Study On Characteristics Of Enhanced Coalbed Methane Drainage By Nitrogen Injection Instead Of Gas In Low Permeability Coal Seams

China is a country rich in coal and poor in oil and gas.Its coal reserves rank fourth in the world,second only to the United States,Russia and Australia.At the same time,coal is also one of the most important fossil energy in China.At present,coal and gas outbursts often occur in the process of mine mining,which seriously threatens the safe mining of coal and the economic benefits of the mine.How to ensure mine mining safety and how to desorb underground gas faster has become a hot topic in academic circles.At the same time,compared with the coal of major coal producing countries such as the United States,Australia,Russia and South Africa,China's coal generally has the characteristics of high ash content,deep buried formation,low permeability,low saturation and strong heterogeneity.According to the gas adsorption theory,injecting heat into the coal seam can improve the coal seam temperature,promote the gas desorption of the coal seam and improve the gas extraction rate,Nitrogen injection into coal seam can also be used as protective gas,which is conducive to mining safety.Therefore,it is of great significance in industry to study the gas flow law of coal and strengthen the gas drainage characteristics of heat injection mining.After consulting the relevant literature,this paper summarizes the theoretical knowledge of coal microstructure characteristics,basic thermodynamic properties and the mechanism of nitrogen displacing coalbed methane,which lays a solid foundation for the research.On this basis,the following work is carried out in this paper: coal sample pretreatment,industrial analysis and vitrinite reflectance experiment,coal sample thermogravimetric experiment,nitrogen displacement coal seam gas experiment and coal sample permeability experiment are completed;On the basis of fully considering the gas diffusion adsorption desorption equation and permeability equation,the mathematical model of nitrogen injection displacing coal seam gas is established;The computer simulation is carried out by using COMSOL multiphysics software in the way of multi physical field coupling,and the simulation of hot nitrogen injection to displace gas in different environments is carried out respectively.The main conclusions of this paper are as follows:(1)The greater the nitrogen pressure and temperature injected into the coal seam,the higher the maximum gas permeability of the coal sample under full displacement;Nitrogen can be adsorbed competitively with gas to replace some adsorbed gas;(2)In the experiment of nitrogen displacement of coal seam,the injection of nitrogen will make the coal seam temperature rise slowly in the initial stage.After the reaction is sufficient,the coal seam temperature rises rapidly.The heating rate of200 ? nitrogen is 2.5 times that of 100 ? nitrogen;At the beginning of the experiment,the heating rate of coal samples was slow,and then the heating rate began to increase slowly;The higher the temperature of purge nitrogen,the faster the heating rate of coal sample;(3)Referring to the literature,it can be seen that when nitrogen is initially injected into the coal,it will convection and diffuse with the seepage in the coal,the partial pressure of free methane will be reduced,the coal fracture will have local regional deformation,and the adsorption expansion and thermal expansion will occur on the fracture surface,so the pore fracture reaction in the coal pore will become smaller in the initial stage;With the further increase of nitrogen injection pressure,nitrogen fully enters the coal seam for displacement and brings heat to the coal seam at the same time.The heated coal matrix will initially expand under the influence of thermal stress to reduce the compression of coal cracks,so the permeability of coal samples will be reduced.The higher the nitrogen temperature in the initial stage of reaction,the lower the permeability of coal samples.When the heat absorption is sufficient and the thermal stress and nitrogen gas expansion can reach a certain degree,the internal cracks of coal will crack and open,and the gas migration rate in coal will also increase,Therefore,the permeability of coal samples began to rise rapidly;(4)Under the same working condition,with the increase of formation pressure,the permeability of nitrogen in coal samples will gradually decrease,but its reduction rate will continue to slow down.The increase of formation pressure will reduce the relative pressure between nitrogen and the environment,and squeeze the pores in the coal to make the coal fractures closer,so as to reduce the gas permeability.In the actual industrial mining,the deeper the coal seam reservoir depth is,the greater the formation pressure is,and the more difficult it is to mine coalbed methane.The experimental results are consistent with the reality of industrial industry;(5)Thermal nitrogen injection displacement of coalbed methane can effectively increase the permeability of coal samples and increase the amount of gas desorption,which is conducive to coalbed methane mining;(6)The temperature effect of high-temperature nitrogen flowing in the coal body becomes more and more significant with the increase of its temperature.The higher the nitrogen temperature,the more heat it carries,which is more conducive to heat diffusion and heating the coal seam;(7)Through simulation and experiment,it is demonstrated that heat injection n2-ecbm technology is a promising technology to promote coal seam gas production.It combines heat injection mining with gas injection mining,and nitrogen as protective gas,which can effectively enhance the safety of mine mining;(8)The process of nitrogen injection displacing coal seam gas is divided into two stages.In the first stage,the nitrogen injected into the coal seam will seep,diffuse and then adsorb in the coal seam,and in the second stage,the gas in the coal seam will undergo desorption diffusion and seepage.Temperature rise and pressurization can effectively improve the nitrogen seepage rate in the first stage.Through the combination of simulation and experiment,this paper studies the relevant characteristics of high-temperature nitrogen displacement of coal seam gas,demonstrates that nitrogen injection displacement is a promising way to promote coal seam gas desorption in industry,and also provides theoretical support for the research and development of new technology of mine safe mining.