| The disturbance caused by coal seam construction or gas pressure relief will cause the damage of surrounding coal and rock.The damage not only makes the structure properties and mechanical behavior of coal and rock mass changes,but also change the physical parameters of coal and rock mass(such as porosity,permeability,etc.)are eventually caused a great impact on gas occurrence and migration.In addition,the migration of gas not only changes the pore pressure,the effective stress of coal seam and adsorption swelling deformation,resulting in evolution and the accumulation of damage,but also takes away a part of the heat,changing the temperature field of coal seam.The change of temperature will not only affect the stress distribution of coal and rock mass,but also affect the adsorption and diffusion ability of gas.The gas drainage is actually a multi field coupling process of coal rock damage field,temperature field,seepage field of gas.Based on the coal unloading damage theory,the principle of effective stress and porous medium heat and mass transfer mechanism and finite element knowledge,this paper studies the distribution and evolution law of coal rock damage field and gas seepage field under multi field coupling by theoretical derivation and numerical simulation,which has important theoretical significance to guide the design of gas drainage and improve the gas drainage rate.The specific research contents are as follows:(1)Coal rock is regarded as the pore fissure double medium,according to the characteristics of coal and rock damage,this paper defines the damage variable of coal and rock by the effective strain,and the constitutive equation of elastic damage of coal and rock,which takes into account the influence of gas pressure and temperature.(2)According to the degree of damage to describe the degree of fragmentation of coal,the Weibull distribution is used to simulate the heterogeneity of coal and rock.Based on the influence of Fick diffusion,Knudsen diffusion and slip effect on gas flow,the diffusion and seepage coupling equation of gas in coal seam is established.In addition,the influence of gas flow on the change of internal energy,heat of adsorption and deformation of coal and rock on the temperature field is considered.(3)In this paper,the finite element source program is second developed using FORTRAN language and the multi field coupling is developed considering temperature,gas seepage and deformation of coal rock damage analysis program,and the correctness of this procedure is verified.(4)This program is used to simulate the process of gas drainage in the coal mine.The results show that,in the early stage of drainage,the damage range around the borehole is about 3.5 m,and with the continuous development of the drainage time,the damage scope of coal is gradually expanded,and the permeability of coal seam increases by an order of magnitude.(5)The program is used to calculate the process of CBM by heat injection.The results show that heat injection can significantly promote gas desorption and gas diffusion and improve the gas production rate and gas production efficiency.Under the constant temperature boundary,the influence range of heat injection is about 0.5 m,and the production rate of gas production and gas production are more than 110%,more than 90%,respectively. |
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