Abstract
Water drainage and retention in clay-bearing low-permeability sandstones are controlled by capillary thresholds within multiscale pore-throat networks, yet their responses in nuclear magnetic resonance (NMR) transverse relaxation time (T-2) distributions remain difficult to quantify. In this study, low-temperature nitrogen adsorption (LTNA), mercury intrusion capillary pressure (MICP), and stepwise centrifugation-NMR measurements were combined to investigate selective water drainage and persistent retention under capillary forcing. The results show that, as the centrifuge-equivalent capillary pressure increases stepwise, long- T(2 )components decay preferentially, indicating that water in larger and better-connected pore throats is expelled first. In contrast, short- T-2 components persist at high centrifuge-equivalent capillary pressures, demonstrating that fine pore throats and strongly confined interfacial regions dominate water retention. A critical bound-water saturation (S-wc) was identified from the main curvature transition of the fully water-saturated T2 distribution, whereas a limiting bound-water saturation (S-wu) was estimated by asymptotic extrapolation of the capillary pressure-mobile-water relationship. The average S-wc values of Type I, Type II, and Type III samples are 37.6%, 69.8%, and 87.4%, respectively, and the corresponding S-wu values are 17.7%, 29.9%, and 39.7%. These trends indicate stronger water retention from coarse- to fine-pore-throat-dominated systems. Compared with conventional fixed T-2 cutoff methods, the proposed framework links NMR response to centrifugation drainage pathways and LTNA-MICP-constrained pore-throat distributions through a scale-dependent effective surface relaxivity model.
Paper Information：
He Yang, Xinmin Ge*, Di Tang, Yixiong Wu, Bohan Wu, Yuqi Wu. Capillary-controlled water drainage and retention in multiscale pore–throat networks of clay-bearing low-permeability sandstones. Physics of Fluids, 2026, 38: 086607.https://doi.org/10.1063/5.0345241
