Real‐time multiscale characterization of water migration in deep coal under stepwise pressure‐increasing injection: Combined low‐field nuclear magnetic resonance and multifractal approach

Abstract Deep coal seams are subject to pronounced risks of dynamic disasters, and coal seam water injection is a key technology for achieving synergistic multi‐disaster prevention. However, the cross‐scale dynamic behavior of water migration within coal seams during injection remains unobservable in real time with sufficient precision, leaving field regulation without a scientific basis and preventing accurate control of water injection effectiveness. To address this challenge, this study integrates low‐field nuclear magnetic resonance (LF‐NMR) technology with a triaxial loading system to conduct experiments on stepwise pressure‐increasing water injection in dry coal seams under in‐situ hydrostatic pressure conditions, achieving real‐time multiscale monitoring and quantitative characterization of water migration. Specifically, nuclear magnetic resonance imaging and hard‐pulse one‐dimensional imaging (HSE) were employed to track the macroscopic migration of the water invasion front, and LF‐NMR T 2 spectra combined with multifractal theory were used to quantitatively characterize the co‐evolution of pore water distribution heterogeneity and hydraulic connectivity. The results indicate that the stepwise pressure‐increasing water injection process in coal seams can be divided into two stages—rapid seepage and progressive filling—with the evolution of pore water distribution heterogeneity and hydraulic connectivity exhibiting synchronized two‐stage characteristics. An exponential prediction model derived from the experimental results achieves a coefficient of determination ( R 2 > 0.97), accurately characterizing the evolution of water volume increment throughout the injection process. This study provides a scientific basis for the dynamic optimization of stepwise pressure‐increasing water injection technology for deep coal seams.