Microscopic distribution and dynamic variation of water under stress in middle and high rank coal samples

Determining the process of water migration and transformation in a pore-fracture system under stress is essential to explain the effect of water saturation on permeability and adsorption in microstructures of coals. In this study, the immovable and movable water saturation values (Sir and Sm, respectively) of middle and high rank coal samples are calculated using saturation and centrifugation methods based on nuclear magnetic resonance (NMR) technology. Micro-distribution and influencing factor of water in the initial state are analyzed by low-temperature liquid nitrogen tests. Movable and immovable saturation variations under stress are elucidated by measuring the corresponding T2 spectrum of pores with a range of diameters. The results show that the heterogeneity of irreducible water in coal samples are considerably higher than that of movable water in all the samples. The irreducible state thereafter controls the micro-distribution of water in coal samples. With the increase in coal metamorphism, the distribution heterogeneity of immovable and movable water increases and decreases, respectively. The water content variations in adsorption pore, seepage pore, and fracture all decrease linearly with the increase in Sir, indicating that the amount of immovable water determines the dynamic variation, and the distribution of initial water mainly affects the water migration process. The immovable water saturation, Sir, has distinct stress sensitivity, and the capillary bound water present in seepage pore can be transformed into movable water. The effect of stress on immovable water in larger pore, such as seepage pore, is distinctly stronger than that on immovable water in adsorption pore. This study indicates that Sir obtained by the saturation–centrifugation method is significantly higher than the actual irreducible water saturation. The bound water saturation calculated by this method is only suitable for high rank coal samples with poorly developed seepage pore.