Effects of creep deformation on the spatial evolution of pore and fracture structures in coal under unloading confining pressure

Comprehensively understanding the influence of creep deformation on the spatial evolution of pore and fracture structures (PFS) in coal is of significant importance for coal mine gas extraction. In this paper, the spatial evolution of the PFS of coal under the influence of creep deformation, in the presence of unloaded confining pressure, is observed online and in real-time by utilizing stratified nuclear magnetic resonance (NMR) technology in conjunction with a mechanical loading system. Stratified T2 spectra of coal samples are obtained. Simultaneously, the impact of creep deformation on the spatial evolution of coal's PFS is analyzed. The fractional-order calculus theory is introduced, and a fractional derivative seepage pores and fractures (SPF) porosity evolution model, accounting for the influence of creep, is established. The results indicate that during the creep process, both compacted and fractured states coexist within the coal, and the water in the pores migrates due to localized compression within the coal samples. Furthermore, the SPF and adsorption pores (AP) within each layer of the coal exhibit a mutual transformation phenomenon. When the coal is subjected to loading beyond its long-term strength, the degree of fracturing within the coal exceeds that of compacting. This disparity leads to the rapid development and expansion of PFS, ultimately resulting in coal failure. Additionally, the fractional derivative SPF porosity model effectively describes the evolution of SPF porosity in coal subjected to the creep of unloaded confining pressure. Within the model, the fractional order is employed to characterize the degree of connectivity of PFS in coal. Notably, a larger value of fractional order corresponds to a heightened degree of connectivity.