has obvious response to the whole deformation process.(4)Based on the methane isothermal adsorption experiment,it is discovered that the influence of micropore-mesopore of deformed coal on methane adsorption is closely related to the degree of metamorphism.With the increase of deformation,the micropore is always the main control factor of methane adsorption capacity in low-middle rank bituminous coal,while the mesopore plays an important role in the stage of strong deformation.The influence of micropore-mesopore structure in middle-high rank bituminous coal on methane adsorption capacity is similar to that of low-middle rank bituminous coal in general.The difference is that the mesopore has begun to play an important role in the weak deformation stage.(5)The influence mechanism of coal molecular structure with different deformation strength on methane adsorption is illustrated by the synthesis of molecular structure and methane adsorption capacity.In the early stage of weak deformation,the factors affecting methane adsorption capacity are aliphatic chains and oxygen-containing functional group,and in the later stage of weak deformation,it is transformed to aromatic structure.In the stage of strong deformation,the interlayer pores between aromatic sheets are the main factors affecting the methane adsorption capacity.(6)Based on the molecular simulation technology,the methane adsorption characteristics of different structural defects in deformed coal are obtained.Boundary defect(SW)and vacancy-like defects are widely present in deformed coals,including single hole defects(SV),double hole defects(DV),and multi-hole defects(MV1 and MV2).According to the formation mechanism and formation energy of the defects,the brittle deformed coal is mainly composed of SV and DV defects,and the ductile coal contains mainly SW,MV1 and MV2 defects.Compared with the carbon-carbon bond and the aromatic ring center in the defect,the higher electron density above the carbon atom in the defect is more conducive to the adsorption of gas molecules.From the gas density distribution,gas molecules are more easily enriched near the defects.The adsorption isotherms of gas molecules on the surface with different defects are consistent with the characteristics of type I adsorption curve.The adsorption capacity of gas molecules with different defects is shown as follows:the size order of Langmuir volume(VL)is MV1>MV2>DV>SW>SV,and the size order of Langmuir pressure(PL)is SW>SV>DV>MV2>MV1.Research shows that the defects from the internal structure of the deformed coal under the action of the geological tectonic dynamics are conducive to the occurrence of gas.