Quantitative characterization of water transport and wetting patterns in coal using LF-NMR and FTIR techniques

Water injection in coal seams is an effective technique for preventing and controlling dust disasters in coal mines. However, there is a lack of quantitative evaluation of water transport in coal and its wetting effect. In this paper, the wetting experiments of coal samples with different coal rank and particle size at a given water volume were characterized using low-field nuclear magnetic resonance (LF-NMR) tests. Additionally, the physicochemical properties of coal were investigated by Fourier transform infrared (FTIR) and low temperature N2 adsorption (LTNA) experiments, establishing quantitative relationships between these properties and contact angle, T2-weighted average value (T2g), and peak area of adsorbed water (ATotal). The relationships reveal the dynamic transport process of water in coal samples and its wettability law. The results show that the relaxation time of free state water in porous media decreases with stronger wetting of coal samples, resulting in a faster relaxation rate of free state water and a leftward shift of corresponding peaks. In the range of coal particle sizes from 0.18 to 2 mm (10–80 mesh), the wettability of coal deteriorates with the increasing degree of deterioration of coal samples, with larger particles of lower rank coal samples exhibiting better wettability. The better the wettability of the coal, the shorter the relaxation time of the corresponding peaks, the larger the total area of the adsorption peaks, and the smaller the T2g,24h values. The FTIR structural parameters of hydroxyl (–OH) and carboxyl (–COOH) groups have the greatest influence on the hydrophilicity of the coal. Correlations of factors affecting the wettability of the coal body were analyzed, and three equations for quantitative characterization of the wettability of the coal body were constructed. Overall, the low-field NMR method presented in this paper provides an efficient means of quantitatively characterizing coal wettability and revealing the macro and microscopic mechanisms underlying the interaction between coal and water, as well as the associated wettability patterns.