Pore-fracture response characteristics of coal under triaxial stress paths revealed by in-situ LF-NMR and fractal theory

Abstract

To investigate the evolution of pore-fracture structure (PFS) in coal under stress constraints, real-time nuclear magnetic resonance tests were conducted on coal samples subjected to four stress paths (SP-I to IV). The PFS evolution, fractal characteristics, failure morphology, and permeability contribution were analyzed using T₂ spectra, nuclear magnetic resonance images (NMRIs), and fractal theory. The results show that stress paths significantly influence pore compaction, dilation, and fracture development. SP-III exhibits the most significant promotion of pore dilation before coal failure, followed by SP-IV and SP-II, whereas SP-I suppresses pore dilation. Confining pressure unloading promotes pore dilation, whereas the effect of axial loading depends on the confining pressure reduction. Coal failure significantly reduces pore heterogeneity, accompanied by a decrease in the pore fractal dimension (D_b). Multifractal parameters Da and A_c were associated with pore compression, dilation and microfracture nucleation, while H_c served as a robust index of pore connectivity. The failure mode of SP-I was shear-dominant, while the remaining stress paths manifested a combined tensile-shear failure. The failure zone of SP-III exhibited the highest morphological complexity, followed by SP-IV, SP-II, and SP-I. The stress path significantly influenced the permeability contribution of PFS within the 100–1000 ms, highlighting distinct permeability responses under different stress paths. These findings provide valuable insights into the mechanisms governing gas migration in coal seams.

Keywords

In-situ LF-NMR / Triaxial stress path / Pore-fracture structure / Fractal dimension / Permeability contribution

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