Imbibition characteristics and mechanisms of coal using digital image subtraction technique based on Micro-CT.

• X-ray CT and digital subtraction efficiently reveal internal fluid in coal specimens. • Using fluid information data to characterize effective imbibition pore-fractures. • Imbibition model fits well, surpassing Lucas-Washburn in viscous-inertial stage. • Initial imbibition occurs in smaller pores, shifting to larger ones at equilibrium. • Capillary radius most significantly affects imbibition model sensitivity. Flow and distribution of fluids within coal is of great significance for studying the enhancement of coal seam water injection and permeation-wetting effects, achieving dust pollution control, and gas disaster management. In this regard, relying on the theory of imbibition dynamics, the imbibition model based on microscopic structural parameters is derived. X-ray computed tomography (CT) scanning imaging experiments are conducted on coal in both dry and imbibition-saturated states. Utilizing digital image subtraction technique, pore-fractures structure characteristics and fluid distribution are extracted. A comprehensive investigation is conducted to explore the fluid ascent behavior in individual capillaries and flow characteristics within the pore-fracture networks, thereby revealing the mechanisms through which pore-fractures influence fluid migration during imbibition. The results demonstrate that digital image subtraction technique accurately reveals the fluid distribution within coal. The volume of pore-fractures in different regions is a critical factor influencing fluid distribution, with large fractures playing a major role in maintaining and supporting spontaneous imbibition equilibrium. A microscopic-scale imbibition model based on shape factors and curvature correction is established, and effective pore-fractures are characterized using imbibition fluid data. The established imbibition model exhibits a good overall fit at various imbibition stages, especially during the viscous-inertial stage, where the model significantly outperforms the traditional Lucas-Washburn model. Furthermore, the proportion of effective pore-fractures involved in imbibition is not constrained by the distribution pattern of total pore-fractures. Pore-fractures with equivalent radii ranging from 100 to 500 μm have the highest proportion, serving as the main pathways in the initial stage of imbibition rate increase. The driving force required for the rise of the liquid meniscus inside capillaries is positively correlated with the equivalent radius, making it easier for liquid to ascend in smaller-radius capillaries. Among the various factors influencing imbibition evolution, the imbibition model exhibits the highest sensitivity to capillary radius, demonstrating that capillary structural parameters are crucial factors affecting imbibition. [ABSTRACT FROM AUTHOR]