| In gas extraction projects,the diffusion of adsorbed gas,which accounts for 80%to 90% of the total volume,into the free state from the coal matrix into the fracture is an important part of maintaining continuous gas extraction.Sorption time is a key parameter that describes how quickly gas diffuses from the matrix into the fracture,and is generally defined as the time it takes for coal gas desorption to reach 63.2% of the total gas adsorbed.The sorption time is usually measured by gas isothermal desorption experiments on granular coal or lump coal,and the results are affected by the particle size effect of granular coal or gas seepage behavior in lump coal;meanwhile,due to the existence of adsorption-desorption hysteresis,the measured value of adsorption time by traditional adsorption time measurement methods may appear to be infinite.To solve the above problems,this thesis proposes an innovative method to decouple the "diffusion-seepage" behavior by changing the seepage channels of coal samples to obtain the sorption time parameters and analyzes the effect of modified seepage channels on the pore and fracture structure by using Micro-CT.Isothermal gas adsorption/desorption experiments were carried out to study the evolution of sorption time under different load levels,and the influence of sorption time on the gas transport characteristics of the coal and gas extraction in deep coal seams was analyzed by numerical simulation methods,and the following conclusions were obtained:(1)The 3D pore-fracture reconstruction results of the same coal column before and after the transformation of the seepage channel using Micro-CT scan analysis show that the average value of similar comparison distinction between the images of the internal sections of the coal column before and after the prefabricated 3 mm penetration hole is 2.25%,and the spatial distribution and volume ratio of the pore-fissure structure and mineral components remain basically unchanged;the difference of the total porosity between the original coal column and the prefabricated hole coal column is0.0452‰,and the overall similarity between the two 3D pore volume renderings is extremely high,and the average value of pore volume difference in eight intervals is5.8672%.The above data indicate that the 3 mm penetrating pore only serves as a seepage channel to increase the seepage capacity of the coal body and hardly changes the microscopic pore structure around the pore.(2)The effect of seepage channel modification on the adsorption and desorption characteristics of coal bodies was analyzed by conducting isothermal coal adsorption and desorption experiments with coal columns at SH and SHK mines under no and different surrounding pressures.Through the isothermal adsorption-desorption experiments of coal column without surrounding pressure,it was found that the adsorption curves of the original sample and the prefabricated hole sample showed the same trend,the larger the diameter of the hole of the prefabricated hole sample the larger the desorption of the coal body in the same time,and the limit desorption time of the samples from SH,SHK-3S and SHK-3X under the same desorption conditions were different.The isothermal adsorption desorption experiments of coal column under different surrounding pressures showed that the adsorption equilibrium pressure of the original sample was smaller than that of the prefabricated hole sample under the same initial gas pressure and the same surrounding pressure;the larger the surrounding pressure was,the larger the desorption amount of the coal was in the same time,and the desorption amount of the original sample was larger than that of the prefabricated hole sample in the same time,and the ultimate desorption time of the sample showed a trend of first increasing and then decreasing with the increase of the surrounding pressure.(3)By comparing the adsorption equilibrium time of helium and methane,the pressure of the coal body reaches a constant value in about 12 min after filling with helium,while it takes 196 h-213 h to reach the adsorption equilibrium after filling with methane,which indicates that the non-adsorbed gas fills the pore space of the coal column in a short time,and the diffusion process is the controlling factor of the methane adsorption equilibrium in the late dominant coal.Based on the analysis of the effect of coupled mass transfer of "desorption-diffusion-seepage",a new method of decoupling the behavior of "diffusion-seepage" by changing the seepage channel with prefabricated holes in the coal is proposed.By comparing the methane desorption rates of the coal column before and after the modification of the seepage channel,it was found that the differences between the samples in the early period were large,while the desorption rate data were "entangled" afterward,inferring that the differences in the first period were controlled by the seepage behavior,while diffusion dominated the gas transport in the coal column in the later period.The difference in methane desorption rate between different seepage channels in the coal column is divided into seepage and diffusion-dominated phases.(4)This thesis proposes to back-calculate the interporosity flow rate based on the diffusion-controlled methane release content and calculate the sorption time of samples by using the "proposed steady-state" interporosity function.The sorption times of the samples from SH SHK-3S and SHK-3X under the no-pressure conditions were 271.4min,523.6 min and 242.8 min,respectively,with a difference of 2.15 times between the sorption times of coal samples from different locations.The sorption time of the samples under the circumferential pressure of 5 MPa,10 MPa,15 MPa and 25 MPa were 255.3 min,391.5 min,268.6 min and 1107.4 min,respectively;with the increase of the circumferential pressure,the sorption time calculation results showed an exponential increase and the trend of increasing adsorption time was more obvious.The sorption time of the samples under SHK-3X at 25 MPa was 4.33 times of that at 5 MPa.Due to the hysteresis effect of adsorption and desorption,the percentage of gas residual in each sample was above 40%,which was greater than the total adsorption amount of36.8%,leading to the failure of the calculation of the conventional sorption time measurement method,which resulted in infinity.(5)The effects of different sorption times on coal gas flow were simulated and analyzed from laboratory scale and deep coal seam gas extraction perspectives.The results of coal column gas desorption simulation showed that the rate of coal column gas desorption decreases and the time required to reach a certain value increases as the sorption time increases,in which the initial rate of coal column gas desorption decreases by 4.11 times when the sorption time changes from 0.1 d to 0.5 d and the time required to reach 8.50 m L/g increases by 3.08 times.The desorption rate is more sensitive to the change of sorption time.The simulation results of deep gas extraction showed that the time required to predict gas extraction in the range of 2 m from the borehole increased by 1 d,5 d and 86 d when the adsorption time was increased by 10 times,respectively,and the larger the value of sorption time was,the longer the time required to predict the extraction time,and the greater the gas flow rate and accumulated gas extraction in the same extraction time.The accuracy of the sorption time parameter has a great influence on the prediction of gas extraction effect in deep coal seams.There are 66 figures,29 tables,and 92 references in this thesis. |
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