Capillary-controlled water drainage and retention in multiscale pore–throat networks of clay-bearing low-permeability sandstones

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 (T2) 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-T2 components decay preferentially, indicating that water in larger and better-connected pore throats is expelled first. In contrast, short-T2 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 (Swc) was identified from the main curvature transition of the fully water-saturated T2 distribution, whereas a limiting bound-water saturation (Swu) was estimated by asymptotic extrapolation of the capillary pressure–mobile-water relationship. The average Swc values of Type I, Type II, and Type III samples are 37.6%, 69.8%, and 87.4%, respectively, and the corresponding Swu 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 T2 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.