Investigation Of Gas Adsorption And Diffusion Behavior Based On Three-dimensional Pore Structure Of Coal

Investigation of coal structure characterization and gas adsorption and diffusion behavior in coal is of great significance for the coal mining safety and the reduction of greenhouse gas emissions.Through the analysis of current research,due to the limitation of coal three-dimensional pore structure characterization methods,the morphology and spatial distribution of three-dimensional pore structure of coal can not be obtained,which leads to the fact that the research on gas adsorption and diffusion behavior based on three-dimensional pore structure is insufficient.This thesis explored the new methods to characterize three-dimensional pore structures in coal.Moreover,gas adsorption and diffusion behavior in coal was comprehensively and systematically studied based on three-dimensional pore structure with the combination of theoretical analysis,experimental researches and numerical simulation.The purpose of this thesis is to study the gas adsorption and diffusion behavior in coal based on the three-dimensional pore structure obtained by synchrotron radiation nano-CT.In order to evaluate the advantages and disadvantages of synchrotron radiation nano-CT in characterizing pore structure,nuclear magnetic resonance?NMR?cryoporometry method was firstly explored to characterize the pore structure of coal.NMR cryoporometry can directly obtain the pore size distribution?PSD?from the linear relation between pore volume and signal intensity and the relationship between the melting point of the frozen liquid in pore and the pore size.Besides,NMR cryoporometry obtains the pore information from melting process and will not be affected by the background signals.During the NMR cryoporometry experiment,the temperature variation will affect the physical properties of samples and NMR cryoporometry probe,which could lead to the deviation between the measured NMR signal intensity and its true value.Thus,we explored the effective methods to calibrate the original signal.Based on the experimental method of NMR cryoporometry and the method of signal calibration,the pore size distribution?PSD?of six coal samples with different coal rank was measured by NMR cryoporometry and was compared with the results from Low-temperature N₂ adsorption and desorption?LTNAD?.It was also found that there was a good correlation between the results obtained by NMR cryoporometry and LTNAD.It was found that NMR cryoporometry yields higher pore volume value than LTNAD.Pores shrinkage/collapse induced by sample drying in LTNAD measurement is the main reason causing pore volume measured by NMR cryoporometry is larger than by LTNAD.The main difference between CT?computed tomography?and other methods in characterizing pore structure is CT can obtain the three-dimensional morphology and spatial distribution of pore structure,which is the basis of studying gas adsorption and diffusion behavior in coal.Since the lab-based CT does not benefit from a monochromatic beam and high X-ray coherency,lab-based CT cannot obtain the three-dimensional pore structure in coal.To study the three-dimensional pore structure in coal,this thesis explored the nanoscale imaging methods by synchrotron radiation nano-CT and the method of nano-CT images processing method.The segmentation of different components?mineral components,organic components and pore?in coal was also realized.The consistence between the segmentation results and proximate analysis results verify the image segmentation algorithm?Between-class Variance Maximisation?BCVM?algorithm?.Based on the acquired three-dimensional pore structure,pore-throat structure was partitioned and the statistics of pore and throat size were conducted and the pore connectivity was also analyzed.The PSD measured by synchrotron radiation nano-CT was compared with the results by LTNAD and NMR cryoporometry.It was found that synchrotron radiation nano-CT has advantages in characterizing closed pores,but there is disadvantage in characterizing pores with smaller pore diameters due to the limitation of resolution.Similar pore size distribution and throat size distribution were found in two coals with same coal rank by synchrotron radiation nano-CT.On the basis of obtaining the three-dimensional pore structure,the method for quantifying pore structure heterogeneity and anisotropy was investigated and the results from the new method for quantifying three-dimensional pore structure heterogeneity are not affected by the pore structure measurement method and the number of sub-volumes.The eigenvalues and eigenvectors of permeability tensor of three-dimensional pore structure calculated by the lattice Boltzmann method?LBM?can quantify the anisotropy of coal three-dimensional pore structure.On the basis of three-dimensional