Reconstruction of post-mining porosity-permeability field in abandoned coal mine goaf and its implication for CO2 sequestration

The post-mining pore–fracture structure significantly influences fluid migration and storage within the goaf, yet it remains inadequately represented in most existing models. This study proposes and field-validates a mechanics-based method for reconstructing the three-dimensional porosity–permeability field of a goaf by coupling its stress and fracture fields. The method integrates an FLAC 3D stress-recovery field, obtained using an upscaled double-yield model, with a 3DEC discrete-fracture field. These fields are coupled under an orientation- and aperture-weighted fracture-tensor reconstruction through a matrix-compaction pathway constrained by mass conservation and a fracture-flow pathway based on Oda's fracture-tensor formulation. The reconstructed field reproduces the O-shaped annular permeability structure generated by the abutment-pressure arching mechanism and its pronounced vertical permeability anisotropy. A relatively stable stress-recovery plateau developed in the central goaf, with a maximum recovery ratio of 60.1%. Transport simulations show that 65%–72% of the peak CO 2 mass remains within the caved zone after 100 years of storage, with retention increasing as the height of the fractured zone decreases. By providing a mechanics-based representation of the complex and heterogeneous three-dimensional porosity–permeability structure of goafs, this study offers new insights into the spatial resource utilisation of goafs in abandoned coal mines.