Response Mechanism Of Coal Macromolecular Structure And Nanopores To Igneous Intrusion

The high temperature and pressure effect caused by igneous intrusion can contribute to the increase of coal seam metamorphism and change the coal quality.Since the Middle and Cenozoic,multiple phases of magmatic activities of different scales have been widely developed in the eastern part of China,resulting in the coal seams in the region being affected to different degrees.Compared with the deep-formation thermal metamorphism,this transient high-temperature thermal metamorphism has more influence on the maturity of coal reservoirs,the physicochemical properties of coal bodies and the evolution of coal hydrocarbon generation.Therefore,it is of great theoretical value and practical significance to carry out the study of coal macromolecular structure and nano-pore structure evolution in response to igneous thermal action.In this thesis,the influence of magmatism on the petrographic and geochemical evolution pathways of coal was systematically analyzed with the aid of optical microscopy and scanning electron microscopy（SEM）,and X-ray diffraction（XRD）,Raman spectroscopy（Raman）,infrared spectroscopy（FTIR）,XPS and high-resolution transmission electron microscopy（HRTEM）were used to investigate the influence of magmatism on the evolution of coal.The control of igneous thermal action on the evolution of coal molecular chemical structure was investigated,and the three-dimensional coal molecular structure influenced by igneous action was successfully constructed;the pore structure development of coal under igneous action was quantitatively characterized by combining high-pressure mercury pressure,low-temperature liquid nitrogen adsorption,carbon dioxide adsorption,smal-angle X-ray scattering and atomic force microscopy,and the control mechanism of coal molecular structure on pore development was investigated;on the basis of the constructed Based on the constructed model,we simulated the methane adsorption characteristics of igneous coal based on Monte Carlo molecular dynamics,and compared the results with the isothermal adsorption experiments of the samples to verify the authenticity and reliability of the constructed model.The main insights were obtained as follows:（1）The mechanism of coal geochemical evolution in response to igneous thermal action was found.The reflectance value R_o of the specular group of coal samples increases from 1.60%to 5.73%as the location of the magmatic intrusive body is approached.The microscopic components in the samples changed,with the specular group gradually scorched and the inert group decreased,and the crustal group disappeared when R_o>1.35%.A variety of thermally altered components were generated,including intermediate phase microspheres,pyrolytic carbon,isotropic and anisotropic coke.In addition,industrial analysis and elemental data analysis indicate that the geochemical evolution pathway of coal is changed by igneous,and the parameter changes depend on environmental conditions such as heating rate,heating time,maximum temperature,pressure and hydrothermal.（2）The control effect of igneous transient high temperature and pressure on the macromolecular structure of coal was revealed.The evolution path of its chemical structure was classified into three stages:anthracite,metamorphic anthracite,and semi-graphitized coal by several spectroscopic experiments and HRTEM image method,and the characteristics of lattice stripes of igneous action coal samples were summarized.The pattern shape,length and degree of orientation of the lattice stripes are obviously different in different stages.Especially in the anthracite stage,the lattice stripes of the coal samples show a typical"onion concentric circle structure"under the influence of magmatism.During this process,the lattice stripe length increases with the degree of coal metamorphism and the degree of lattice stripe orientation also increases significantly,accompanied by the phase changes of chemical structural units such as BSU growth,BSU lateral connection growth and BSU lateral straightening.（3）The evolutionary characteristics of the nanopore structure of coal samples under the influence of magmatism were quantified.The fluid injection method shows that the igneous thermal action promotes the development of macropores,mesopores and micropores in coal.The total pore volume and specific surface area of macroporous pores show a jumping trend of decreasing as the coal approaches the igneous intrusion position,while the total pore volume of mesopores first decreases and then increases,and the specific surface area first increases and then decreases;the total pore volume and specific surface area of micropores show a"two-stage"development characteristic of first increasing and then decreasing.（4）It is clear that the development of micropores in coal affected by igneous is controlled by the development of the basic units of the coal molecular structure.The CO₂ adsorption experiments show that the pore volume and specific surface area of micropores increase in the metamorphic anthracite stage,which is due to the fact that the total pore volume and specific surface area of micropores formed by the folded stacking of aromatic lamellae of basic structural units of coal are larger at this stage,while the total pore volume and specific surface area of micropores formed at this stage tend to decrease due to the straightening of aromatic lamellae and the decrease of stacking spacing at the semi-graphitization stage.（5）A three-dimensional molecular structure model of coal was constructed to elucidate the effect of magmatism on the methane adsorption capacity of coal.Firstly,based on the results of chemical molecular structure tests such as high-resolution transmission electron microscopy（HRTEM）,carbon 13 nuclear magnetic resonance spectroscopy(¹³C NMR),infrared spectroscopy（FTIR）and X-ray diffraction（XPS）,a two-dimensional molecular structure model of coal influenced by magmatism and unaffected by magmatism was constructed.Based on the two-dimensional molecular structure model,the corresponding three-dimensional molecular structure model of coal was successfully constructed based on the geometry optimization and energy minimization principles,CO₂ porosity test and sample true density test results.Based on the Monte Carlo molecular simulation theory and technique,the adsorption of methane molecules in coal seams subject to igneous action and normal coal seams were compared and simulated.The molecular simulation and methane physical adsorption experiments corroborate each other,and the realism of the 3D molecular structure model and the accuracy of the adsorption simulation are verified.It is further shown that the adsorption of methane by coal is significantly increased under the influence of igneous thermal action,and the positive effect of magma on the methane fugacity of coal is clarified.The thesis has 90 figures,25 tables,and 224 references.