| High temperature,high in situ stress and high pore pressure are major common problems faced by deep coalbed methane development in China.By considering the reservoir condition of deep coal seams,this dissertation developed custom-made instruments to study systematically methane diffusion behaviour in four representative coals under high pressure and high temperature conditions.These coal samples consist of the low rank long flame coal,the medium rank gas coal,and coking coal,the high rank anthracite.The influence of coal particle size,gas pressure,temperature and coal rank effect on methane diffusion in coals were investigated.A novel high temperature and high pressure methane diffusion model in coal was proposed and the simplified solution of this model was derived.These works therefore allow the author to explore the methane diffusion mechanism in coals.The following findings can be obtained from this dissertation:(1)Four coal samples have observable fractal characteristics in both the low pressure section of mercury intrusion test and the high pressure section of liquid nitrogen adsorption test.The fractal dimension decreases first and then increases with the increase of coal rank.(2)A suite of high temperature and high pressure methane diffusion simulation test device was built,which can measure four coal samples at the same time.The maximum pressure of the device is up to 60 MPa,and the maximum temperature is around 150 ?.(3)The experimental results of methane diffusion under high temperature and high pressure conditions show that with the increase of coal particle size,the methane diffusion amount,initial diffusion velocity and diffusion coefficient decrease gradually,and 0.17-0.25 mm is selected as the most suitable experimental particle size.Both the cumulative methane diffusion amount and the methane diffusion coefficient increase with the increase of pressure and temperature.The initial diffusion velocity of coal sample is the largest,and the diffusion velocity decreases gradually as time goes.The relationship between diffusion velocity and time is a power function,the higher the degree of coal rank,the greater the cumulative diffusion amount of methane within the same diffusion time.The diffusion coefficient first decreases and then increases while the diffusion velocity monotonically decreases with time.The initial diffusion velocity of methane in high rank coal samples is much higher than that in low rank coal samples.Both pressure and temperature have negative effects on methane diffusivity in coal.In the low pressure stage(2-6 MPa),the pressure controlling diffusion effect dominants.In the stage of medium and high pressure(6-20 MPa),the temperature controlling diffusion effect dominants.(4)Molecular dynamics simulation of methane adsorption and diffusion based on long flame coal shows that with the increase of temperature,adsorption amount and isosteric heat decreases gradually,and the self-diffusion coefficient,the corrected diffusion coefficient and the transfer diffusion coefficient of methane increase.(5)Based on the dynamic change of diffusion coefficient,a high temperature and high pressure methane diffusion model was proposed,and a simplified solution of the new model was deduced.(6)The mechanism of methane diffusion in coal particles was explored.The results show that the more complex the pore structure and the surface of coal,the greater the corresponding influence on methane diffusion coefficient.The increase of gas pressure promotes the increase of the concentration gradient of adsorbent surface which leads to the increase of the diffusion coefficient of methane.The increase of temperature leads to the increase of average free path of gas molecules,the increase of the average kinetic energy of gas molecules and the thermal expansion of pores in coal,which makes methane easy to diffuse in coals.The attenuation characteristics of methane diffusion coefficient in coal are mainly controlled by its diffusion path and resistance in pores of coal. |
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