Experiment And Numerical Simulation Analysis Of Displacing Ch4 By CO₂ Injection

CO₂ is the main cause of the greenhouse effect, CH4 is the main harm during the production of coal mine, but it is also a new type of clean energy. However, the permeability of the coal seam is generally low in China, which is not conducive to the gas drainage. CO₂ injection can significantly improve CH4 recovery. Therefore, displacing CH4 by CO₂ injection is important from the aspect of environmental protection, safety and energy. In this paper, we researched the mechanism of coal pore structure characteristics, mode of occurrence and migration of coalbed methane. On this basis, we conducted experiment and numerical simulation research of displacing CH4 by CO₂ injection, and obtained dynamic variation in the displacement process under different injection pressure and permeabilities.Experimental platform was developed independently to use to research about the permeability of the CH4 and CO₂. Through the experiment it can be concluded that permeability of coal are affected by vertical stress and injection pressure.The permeability is lower when the vertical stress increases.When the injection pressure is low permeability decreases with pressure increasing,and when the injection pressure is high permeability increases with pressure increasing. Permeability changes of the two gases are basically the same, the difference is that just permeability of CO₂ higher than CH4.On the basis of understanding the permeability of coal specimen, we carry out displacing coalbed CH4 by CO₂ under different injection pressure（1.0, 1.2, 1.5MPa）. The research results show that replacement efficiency are all above 95% at different injection pressure. By the change trend of concentration of each component, concentrations of CO₂ increases while concentrations of CH4 decreases. After injection of 0.433⁰.616 displaced volume CO₂ break through outlet, after CO₂ breakthrough the gas outlet, its concentration increased slowly to 100%. As the injection pressure increases, the breakthrough and displacing time gradually advance. Increasing injection pressure can improve the displacement efficiency.Based on Darcy’s law, Fick’s law and Langmuir’s law, a set of numerical models were established under multi-physics field coupling conditions, and coal seam gas injections under different injection pressure and permeability conditions were simulated make use of the Comsol Multiphysics software. Numerical simulation results are consistent with the experimental trend, and the displacement effect is good. CO₂ breakthrough outlet time shorten from 470 min to 230 min and displacing time advance from 550 min to 280 min when injection pressure increase from 1.0 MPa to 1.5 MPa. Displacing time delaye from 420 min to 840 min when the permeability decreased from 0.14×10-15m2 to 0.054×10-15m2. Injection pressure and permeability significantly affect the displacement efficiency. Increasing injection pressure can advance CO₂ break through outlet and displacement time. Longer time is required for replacement under lower permeability, and displacing progress is slower under low permeability.