Wettability Variations among Macroscopic Components of High-Rank Coal and Their Effects on Permeability-Stress Correlations: An Experimental Study

The pivotal petrophysical attribute of coal seams, i.e., their wettability, profoundly influences coal mining productivity and gas prevention/exploitation strategies within mines, stemming from intricate coal-water and coal-gas interactions. To reveal the intricacies of the surface wettabilities of the macroscopic components of coal, an in-depth study, including microscopic characterization, proximate analysis, contact angle measurements, coal facies analysis, and permeability-stress correlation analysis under both dry and wet states, was conducted on high-rank coal samples from the Qinshui Coalfield in North China. The key findings revealed the occurrence of a sequential decrease in the contact angle and thus enhancement of the wettability from the vitrain to clarain to durain coal components based on tests conducted using distilled water on high-rank coal surfaces. This wettability enhancement was positively correlated with the coal ash yield and mineral matter abundance. The spatial and vertical variations in the coal wettability were primarily controlled by the historical evolution of the peat swamp environment. Notably, the contact angle measurements utilizing coal fine tablets exhibited greater sensitivity compared to polished sections, bolstering the scientific rigor and precision of the wettability assessments. Furthermore, as the effective stress intensifies, the permeability of the coal cores composed of macro-components (durain, vitrain, and clarain) decreased exponentially due to the spontaneous imbibition capability. This comprehensive study provides a reference for the optimal selection of water management strategies in coal mining operations, enhancing overall mining safety and profitability.