| A deep study of coal seam gas occurrence and flow mechanism is an important theoretical basis to guide gas drainage engineering and gas disaster prevention.During gas extraction,the adsorbed methane desorbs from the coal surface and diffuses to the fracture through the pores,then flows into the borehole from the fracture.The desorption characteristics determine the gas emission from the matrix to the fracture.The methane-coal isothermal adsorption and desorption curves indicate that there is a hysteresis phenomenon in methane desorption.The desorption hysteresis effect is not considered in the current coal seam gas migration model.The gas exchange between matrix and fracture is driven by pressure(concentration)gradient,but the concentration difference between the adsorption and desorption process is not distinguished.Therefore,it can not accurately reflect the gas migration characteristics during coal seam drainage.To study the influence of desorption hysteresis effect on gas migration,the isothermal adsorption/desorption experiments of methane-coal were carried out,and then the desorption threshold pressure obtained from the experiment was introduced into the methane mass exchange function.The coal-gas coupling model with considering the desorption hysteresis effect was established,and finally,the influence of the desorption hysteresis effect on gas migration and deep coal seam gas drainage was analyzed by the established models.The main conclusions are as follows:1)The pore morphology characteristics of coal samples were obtained by scanning electron microscope,mercury intrusion method and physical adsorption method.It is found that there are a large number of ink bottle pores in the coal samples.The specific surface area and pore volume of micropores increase with particle size,while those of mesopores and macropores decrease with particle size.The specific surface area and pore volume of micropores account for themost of the total value,which indicates that micropores are the prominent pores in the coal sample,and gas mainly occurs in micropores.The FHH fractal dimension has no apparent relationship with coal particle size,while the Sierpinski fractal dimension decreases with the increase of coal particle size.2)Experiments of gas desorption at atmospheric pressure and isothermal adsorption-desorption were carried out with different-particle-size and cylindrical coal samples,and the results show that the larger the matrix size is,the more difficult the gas desorption is.The isothermal adsorption-desorption results show that the desorption hysteresis degree of particle coals ranges from 14.29% to 23.08%,and the desorption hysteresis increases with the increase of coal particle size.The desorption hysteresis degree of cylindrical coal ranges from 33.54 to 39.65%.,which indicates that the desorption hysteresis is positively correlated with matrix size.The hysteresis degree was positively correlated with specific surface area and pore volume of micropores,and negatively correlated with that of mesopores and macropores,indicating that desorption hysteresis was mainly caused by micropores.Furthermore,the mechanism of hysteresis was analyzed from pore deformation theory and adsorption potential theory.3)Taking the gas content as the criterion,the relationship between the equilibrium pressure during the adsorption and desorption process was analyzed,and the pressure difference was defined as the desorption threshold pressure,which increased with the increase of equilibrium gas pressure and coal particle size.The desorption threshold pressure was introduced into the gas exchange function,and the coal seam gas migration model with threshold pressure was constructed.Finally,the coal-gas coupling model considering the desorption hysteresis effect was established,which is more suitable for simulating the gas desorption-diffusion-seepage process and can reflect the coalbed methane flow characteristics during gas drainage.4)Based on the established model,the influence of desorption hysteresis on the gas migration characteristics was analyzed using COMSOL numerical simulation software.It is found that the desorption threshold pressure decreases gradually with gas desorption,and the desorption hysteresis has a more significant effect on the coal seam with low permeability and large matrix size.The simulation of deep coal seam gas extraction shows that the desorption hysteresis effect makes gas extraction more difficult,and the time of outburst risk elimination is longer.For gas extraction in a deep coal seam,it is of great importance to take measures to reduce the desorption hysteresis of coal seam gas.There are 54 figures,14 tables and 115 references in this paper. |
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