Study On Enhanced Coal Seam Gas Drainage Technology Of High Pressure Nitrogen Iniection And Its Field Application

Gas?methane and carbon dioxide?disaster is an important factor restricting the safe production of coal mine,by injecting other gases into the coal seam to promote the accelerated flow of coal seam gas,improve the yield of coalbed methane,that is,ECBM and CO₂-ECBM,for coal mine underground gas extraction provides a new technical means and application model.Based on this,many scholars have carried out the mechanism of carbon dioxide or nitrogen displacement coal seam gas and downhole test research,most of the experiments have verified the existence of multivariate gas?CO₂-CH₄,N₂-CH₄?competitive adsorption?or replacement adsorption?,field engineering test also investigated the feasibility and effectiveness of gas injection displacement technology.For the previous research and analysis,it can be found that the research mainly focuses on the "static" behavior of competitive adsorption,the study of gas bidirectional diffusion law in the dynamic flow of gas injection displacement "injecting gas-producing gas" is basically in a blank state,in order to solve the above problems,the main research contents and research results of this paper include:Four samples with different coal ranks were collected and diffusion experiments were conducted at different pressures through adsorption-desorption processes.Three widely used models,i.e.,the unipore diffusion?UD?model,the bidisperse diffusion?BD?model and the dispersive diffusion?DD?model,were adopted to compare the applicability and to calculate the diffusion coefficients.Results show that for all coal ranks,BD model and DD model can match the experimental results better than UD model.For the fast diffusion coefficient D_ae of BD model,three samples have a decreasing trend with increasing gas pressure while the other sample shows a V-type trend.The slow diffusion coefficient Dte of BD model increases with gas pressure for all samples,while the ratio ? is an intrinsic character of coal and remains constant.For DD model,the characteristic rate parameter k? does not show sharp change and the stretching parameter a increases with gas pressure.Both D_ae and D,e are in proportion to k?,indicating they all reflect the diffusion rate of gas in coal.The one-way diffusion and bidirectional diffusion experiments are carried out by using the gas bidirectional diffusion experimental device developed independently,and the results show that for the one-way diffusion of a single gas,the greater the adsorption equilibrium pressure,the longer the discharge time,the greater the total emission and desorption.Under the same adsorption equilibrium pressure,the emission time of carbon dioxide is longer than that of methane,and the total emission and desorption amount are also greater than methane.For nitrogen displacement of carbon dioxide or nitrogen to drive methane,with the increase of experimental pressure,the breakthrough time of nitrogen gradually lengthens,the displacement time increases gradually,the total carbon dioxide emissions and desorption amount also increase.Compared with the results of unidirectional diffusion experiment,under the same adsorption equilibrium pressure,the total carbon dioxide emission increased by 1.31?1.66 times and the desorption amount increased by 1.69?2.1 times,and the total methane emissions increased 1.42?1.59 times during nitrogen displacement methane.The desorption amount has been increased by 2.22?2.46 times.This indicates that the injected nitrogen can replace the gas which is difficult to extract under normal pressure in coal body,and plays the effect of "displacement".The fitting effect of the dispersion diffusion model is better than that of the double-hole diffusion model,and with the increase of displacement pressure,the characteristic rate parameters k,of carbon dioxide and methane decrease gradually,and the stretching parameters a do not change obviously.By using the numerical method of gas diffusion model,the diffusion coefficient of single gas unidirectional diffusion and two-way diffusion of gases is calculated,and the applicability of the numerical solution of diffusion model is analyzed.For unidirectional diffusion and bidirectional diffusion,the fitting effect of the two-hole diffusion model ? and the double-hole diffusion model ? is better than that of the single-hole diffusion model,and the fitting effect of the two-hole diffusion model ? and the double-hole diffusion model ? is basically close.Whether it is a single-hole diffusion model,a double-hole diffusion model ?,or a double-hole diffusion model ?,the corresponding diffusion coefficients increase with the increase of displacement pressure.Functions y=Ax/B+x can be a good description of the relationship between pressure and D_a1?Du1.The effect of pressure on bidirectional diffusion may be misunderstood by using the analytic solution calculation of diffusion diffusion model.Under the same pressure condition,the diffusion coefficient(D_a1,D_i1)of carbon dioxide and methane unidirectional are greater than that of bidirectional diffusion.Injection of nitrogen in the process of "diffusion-adsorption" to the coal body to a certain extent to reduce the coal body pre-adsorption of carbon dioxide or methane outward "desorption-diffusion" rate,but as long as sufficient displacement time to ensure that the efficiency of the discharge can be greatly improved.A mathematical model for the gas process of carbon dioxide/nitrogen displacement in coal seam considering adsorption desorption,diffusion and apparent permeability is established.By solving the coupling model,the variation of different gas molar concentrations in coal seam over time can be calculated,so that the content,pressure change,injection and output gas quantity,gas composition and so on of different gases in coal seam can be further analyzed.A mathematical model for the gas process of injecting gas displacement in coal seam considering adsorption desorption,diffusion and apparent permeability is established.By solving the coupling model,the variation of different gas molar concentrations in coal seam over time can be calculated,so that the content,pressure change,injection and output gas quantity,gas composition and so on of different gases in coal seam can be further analyzed.In Henan shredded soft and low permeability coal seam,the nitrogen displacement project test was carried out,and the field test results showed that the gas emission was 3.46 times,2.53 times and 3.29 times the conventional emission,respectively,and the gas content in the coal seam before the gas injection test at No.lth test point was?6.07?8.36?m³/t,the gas content in the range of pumping was reduced to?5.41?5.76?m³/t after the test.The maximum gas purity during the injection of 3 stomata was 4.2 times,6.2 times and 6.3 times before the test.The gas emission of 2 stomatal gas injection during the 3-1 group was 3.49 times and 2.97 times that of the conventional emission respectively,and the maximum gas purity during the gas injection was 2 times before the test,5.12 times,and the gas content of the coal seam was 7.56m³/t before the gas injection test in the 3.46 times group of test points.The gas content in the pumping range was reduced to?4.21?5.75?m³/t after the test.When the injection pressure is higher than 0.8MPa,the injected nitrogen can break through the gas injection hole into the gas production hole in the crushed soft and low permeability coal seam,and with the increase of drilling distance,the effect time of promoting pumping is prolonged,and with the increase of injection pressure,the mixing amount and pure quantity increase,but the concentration of gas production also decreases.After stopping,it is still able to effectively improve the pure flow of drilling gas for a period of time.