Numerical Simulation Of CO₂ Injection Displacing Coalbed Methane And Sequestration In The Deep Coal Seam

The paper takes the block of Shizhuang North Block in Qinshui Basin as the research area and samples the No.3 coal seam of Changping Coal Mine（CP）,Zhaozhuang Coal Mine（ZZ）and Shizhuang Coal Mine（SZ）in the study area.The results of industrial analysis showed that the coal samples were high fixed carbon coal with low water and low ash,and the coal rank was close to anthracite.The content of vitrinite in coal samples exceeds 65%,and the organic matter type is type III kerogen.Ro is above 2.4.The analysis of the whole rock mineral indicates that the coal samples are dominated by clay minerals and quartz.Mercury injection experiments show that the porosity of coal rock is between 4.7%and 6.2%,and the permeability is between0.13 and 0.76mD,which is obviously characterized by low porosity and low permeability.Low-temperature nitrogen adsorption experiments showed that the pore size of coal samples ranged from 6.379 to 30.159 nm,the BET surface area ranged from 2.126 to 10.053 m ²/g,and the BJH total pore volume ranged from 0.0052 to 0.0134 mL/g.Pure gas isothermal adsorption experiments showed that the adsorption order of the three gases for coal samples was CO₂>CH₄>N₂,while CH₄V_L SZ>CP>ZZ,N₂V_L SZ>CP>ZZ,CO₂V_L CP>SZ>ZZ,ZZV_L is low to prove that there are more closed pores in the mercury intrusion.The multi-gas adsorption and desorption experiment showed that the Langmuir parameter of the gas mixture is generally lower than the Langmuir parameter of the pure gas.The preferential adsorption of CO₂ molecular layer by the CO₂ hinders the continuous adsorption of the CH₄ and proves that the CO₂ and The incompatibility of CH₄ facilitates CO₂ flooding the coal seam CH₄.The Langmuir parameter of the desorption experiment is generally lower than that of adsorption,indicating that the desorption of the coal sample has hysteresis.Using Simedwin to analyze the influence of reservoir permeability and coal diffusion rate on the mixture injection,it was found that the high permeability reservoir N₂ is easy to break through,while the high CO₂ component has a higher yield and yield of CH₄ in the later period.Therefore,it is necessary to inject a high proportion of CO₂ gas mixture（95%CO₂-5%N₂）.The high CO₂ component gas production peak of the medium-permeability reservoir is too late.Although the high-N₂ component has high CH₄ yield and yield,it is easy to break into the production well,so it needs to inject 50%CO₂-50%N₂ mixture.The low-permeability reservoirs do not break through regardless of the composition of the injected component.The high-N₂ component has advantageous yields and yields,so it is necessary to inject a high proportion of N₂ mixture or even pure N₂.The diffusion rate will not affect the final production of CH₄,but directly controls the diffusion rate of the three gases in the coal seam.The coal bed has a high diffusion rate for CH₄ and CH₄ has a high gas production rate.The low diffusion rate of N₂ in the coal seam will lead to an early breakthrough in pressure gradient migration N₂ without displacing CH₄.The Langmuir parameter of CH₄ directly influences the final yield and daily yield of CH₄,while the lower the Langmuir parameter of CO₂ and N₂,N₂ will break through to the production well earlier.Taking the Shizhuang North Block as an example,if the permeability of the reservoir is below the limit,and the target CO₂ is sequestered,pure CO₂ can be injected and the matrix will not expand;this will increase the CBM recovery.In order to achieve this goal,high N₂ or even pure N₂ needs to be injected to increase the partial pressure of N₂,increase reservoir permeability,and obtain high CH₄ total production and daily gas production.To take both CO₂ storage and CBM recovery into account,only reservoir modifications can be carried out,and the permeability of the coal seam can be increased by means of fracturing to reach about 1md,and 50%CO₂-50%N₂ can be injected;Above 3md,not only can we ignore the low gas production in the early stage of injection,but also can inject pure CO₂ to obtain high CO₂ storage,CH₄ production and CH₄ daily gas production rate.