Expelling/Displacement Effect Of Gas Injection On Methane Desorption In Tectonic Coal Under Different Gas-diving Modes

Nappe structures of coal seams in the south of China are generally developed and complex,resulting in high gas content in coalbed and rapid gas release exposed to air.In view of the problems of high gas pressure,poor permeability,and fast attenuation of in gas drainage in Puxi V coal seam,Jiahe Mining Area,Hunan Province,underground gas injection is proposed to realize rapid outburst elimination in tectonic coal seam.In the process of underground gas injection to displace gas,it includes the seepage movement（high pressure injection）of the injected source gas to coal fractures and large pores under the effect of pressure difference;There is also a phenomenon of gas diffusion（isobaric diffusion）at the end of coal micropores depending on the concentration difference.To explore the gas displacement mechanisms in tectonic coal under different gas driving modes,this paper mainly aims at the pore structure characteristics of tectonic coal,the difference of CO₂/N₂gas displacement effects under isobaric diffusion and high pressure injection conditions,the molecular dynamics responses of CO₂/N₂and CH₄competitive adsorption in coal molecules,the diffusion and displacement characteristics of injection gas in coal seams,and the influences on gas drainage.Results of pore structure of coal samples by combining mercury injection and low temperature nitrogen analysis（LNA）show that the cumulative pore area of Puxi well anthracite samples measured by mercury intrusion method is between 5.93 and 7.06 m²/g,the proportion of microporous surface area in coal is the largest,and the proportion of small pore is the second,while the contribution of large pore and fracture to total pore volume is the largest.The total pore area and pore volume of anthracite measured by LNA method gradually increase with the decrease in protodyakonov coefficient of samples,and the distribution characteristics of specific surface area are the largest proportion of micropores.The anthracite samples have large hysteresis loops in both high and low pressure regions,and the nitrogen desorption curve has an obvious downward inflection point,indicating that there are semi-open thin necked bottle holes in coal samples.The coal samples have obvious fractal characteristics both in the low pressure range by mercury intrusion and in the high pressure section by LNA method.Furthermore,the fractal dimension of V coal is larger owing to a lower hardness than VI coal,and the roughness and complexity of pore surface of V coal are greater.Under the isobaric diffusion driving mode,the total displacement amount of CH₄by different gas injections is caused by actions of gas partial pressure and competitive adsorption effect.In the isobaric diffusion process of N₂,the desorption amount of CH₄is dominated by gas partial pressure.As the initial adsorption pressure of CH₄increases,the total desorption amount of CH₄by CO₂/N₂injection increases,but the CH₄desorption efficiency and the injection/displacement ratio of each gas to CH₄gradually decrease.When the gas injection pressure increases from 0.5MPa to 2.5MPa,the desorption rate of CH₄by CO₂diffusion decreases from 42.1%to 33.57%;and the CH₄desorption rate by CO₂diffusion decreases from 30.06%to 24.8%,indicating that the displacement efficiency of gas isobaric diffusion is continuously weakened with the increase of adsorption pressure.CH₄desorption rate and injection/placement ratio of CO₂injection are both greater than those of N₂injection,so the displacement ability of N₂isobaric diffusion for CH₄is weaker than that of CO₂injection.After N₂injection,the total pressure of system tends to rise;while after CO₂injection,the total pressure gradually decreases.Under the high pressure gas driving mode,the phenomenon of CH₄displacement by different gases is absolutely dominated by competitive adsorption effect,while the partial pressure of gases plays a inhibiting role in the CH₄displacement.The total desorption amount of CH₄by different gas injections also increases with the increase of initial equilibrium pressure,but the displacement rate and injection/displacement ratio of CH₄decrease accordingly.Under the same initial pressure,the desorption amount,desorption rate and injection/displacement ratio of CO₂injection are significantly greater than those of N₂injection.The maximum values of CH₄desorption rate and injection/placement ratio by CO₂driving are 18.07%and 0.232,respectively;When high-pressure N₂driving,maximum values of CH₄desorption rate and injection/placement ratio are only 10.63%and 0.082,respectively.Therefore,the CO₂displacement performance is better than N₂during high-pressure gas injection displacement.According to molecular dynamics simulation analysis,the dispersion interaction between CO₂/CH₄/N₂and coal molecules is dominant,with CO₂possessing the largest dispersion interaction energy,followed by CH₄and N₂.The total displacement of CH₄is controlled by the competitive adsorption capacity and partial pressure of the gas.At the initial stage of gas injection,the high-pressure driving effect is dominant,while it rapidly decays.After the system reaches equilibrium,the displacement action plays a major role by gas isobaric diffusion.Overall,the displacement effect of CO₂injection is stronger than that of N₂injection.The higher the adsorption pressure before gas injection,the faster the displacement rate during high-pressure driving.After the system equilibrates,it undergoes a displacement action dominated by isobaric diffusion,during which a large amount of adsorbed gas is desorbed and produced.The amount of CH₄displaced at this stage determines the total displacement capacity of the system.The total displacement amount of CH₄is controlled by the competitive adsorption capacity and partial pressure action of injection gas,and the displacement effect of CO₂on CH₄is also stronger than that of N₂.Moreover,during isobaric diffusion,the displacement effect of the partial pressure action of the injected gas is also greater than that by high-pressure injection mode,resulting in a greater amount of CH₄ultimately displaced by isobaric CO₂injection.The higher the adsorption pressure of CH₄during gas injection,the lower the displacement efficiency of CH₄.The gas driving simulation results of CO₂/N₂injection in the V coal seam of Puxi coal mine show that the diffusion radius of CO₂and N₂increases linearly with the gas injection time under different injection pressures;The diffusion radius of N₂is significantly larger than that of CO₂in both the case of isobaric diffusion and high-pressure injection.After 8 hours of gas injection at 1.5MPa,the diffusion radii of N₂and CO₂are 0.96m and 0.30m,respectively.Although the diffusion capacity of CO₂is less than that of N₂,the CH₄pressure relief range of CO₂displacement is significantly larger.After the CO₂/N₂underground injection tests were carried in V coal boreholes,the pure gas drainage flow of each group of inspection boreholes increased significantly.Whether CO₂or N₂is injected,the average gas extraction volume of high-pressure gas injection is greater than that of isobaric diffusion.However,with the increase of CO₂/N₂injection pressure,the gas injection/displacement ratio gradually decreases.The closer the borehole is,the shorter the breakthrough time of CO₂/N₂diffusion in V coal seam is,and the better the displacement effect is.Under the condition of isobaric diffusion,the average gas flow of CO₂injection into Group A holes and N₂injection into Group C holes are maintained at about 0.01m³/min;while under the condition of high-pressure gas injection,the average gas flow of CO₂injection into Group B holes is maintained at 0.0165m³/min,and the average gas flow of N₂injection into Group D holes is about 0.013m³/min,which shows that the displacement performance of high-pressure CO₂injection is better than that of N₂injection.