Real-Time Monitoring of Pore-Fracture Structure Evolution during Coal Creep Based on NMR

Abstract

Understanding the structural change of coals at micro- and mesoscales during the creep process is essential for exploring the time-dependent properties of deep coal and the long-term safe and effective operation of deep underground engineering. The conventional triaxial compression and creep tests on coal are carried out using a nuclear magnetic resonance (NMR) apparatus with a complete mechanical loading system at different confining pressures. The variations of parameters such as peak areas of various pore distributions, pore percentage, and porosity increment are quantitatively analyzed by real-time monitoring of T 2 spectra during loading. Combined with nuclear magnetic resonance imaging (NMRI), accurate characterization and visualization of microscopic pore structures in coal are realized. The experimental results show that the adsorption pore proportion in the pore structure of the deep coal sample from Pingdingshan, Henan Province, is more than 80%. The changes in seepage pores and microfractures are most obvious in the triaxial compression and creep test. The creep deformation leads to the development of pore structure, and the creep porosity growth rate M increases gradually with increasing stress levels. M can accurately describe the rapid development of the pore structure and the damage accumulation due to time effects during creep. During creep, the most obvious changes in pore structure are the first and last levels of creep. The fractal dimension shows an “inverted V” trend during creep and is more sensitive to the inhibition of pore-fracture structure evolution due to increasing confining pressure. The creep strain increment and porosity increment during graded creep appear to be a similar behavior, indicating that the change of pore structure in the creep process based on the NMR system test can reveal the creep damage mechanism and indirectly characterize the deformation behavior of coal creep.

Coal Molecular Structure Evolution for Methane Adsorption Molecular Mechanism

“…Moreover, the composition of different aliphatic and aromatic chemical structure groups in the coal macromolecule models may also affect the adsorption properties of methane [6,7]. Some useful experimental techniques can be used to investigate the coal chemical structure, which affects the adsorption capacity, e.g., FTIR spectroscopy, Raman, XRD, 13 CNMR, and HRTEM [8][9][10][11]. These experiments can provide a lot about the types of functional groups, crystal structures, carbon types, and aromatic stripes in the chemical structure of coal.…”