The Effect\nof Microstructure on Spontaneous Imbibition\nof Water in Coals

Studying the effects of pore structure on spontaneous\nimbibition\n(SI) is of great significance for optimizing coal seam water injection\nmeasurements. To investigate the influence of reservoir microscopic\npores on imbibition, we studied pore structure in selected coal samples\nusing Mercury intrusion porosimetry (MIP) and nuclear magnetic resonance\n(NMR), along with multifractal analysis. SI experiments were conducted\non coal cores to investigate the dynamic imbibition process and quantitatively\nanalyze imbibition at different time intervals, as well as the factors\ninfluencing imbibition. The results show that the adsorption pores\nprimarily function as water storage spaces, while the seepage pores\nhave dual functionality in both water conduction and storage during\nimbibition. Water is imbibed more quickly in seepage pores than in\nadsorption pores, as the effective driving force is lower in adsorption\npores due to the high frictional resistance caused by strong pore\nheterogeneity. Due to the strong microscopic heterogeneity of coal,\nthe imbibition process is predominantly governed by its microscopic\nproperties. The imbibition rate of coal is mainly controlled by the\npore connectivity (<i>H</i>), development degree of seepage\npores (<i>D</i><sub>max</sub>), concentration degree of\npore size distribution (PSD) (<i>D</i>(1)) and uniformity\nof PSD (<i>D</i>(2)), but less effected by the adsorption\npores (<i>D</i><sub>min</sub>) and PSD heterogeneity (Δ<i>D</i>). The imbibition capacity is mainly affected by wettability\n(θ), while it is relatively weakly influenced by pore-related\nparameters. Considering the heterogeneity of the pore network in coals,\na new model for SI was proposed, which shows better fitting performance\ncompared to conventional models. This study can help enhance the comprehension\nof the underlying mechanism of SI and contribute to its practical\nimplementation.