| Coal seam gas is an important clean energy,and the two main parameters that affect its migration are diffusion and seepage.When mining enters the deep high-stress area,it is relatively difficult to extract gas from coal seam,and the gas migration model based on the classic dual pore-fracture coal structure is not effective in predicting gas production.This is mainly due to the transformation of the physical structure of coal under high stress conditions,which affects the laws of gas diffusion and seepage movement.In this paper,rock mechanics,fluid mechanics,seepage mechanics,adsorption science,fractal geometry,diffusion dynamics and other theoretical knowledge are mainly used.Besides,the pore fracture system of coal was analyzed by fluid intrusion method,scanning electron microscope and CT.The self-developed test equipment was used to analyze the coal mass desorption law under the conditions of different adsorption equilibrium pressure and high stress condition.And the coal mass diffusion coefficient and the effective coal mass diffusion coefficient were obtained under different stress conditions on the basis of high stress diffusion model.Besides,the permeability evolution model is constructed according to the different engineering and natural stress-strain response laws in the fracture structure.And a gas-solid coupling model for gas migration in deep high-stress coal seams is proposed.The main conclusions are:1)Many testing methods were used to describe the pore and fracture systems.According to the fractal dimension theory and mercury injection method,the boundary pore size of diffusion and seepage system is determined.As for the specific surface integral and pore volume fractal,it can be seen that the boundary apertures of 1-3 mm coal particle and block coal are basically between 30-40 nm,and the boundary apertures obtained by the two fractal methods are relatively close.The sensitivity of porosity to stress is very low under high stress states,which can reflect that the change of fracture permeability is small and the fracture is in a closed state at this monent.Combined witn the gas migrtation laws in the pore and fracture,the master role conservation model about gas amount was established.2)The difference of coal morphology will lead to the difference of diffusion evolution laws.According to the relationship between gas desorption amount and time in the whole process,the diffusion model of coal particles considering the amount of gas loss in the adsorption state was established.Results show that the diffusion coefficient of 0.2-0.25 mm and 1-3 mm coal particle are about 1×10-11 m2/s-2×10-11m2/s and 1.1×10-9 m2/s-1.7×10-9 m2/s,respectively,which means the larger the particle size,the greater the diffusion coefficient.Besides,the diffusion model of cylindrical and cube-shaped coal is constructed with the cylindrical shape as the matrix unit.And the diffusion coefficient of cylinder and cube coal sample are about 0.8×10-1010 m2/s-3.3×10-1010 m2/s and 0.76×10-1010 m2/s-1.95×10-1010 m2/s,respectively.The diffusion coefficient values of the cube-shaped and 0.2 mm to 0.25 mm coal particle are in the same order of magnitude.However,the desorption amount of coal per unit mass in the same time varies by about 2-2.5 times,which is mainly related to the different matrix shape factors caused by the morphological.The shape factor is also an important factor to affect the amount of desorption,and the shape factor of 0.2-0.25 mm coal particle is about 26.5 times that of cylindrical coal.3)The effect of high stress on the physical structure of coal was discussed.And the evolution characteristics of coal fracture space with the change of stress were studied by CT and permeability experiments,which can obtain the characteristics of the fracture system of coal mass under high stress.According to this,the coal mass is regarded as a topological neural network composed of an infinite number of interchangeable pores plus the coal body.In network,the local pore system is composed of numerous similar local pore systems,and the local pore system is based on the dimension of the diffusion length of gas molecules to intercept the matrix microbodies.Based on this,the structure model of pore network topology microelement is presented.Meanwhile,according to some reasonable assumptions,the theoretical relationship between diffusion length and stress is put forward,which satisfys the Langmuir form.And then the diffusion model of coal mass under high stress condition is constructed.4)The diffusion evolution laws of high stress coal mass under the natural condition and is obtained.The coal mass diffusion coefficient and the effective coal mass diffusion coefficient are about1.83×10-19 m2/s-7.02×10-19 m2/s and 5.6×10-6 1/s-25.41×10-6 1/s under different adsorption equilibrium pressures respectively when the stress are 20 MPa?30 MPa and 40 MPa.In addition,the diffusion coefficient and effective diffusion coefficient of coal mass under high stress condition increased with the increase of adsorption equilibrium pressure.However,under the same adsorption equilibrium pressure,the larger the stress is,the larger the diffusion coefficient is,and the smaller the effective diffusion coefficient is.Compared with the unstressed coal mass,the physic structure of coal is changed under high stress state,which causes the style of diffusion to change the surface diffusion.And this is an important factor that cause the difficulty of gas extraction in deep high-stress coal seam.5)The diffusion evolution laws of high stress coal mass under the step by step condition is obtained.Different desorption pressure gradients also have great influence on gas diffusion.On the experimental basis of 40 MPa stress and 5 MPa gas equilibrium pressure,when the pressure gradient is 1 MPa,the maximum coal mass diffusion coefficient is 9.96×10-1919 m2/s corresponding to 1-0.1 MPa,and the minimum value is3.21×10-19 m2/s corresponding to 5-4 MPa.It shows that the coal mass diffusion coefficient increases gradually in the process of pressure decreasing.For experiments with different desorption pressure gradients of 5-4 MPa,5-2 MPa and 5-0.1 MPa,the corresponding coal mass diffusion coefficients were 3.21×10-19 m2/s,6.37×10-19 m2/s and 7.02×10-19 m2/s,indicating that the larger the pressure gradient,the greater the coal mass diffusion coefficient.6)The permeability evolution model based on different fracture strain response mode was established.According the coal permeability is affected by the role of effective stress and adsorption expansion deformation,the phenomenon of permeability rebound and rebound were obtained.Under the condition of fixed parameters,for the initial pressure of the reservoir and the compressibility coefficient of the fracture,as long as the gas pressure drops to a certain value,the permeability of the coal mass will rebound and recover,while for the internal expansion coefficient,it is the opposite,and the permeability of the coal mass will never rebound and recover.7)Multi-field gas-solid coupling model is used to analyze the law of gas migration in coal seam.The multi-field gas-solid coupling models which are suitable for deep high-stress and low stress coal mass were adopted,and the gas migration laws were obtained under these two stress conditions.The results show that the change of physical structure model of high stress coal mass is the essential reason why the deep coal gas is difficult to extract.As for the study on the conversion of the master role conversion in deep coal seam,it can be seen that the time when the seepage plays the main controlling role is shorter than that of diffusion.At last,the interaction between extraction boreholes is discussed by using the pressure drop coefficient,and the effect of trapezoid pattern is regarded as the best than rectangle pattern and diamond pattern.This dissertation has 95 figures,32 tables and 213 references. |
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