New Policy to Specify Availabilityof Data, Models, and Code

Previously conducted studies have established that the permeability evolution of coal under repeated loading and unloading stress paths and the distribution and changes of its internal microstructure are crucial for the synergistic, efficient, and safe mining of coal and coalbed methane. In this paper, an analysis has been carried out by five cyclic loading and unloading experiments under confining pressure and different axial pressures and six nuclear magnetic resonance tests which was about the permeability evolution law and response mechanism of internal microstructure distribution of coal samples under repeated loading and unloading stress paths with gradually increasing axial pressure. The results indicate that the stress loading and unloading path, which involves increasing axial pressure, leads to two damages to the coal samples. The initial damage occurs at a bias stress of 30MPa, coinciding with the maximum values of permeability and fissure signal intensity. As the stress loading and unloading progresses, the stress sensitivity gradually decreases during the loading stage. Furthermore, there is a significant difference in the unloading curves of permeability for different axial pressures. The microstructural changes within the coal samples are more pronounced during the unloading stage. Both the NMR signal intensity and porosity increase slowly. Finally, the secondary damage occurs at a bias stress of 58MPa. Through repeated cycles of loading and unloading, the loading stage becomes less sensitive to stress, resulting in more distinct differences in the unloading curves of permeability under various axial pressures. During the unloading phase, the coal samples experience significant changes in their microstructure. The NMR signals and porosity gradually increase in intensity, while the coal samples undergo additional damage under a bias stress of 58 MPa. The permeability at each stage is directly related to the intensity of fissure signals, highlighting the influence of porosity on permeability.