Spatio-temporal evolution of pore and fracture structures in coal induced by initial damage and creep behavior: A real-time NMR-based approach

Understanding the impact of mining disturbances and creep deformation on the macroscopic deformation and the microscopic pore and fracture structures (MPFS) of coal is paramount for ensuring the secure extraction of coal resources. This study conducts cyclic loading-unloading and creep experiments on coal using a low-field nuclear magnetic resonance (NMR) experimental apparatus which is equipped with mechanical loading units, enabling real-time monitoring the T 2 spectrum. The experiments indicated that cyclic loading-unloading stress paths initiate internal damage within coal samples. Under identical creep stress conditions, coal samples with more initial damages had more substantial instantaneous deformation and creep deformation during the creep process. After undergoing nearly 35 h of staged creep, the total strains for coal samples CC01, CC02, and CC03 reach 2.160%, 2.261%, and 2.282%, respectively. In the creep stage, the peak area ratio of seepage pores and microfractures (SPM) gradually diminishes. A higher degree of initial damage leads to a more pronounced compaction trend in the SPM of coal samples. Considering the porosity evolution of SPM during the creep process, this study proposes a novel fractional derivative model for the porosity evolution of SPM. The efficacy of the proposed model in predicting porosity evolution of SPM is substantiated through experimental validation. Furthermore, an analysis of the impact mechanisms on key parameters in the model was carried out.