Pore Sensitivity Of CH₄ Adsorption By Coal Particles And Its Control Mechanism On Diffusion Behavior With Applications

Coal is a complex porous medium.In the multi-stage pores of coal particles,there are obvious differences in the adsorption state of CH₄,which has pore sensitivity.When the adsorption equilibrium state is broken,the adsorbed CH₄ is converted into free CH₄and released from the multi-stage pores of coal particles.The adsorption location and adsorption mode of CH₄ in the pores will have an effect on the diffusion behavior of CH₄.In order to study the influence of the pore sensitivity of CH₄ adsorption on the diffusion behavior of coal particles,this thesis firstly tested the basic and pore parameters of four coal samples from Qinan coal mine,and carried out Monte Carlo simulation study of CH₄ adsorption in multi-stage pores in coal particles based on the experimentally obtained parameters,and obtained the relationship between the number of CH₄ molecules adsorbed in pores of different pore sizes and the adsorption equilibrium pressure,and then established time-series bidisperse diffusion model considering the pore sensitivity of CH₄ adsorption in coal particles;after that,the CH₄diffusion coefficients were solved based on the idea of inverse problem modeling and the reliability and applicability of the diffusion model were verified,and finally the diffusion model was developed as software and applied to the accurate prediction of drill cuttings gas desorption index K₁.The main conclusions obtained in this thesis are as follows:（1）The pore structure characteristics of the coal samples were analyzed by physical adsorption method,and the pore structure parameters of the four coal samples were calculated by DFT and BJH analysis methods,and it was found that in the pore volume distribution,the pore volumes of the four coal samples in the pore size range of0.33～1.5 nm pores accounted for 65.06%～85.55%of the total pore volume.In the distribution of specific surface area,the proportion of specific surface area of pores in the pore size range of 0.33～1.5 nm to the total specific surface area of the four coal samples ranged from 98.06%to 98.97%.Therefore,it can be concluded that the pore size interval of 0.33～1.5 nm pores contributed the most to the total pore volume and total specific area.（2）The pores of the coal samples were simplified to a slit model,and the cell parameters of the slit model were determined by the X-ray diffraction experimental results.The pore size of 0.57 nm,0.79 nm,0.87 nm,0.975 nm,1.085 nm,1.2 nm,1.315nm,1.43 nm,and 2 nm slit pores were analyzed by Monte Carlo simulations at different adsorption equilibrium pressures.The number of CH₄ molecules accommodated in a single cell was positively correlated with the adsorption equilibrium pressure.As the adsorption equilibrium pressure increases,the sensitivity of pores to adsorption of CH₄molecules varies with different pore sizes.The pore size of 0.570 nm can adsorb a large number of CH₄ molecules at low pressure and has a significant pore sensitivity.When the pressure increases to 1 MPa,the number of CH₄molecules increases very slightly and gradually stabilizes,while the number of CH₄ molecules in the pores of the remaining simulated pore sizes increases continuously with the increase of pressure.（3）After the adsorption equilibrium state is broken,the resistance to desorption diffusion of CH₄ molecules adsorbed in the complex pore structure of coal is different.Based on the differences in adsorption sensitivity,adsorption potential,and diffusion rate of CH₄ molecules in the pore size range of 0.38-0.76 nm and pore size>0.76 nm,micropores and macropores were defined.The adsorbed CH₄ molecules in macropores are subject to less force,more sensitive to the lowering of pressure,and easily diffuse out of the pores by rapid desorption.CH₄ molecules in micropores are subject to strong adsorption potential traps,are less sensitive to the lowering of pressure,and are difficult to desorb from the pores initially.According to the adsorption characteristics of CH₄molecules in macropores and micropores,time-series bidisperse diffusion mode was proposed to divide the diffusion process into two stages.In the first stage of diffusion,the CH₄ molecules adsorbed in macropores diffuse out rapidly.In the second stage of diffusion,the CH₄ molecules in micropores diffuse out slowly through micropores and macropores in turn.（4）Time-series bidisperse diffusion model considering the pore sensitivity of coal particles for CH₄ adsorption was developed.The diffusion process was divided into two stages based on the macropore adsorption ratio,which was determined based on the Monte Carlo simulation results and the pore structure parameters of the coal.In the first stage of diffusion,the unipore diffusion model was used to solve the macropore diffusion coefficient of CH₄ in the first stage of diffusion.In the second stage of diffusion,the optimization module of COMSOL simulation software was used to invert the CH₄ diffusion coefficients.It was verified that the new model can accurately describe the whole process of CH₄ diffusion in coal samples,and the macropore adsorption ratio in the bidisperse diffusion model does not match with the actual adsorption characteristics of CH₄ and the over-parameterization was solved while satisfying the accuracy of fitting.（5）The K₁ of four coal samples was tested using the WTC gas prominence predictor when the adsorption equilibrium pressure was 2MPa,and the relative errors ranged from 33.96%to 61.54%when comparing the test results with the actual desorption volume at the first min of the desorption experiment.The method of calculating K₁ based on the new model was proposed,and the new model was developed as diffusion model APP in order to facilitate the application of the new model.the first min desorption of four coal samples at the adsorption equilibrium pressure of2 MPa was calculated by applying the diffusion model APP.The relative errors were0.34%～6.83%when comparing the first min desorption calculated by the diffusion model APP with the actual first min desorption of the coal samples.This shows that the calculation of the first min desorption based on the new model is reliable.There are 46 figures,21 tables and 116 references in this thesis.