Capillary water migration characteristics of sandstone with different porosities and calculation methods for pore surface relaxation strength

The relaxation strength of pore surfaces (ρ2) plays a pivotal role in transforming the nuclear magnetic resonance (NMR) T2 spectrum into the pore size distribution, rendering it an indispensable parameter for characterizing the microstructure via NMR techniques. In this study, spontaneous imbibition (SI) experiments, real-time NMR measurements, and mercury intrusion porosimetry (MIP) tests were conducted on three sandstones with different porositirs. We propose a novel approach for calculating ρ2 using the capillary water absorption coefficient while analyzing the characteristics of capillary water migration during SI through entropy theory. The results demonstrate a positive correlation between porosity and the capillary water absorption coefficient, indicating that micropores and mesoporous channels play dominant roles in this process. Moreover, the migration rate of capillary water is influenced primarily by porosity, followed by pore size. Additionally, the equilibrium time of entropy during SI is negatively correlated with both the porosity and the capillary water absorption coefficient. Furthermore, the capillary water absorption coefficient can be utilized to calculate ρ2. Notably, when comparing the difference ratios of ρ2 obtained through the capillary water absorption coefficient method and MIP for samples A1, B1, and C1 (5.075%, 2.746%, and 7.583%, respectively) were compared, except for tight sandstone A1, the difference is less than 10%. This suggests that this method is not only feasible but also reliable for not particularly dense rocks. Moreover, this method is simple, widely applicable, enables nondestructive testing of samples, and incurs a low experimental cost; thus, this method has promising application prospects.