LIU Junlin，LIU Huihu，ZHANG Kun，et al. Diffusion characteristics of CO2 and CH4 in CO2-ECBM process of low permeability coal seam[J]. Coal Science and Technology，2023，51（S1）：112−121
. DOI: 10.13199/j.cnki.cst.2022-1296| Citation: |
LIU Junlin，LIU Huihu，ZHANG Kun，et al. Diffusion characteristics of CO2 and CH4 in CO2-ECBM process of low permeability coal seam[J]. Coal Science and Technology，2023，51（S1）：112−121 . DOI: 10.13199/j.cnki.cst.2022-1296
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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.
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