Study on the diffusion and migration law of CO₂ sequestrated in abandoned coal mine goaf

Abstract

The potential for CO₂ sequestration in the goaf of abandoned coal mines is significant due to the extensive fracture spaces and substantial residual coal present. Firstly, the adsorption characteristics of residual coal in goaf on CO₂ were studied by the isothermal adsorption test of CO₂. Then, to accurately calculate the amount of adsorbed CO₂ within the residual coal in the goaf, the bidisperse diffusion numerical model considering only Fick diffusion was modified in combination with the diffusion mechanisms. The simulation results showed that the modified model can well describe the diffusion behavior of CO₂ in the residual coal matrix. Finally, the numerical simulation of CO₂ sequestration in the goaf of abandoned coal mines was carried out, and the influence of different injection well deployment positions and various thicknesses of residual coal on the migration law and storage effect of CO₂ in goaf was analyzed. The results showed that CO₂ preferentially flowed into the caving zone with higher permeability. The distribution of CO₂ streamlines in the goaf was the most dense in the caving zone and the streamlines in the fracture zone were gradually sparse from bottom to top. When the injection well was deployed at the interface of the two zones, the CO₂ had the best seepage path. The total storage capacity within 90 days was 7.702754 × 10⁶ kg, of which the free state storage capacity in the fracture of the goaf and the adsorbed state storage capacity in the residual coal were 6.611451 × 10⁶ and 1.091303 × 10⁶ kg, respectively. When the injection well was deployed in the middle of the residual coal seam in the goaf and the middle of the fracture zone, the total storage capacity at the same time was 7.613508 × 10⁶ and 6.021495 × 10⁶ kg, respectively. The coal with different thicknesses remaining at the bottom of the goaf significantly affected the adsorbed state storage, but had little effect on the free state storage. When the thickness of the residual coal seam was 0.20, 0.35, and 0.50 m, the adsorbed state storage capacity within 130 days was 4.37623 × 10⁵, 7.65791 × 10⁵, and 1.093406 × 10⁶ kg, respectively.

Keywords

abandoned coal mine goaf / CO₂ geological sequestration / fluid-solid coupling

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2025 The Author(s). Deep Underground Science and Engineering published by John Wiley & Sons Australia, Ltd on behalf of China University of Mining and Technology.