pore structure,single-component gas adsorption and diffusion behavior in coal was analyzed.Gas adsorption capacity is defined as the gas adsorption amount on per unit pore surface area.It was found that the gas adsorption capacity is related to the coal content.There is a negative correlation between gas adsorption capacity and ash content and a positive correlation between gas adsorption capacity and vitrinite content,which is consistent with previous research results.The hysteresis between desorption and adsorption isotherms is related to the proportion of pore-throat structure.The larger the proportion is,the more significant the hysteresis is.Besides,based on the three-dimensional pore structure of coal,gas diffusion behavior in coal was also investigated,and it was found that gas diffusion coefficient is higher in coal with the higher gas adsorption capability.In the process of adsorption,gas diffusion coefficient peaks at the pressure where capillary condensation begins.In the process of desorption,there is a U-shaped relationship between the gas diffusion coefficient and the gas pressure.In order to study the dynamic process of gas adsorption and diffusion in the three-dimensional structure of coal,a gas adsorption-diffusion coupled model based on three-dimensional pore structure was developed and the selection of parameters in numerical simulation was also studied.The dynamic process of gas adsorption and diffusion in coal were visualized in numerical simulation.In order to study the dynamic process that methane was displaced by carbon dioxide and the effect of CO₂ injection pressure on methane recovery and carbon dioxide injection during CO₂-ECBM,a CO₂-ECBM model based on three-dimensional pore structure was developed.In this model,extended Langmuir equation was applied to characterize the competitive adsorption mechanism of carbon dioxide-methane binary gas system,and the coupling of gas sorption and gas diffusion was achieved as well.The CO₂-ECBM model and numerical method based on the three-dimensional pore structure can be directly combined with the three-dimensional pore structure of coal obtained by synchrotron radiation nano-CT to obtain the real-time three-dimensional distribution of methane and carbon dioxide in the CO₂-ECBM process.The research on the impact of CO₂ injection pressure on methane recovery and CO₂ injection showed that the rate of CH₄ recovery and CO₂ injection was enhanced with the increase of CO₂injection pressure,but the influence is decreasing with the increase of carbon dioxide injection pressure.The change of carbon dioxide diffusion coefficient caused by the variation of CO₂ injection pressure can explain why the increase in carbon dioxide injection pressure can enhance methane recovery and carbon dioxide injection rate.The difference in carbon dioxide injection and methane recovery rate between different coals is related to the difference of gas diffusivity in different coals.The rate of carbon dioxide injection and methane recovery in the coal with higher gas diffusion coefficient is larger.The impacts of stress on gas adsorption and diffusion in coal are mainly caused by the deformation and destruction of coal pores.In this thesis,the capability of resistance to deformation?Young's modulus?and the peak stress where coal is destroyed?strength?were used to characterize the stress sensitivity of gas adsorption and diffusion.Numerical simulation method was applied to study the effects of three-dimensional pore structure on the stress sensitivity of gas adsorption and diffusion.Based on the quantitative characterization of three-dimensional pore structure of coal by synchrotron radiation nano-CT,the mathematical model and parameters describing the pore size distribution of coal were obtained through statistical analysis of the PSD of two coal samples.Combined with the improved quartet structure generation set?QSGS?,coal with predefined pore size distribution and predefined anisotropy value was generated and the impacts of the porosity and pore structure anisotropy on the stress sensitivity of gas adsorption and diffusion were investigated based on the generated coal with predefined PSD and predefined pore structure anisotropy value.It was found that gas adsorption and diffusion in coal with higher porosity is more sensitive to stress.The anisotropy of stress sensitivity of gas adsorption and diffusion increases with the pore structure anisotropy.When the porosity is the same and the overall strength is defined as the strength in the direction where the strength is lower in anisotropic coal,the stress sensitivity of gas adsorption and diffusion in anisotropic coal is higher than that in isotropic coal.The difference of the stress sensitivity between isotropic coal and anisotropic coal increases with the increase of pore structure anisotropy.