Visualization characterization of void structure evolution in broken limestone and its influence on permeability

• The innovative experimental system enables 3D void visualization in broken coal and rock. • Medium holes play a crucial role in determining the overall void space development in broken limestone. • The change of permeability is dominated by the internal structure evolution of the sample in the plastic compaction stage. • Locating ground drilling ends at the lower layer of the caved zone enhances coalbed methane extraction efficiency. Optimizing coalbed methane extraction from goafs requires an understanding of permeability evolution. The seepage behavior during broken coal and rock compaction, governed by changes in void structure, critically impacts gas flow characteristics. By applying an innovative second-generation compaction-seepage-CT scanning system, a visible model of void structure under different compressive stresses is obtained, and the relationship between macroscopic permeability and microscopic voids is established. The key findings are: (1) Under 20 mm compression, the permeability decreases sharply, with 74.1 % of the change driven by internal structure evolution during the plastic compaction stage. (2) The visible 3D reconstruction model of voids reveals that the connected void volume is over 400 times larger than the isolated void volume. The permeability is primarily controlled by the development of connected voids within the void network. (3) All void parameters exhibit layered evolution characteristics: upper layer > middle layer > lower layer. The upper layer fractures first due to proximity to the indenter, whereas the middle and lower layers only fracture upon reaching greater compaction under sustained axial loading. (4) Void and throat parameters exhibit staged evolution under stress, resulting in staged changes in permeability. During the initial and linear compaction stages, particle skeleton optimization causes minimal connected void and throat changes, and the average permeability variation amplitude decreases by 3 × 10 -12 m 2 . In the plastic compaction stage, the sample appears to be re-breaking, the connected voids and throats are pressed and gathered, and the average permeability variation amplitude decreases by 8.6 × 10 -12 m 2 .

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