Quantitative determination of CH₄ production and CO₂ storage in coal mine goaf: A CO₂-ECBM recovery model that considers zonal permeability

Abstract

Injecting CO₂ into a coal mine goaf can not only recover residual CH₄ but also satisfy the effect of CO₂ geological storage in abandoned mines. To reveal the gas–solid couplings during CO₂-enhanced coalbed methane (CO₂-ECBM) recovery in abandoned mines, this study summarizes the occurrence of methane in a coal mine goaf, including falling, fracture, and bending subsidence zones, coal pillars, and surrounding rocks. The zonal permeability is then assessed and validated. Furthermore, a fully coupled CO₂-ECBM model of coal deformation and CH₄–CO₂ competitive adsorption and seepage is established. Finally, a series of scenario-based numerical simulations are conducted to quantitatively investigate CH₄ production and CO₂ storage in a coal mine goaf. The results showed that CO₂ injection into the coal pillars enhanced gas production from the coal pillars by 94% after 300 d. The efficiency of CO₂ injection into coal pillars for storage is very low compared with that of gas recovery. The total CO₂ storage in the goaf was 71,200 Nm³ after 3000 d of injection when the main storage spaces of CO₂ were the coal pillars and falling zone. The results of this study have implications for the coupling of coal, CH₄, and CO₂, particularly for the quantitative determination of CH₄ production and CO₂ storage in coal mine goafs.

Keywords

CO₂ storage / CH₄ production / Coal mine goaf / CO₂-enhanced coal bed methane recovery model / Zonal permeability

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