Diffusion characteristics of CO₂ and CH₄ in CO₂-ECBM process of low permeability coal seam

The ability of gas diffusion is one of the key factors that determine the success or failure of CO₂-ECBM. Taking the pore characteristics of coal reservoirs of No. 13 Coal Seam of Liuzhuang Coal Mine and No. 7 Coal Seam of Qidong Coal Mine in the low permeability coal bearing area of Huaihe River and Huaihe River in China as the research object, the diffusion behavior of CO₂ and CH₄ gas in the process of CO₂-ECBM was studied in order to obtain the diffusion law of CO₂ and CH₄ gas under the reservoir conditions and provide a theoretical basis for the study of the diffusion law of the low permeability coal seam in the process of CO₂-ECBM. The research results show that the Fick type diffusion is the main type of gas diffusion, accounting for more than 50%, followed by the transitional type diffusion, accounting for nearly 30%, the Knudsen type diffusion of CH₄ accounts for about 10%, and the Knudsen type diffusion of CO₂ accounts for nearly 20%; When CO₂ and CH₄ gas undergo Fick type diffusion and transitional type diffusion, their diffusion coefficients have no obvious relationship with temperature change. At a pressure of 2 MPa, the gas diffusion coefficient in Coal 13 of Liuzhuang Mine is about 10 m²/s, and the gas diffusion coefficient in Coal 7 of Qidong Mine is above 13 m²/s. With the increase of pressure, the diffusion coefficients of both types begin to decrease, and the drop rate is the fastest in the pressure range of 0~5 Mpa. When the pressure reaches 20 MPa, The diffusion coefficients of the two types of gas diffusion basically tend to be stable, which is nearly 10 times smaller than that at 2 MPa; When the gas diffusion is Knudsen type, the diffusion coefficient increases by 0.2 m²/s for every 6 ℃ increase in temperature, showing a slow linear growth; Comparing the gas diffusion coefficients of different diffusion types, CH₄ has more advantages than CO₂ in Fick type diffusion, and CO₂ has more advantages in the other two diffusion types; By comparing the diffusion rate of CH₄ in different diffusion types, the diffusion rate of CH₄ gas in low permeability coal is faster when Fick type or Knudsen type is used as diffusion type, and slower when transition type is used as diffusion type; In the process of CO₂-ECBM, CO₂ diffuses faster in micropores and small pores than CH₄, which can more effectively replace CH₄ in micropores and improve CH₄ production.