The Effect of Microstructure on Spontaneous Imbibition of Water in Coals

Studying the effects of pore structure on spontaneous imbibition (SI) is of great significance for optimizing coal seam water injection measurements. To investigate the influence of reservoir microscopic pores on imbibition, we studied pore structure in selected coal samples using Mercury intrusion porosimetry (MIP) and nuclear magnetic resonance (NMR), along with multifractal analysis. SI experiments were conducted on coal cores to investigate the dynamic imbibition process and quantitatively analyze imbibition at different time intervals, as well as the factors influencing imbibition. The results show that the adsorption pores primarily function as water storage spaces, while the seepage pores have dual functionality in both water conduction and storage during imbibition. Water is imbibed more quickly in seepage pores than in adsorption pores, as the effective driving force is lower in adsorption pores due to the high frictional resistance caused by strong pore heterogeneity. Due to the strong microscopic heterogeneity of coal, the imbibition process is predominantly governed by its microscopic properties. The imbibition rate of coal is mainly controlled by the pore connectivity (H), development degree of seepage pores (Dmax), concentration degree of pore size distribution (PSD) (D(1)) and uniformity of PSD (D(2)), but less effected by the adsorption pores (Dmin) and PSD heterogeneity (ΔD). The imbibition capacity is mainly affected by wettability (θ), while it is relatively weakly influenced by pore-related parameters. Considering the heterogeneity of the pore network in coals, a new model for SI was proposed, which shows better fitting performance compared to conventional models. This study can help enhance the comprehension of the underlying mechanism of SI and contribute to its practical implementation.