Topological Spatial Response of a Loaded Coal Microstructure: Insights of In Situ Microcrack Network Dynamic Evolution and Porosity Decay Prediction

Topological evolution characteristics of a weakly connected microstructure network in loaded coal determine the behavior of weaker diffusion and lower permeability for an in situ coal seam, governing the mechanisms of gas storage and migration, and contributing to controlling gas outburst disasters in deep complex strata. This study employs Micro-CT integrated with AI-based image segmentation and DVC to visualize 3D reconstruction and skeleton evolution models of coal under different confining pressures (0–20 MPa), which could quantitatively characterize the loaded topological response of the microstructure and establish a simplified model for penny-shaped fractures along with a porosity model. The results indicate that with increasing stress, a 3D reconstruction model of a loaded coal microstructure dramatically exhibits the closure of the fracture network, the increase of the coal matrix scale, and the reduction in connectivity. During low-stress conditions, the relevant structural parameters totally exhibit a predominant diminution, demonstrating that the majority of coal microstructure tends to disappear at the initial stress stage. The aforementioned spatial response could be verified on the skeletonized network model, which may experience a trend of sharp decline at first and then slow closure, with a significant decrease in connectivity. The development of pores and throats shows a rapid decline, which may induce less complexity in the topological network and the reduction of gas migration channels and flow efficiency. The DVC displacement field reveals that microcrack closure manifests as the ellipsoidal-shaped strain concentration zone; meanwhile, the loaded microcracks suffer the elastic compression, closure, and inelastic compression in sequence. In this case, a porosity decay prediction model for loaded coal could be successfully derived and validated by the empirical equation of fracture radius-aspect ratio. The above-mentioned results may contribute to elucidating and predicting the dynamic evolution of in situ coal, and provide theoretical guidance on effective identification of permeability in energy-concentrated disaster-prone coal seam zones.