State-resolved imbibition kinetics under fractal confinement in heterogeneous porous media

Spontaneous imbibition in heterogeneous porous media is traditionally described using continuum-scale capillary theories. However, due to multiscale pore heterogeneity and strong interfacial confinement, imbibition kinetics deviate from single-mechanism descriptions and reflect the coupled evolution of distinct water states that cannot be captured by sequential adsorption models. A state-resolved, fractal-constrained kinetic framework is developed to characterize multi-rate imbibition and is validated by experimentally time-resolved low-field nuclear magnetic resonance spectroscopy combined with saturation-centrifugation fractal analysis. Four representative natural porous specimens provide experimental validation. Rigorous statistical model selection confirms the consistency of the dual-process formulation, and the kinetic parameters are correlated with structural and interfacial properties. A rapid increase in the short relaxation time signal accompanied by concurrent growth in longer relaxation times is observed, indicating coupled multistage imbibition rather than sequential layer-by-layer adsorption. The imbibition process is accurately described by a dual-process mechanism consisting of rapid surface-controlled adsorption and slower diffusion-dominated intra-matrix transport. The saturated fractal dimension constrains the effective sorption capacity, whereas the centrifuged fractal dimension governs the balance between the fast and slow processes. Cross-scale restrictions imposed by pore-throat bottlenecking are quantified using the disparity between the two fractal indicators. This framework establishes a link between multiscale pore structure and dynamic flow behavior in heterogeneous porous media.