Study On The Characteristics Of Coal Physical Field And The Evolution Law Of Gas Occurrence State During Gas Injection

The proposed"double carbon"goal has brought a new opportunity for the development of coal bed methane industry,but also forced the coal industry to accelerate the pace of green transformation,speed up the construction of coal mine gas extraction and utilization technology system is imminent.However,with the increasing of coal mining depth,mining intensity,mining speed and mining scale,gas disasters are becoming more and more serious,and gas control costs remain high.Especially,the low permeability coal seam is the majority in deep mines,which increases the difficulty of prevention and extraction.Coal seam gas injection technology,as a technical means that can not only"increase permeability"and"increase permeability"but also increase flow energy in coalbed methane mining and gas extraction,is of great significance for improving the mining efficiency of coalbed methane and reducing greenhouse gas emissions in the coal mining process.In this paper,the main line of research is the gas injection displacement process under the solid-gas coupling.Centering on the key technical issues such as the characteristics of the physical field inside coal body and the evolution process of gas state during the gas injection process,the influence of different gas injection sources on the gas injection process is deeply studied by combining theoretical analysis,numerical simulation and experimental research.The spatial and temporal distribution characteristics of gas pressure field,temperature field and concentration field during gas injection are analyzed,and the evolution law of gas state inside coal body during gas injection is revealed.The leading role of storage and displacement and dynamic transformation mechanism during gas injection are analyzed.The following main conclusions can be drawn from the above research:（1）Based on the multi-component gas replacement system of gas-containing coal,the change law of gas pressure,temperature and gas occurrence state of CH₄,N₂and CO₂in the process of single gas adsorption,multi-component gas competitive adsorption and gas injection replacement was studied,and the evolution law of gas state in the process of adsorption was revealed.During the adsorption process,the gas pressure and temperature change characteristics of coal are consistent.The adsorption process of the three gases can be roughly divided into two stages:rapid adsorption and slow adsorption.The rapid adsorption stage lasts for a short time,but the adsorption amount is large.The slow adsorption stage lasts for a long time,but contributes little to the increase of adsorption capacity.In the process of competitive adsorption,the variation characteristics of gas pressure and temperature inside coal body are similar to that of single adsorption process,and the variation range of gas pressure and temperature is proportional to the amount of gas adsorption.In the process of gas injection replacement,the phase characteristics of CO₂gas injection pressure are obvious,while N₂injection is not obvious in the process of replacement because of its weak adsorption ability and limited N₂can be absorbed into coal.（2）The distribution characteristics of gas pressure field,temperature field and concentration field of N₂and CO₂source gases in coal body and the changes of gas flow at inlet and outlet during gas injection were analyzed,and the physical field characteristics of weak adsorbent gas N₂and strong adsorbent gas CO₂during gas injection were revealed.According to the spatial distribution law of gas pressure,the gas pressure in coal body is not evenly distributed when the gas injection is 0.7MPa at constant pressure,and the maximum pressure difference in the whole coal body can reach 0.17MPa.From the spatial distribution of temperature,the distribution of temperature in the whole coal body is not uniform,especially in the N₂injection process,the temperature difference is large,the maximum temperature difference in the whole coal body is more than 5℃.From the spatial distribution law of concentration,the distribution of gas concentration in the whole coal body is very uneven,and the concentration difference in some areas can reach more than 90%.In terms of flow rate,as a large amount of CO₂is retained in coal in the form of adsorption state during CO₂injection,the inlet flow rate during CO₂injection is much higher than N₂injection.In the process of N₂injection,since most N₂exists in the form of free state,the flow rate of N₂injection at the outlet is significantly higher than that of CO₂injection,no matter the total flow rate of the outlet gas or the flow rate of the source gas.（3）The temporal and spatial differences of gas evolution during gas injection are studied.The aging characteristics and field effects of free gas and adsorbed gas in coal during gas injection are studied.In order to study the aging characteristics,the difference method in calculus was adopted to analyze the change rule of free gas and adsorbed gas over time by using the real-time monitoring data of gas pressure field,temperature field,concentration field and flow at inlet and outlet.Since CO₂’s adsorption capacity was significantly stronger than N₂,the total amount of adsorbed gas in N₂injection process continued to decline.However,the total amount of adsorbed gas in the process of CO₂injection continues to increase.In order to study the field effect,a mathematical model of thermal-fluid-solid coupling was constructed to analyze the spatial distribution characteristics of free gas and adsorbed gas.As more CO₂is converted into adsorbed gas during gas injection,its flow velocity is slower than that of N₂,and its influence range in coal is smaller than that of N₂.However,because CO₂has stronger adsorption,its displacement effect on the original gas is stronger than N₂.Therefore,in the process of CO₂injection,properly extending the gas injection time can better improve the gas injection effect.In the process of N₂injection,increasing the gas injection pressure and increasing the fluidity of N₂in coal can also effectively improve the gas injection effect.（4）On the basis of the experimental study of gas injection displacement,the physical field characteristics,the gas state variation rule,the leading role of setting and driving and the dynamic transformation mechanism in the process of gas injection pumping were studied by numerical simulation.In the actual underground gas injection process,with the continuous desorption and outflow of gas,the gas pressure in coal body shows a downward trend.With the progress of gas injection,although the temperature near the injection hole and extraction hole changes greatly,it has little influence on other areas of coal body.During the whole gas injection process,the gas concentration continues to decrease,while the gas concentration of the injection source continues to increase.In the process of N₂injection,the internal permeability of coal body shows an overall trend of increasing,while in the process of CO₂injection,the internal permeability of coal body continues to decrease.According to the change law of free gas,the change characteristics of free gas in coal body during N₂injection and CO₂injection are similar.However,from the change rule of adsorbed gas,although the gas content of the injection source continues to increase and the gas content continues to decrease,from the change of the total adsorbed gas,the total adsorbed gas content in the N₂injection process continues to decrease,while the total adsorbed gas content in the CO₂injection process continues to increase.The analysis of the leading role of placement and displacement and the dynamic transformation mechanism in the process of downhole gas injection shows that the leading role of N₂injection is carrier displacement,while the leading role of CO₂injection is displacement and desorption.N₂,as a representative of weakly adsorbed gas,first"takes out"the free gas in coal through carrying displacement after entering coal,and then promotes desorption of adsorbed gas under the combined influence of dilution diffusion and partial pressure.As a representative of strongly adsorbent gas,CO₂first competes for the adsorption vacancy with the adsorbed gas after entering the coal body,replaces and desorbs the adsorbed gas to transform it into a free state,and then is"taken out"of the coal body by subsequent incoming CO₂gas.There are 151 figures 18 tables and 206 references in this thesis.