Spontaneous Imbibition Characteristics and Kinetic Modeling of Tight Sandstones in the Qingshen Block, Sichuan Basin

Six tight sandstone cores from the Shaximiao and Xujiahe formations in Well Dongpo 1, Qingshen Block, Sichuan Basin, were investigated using micro-CT imaging, NMR-monitored spontaneous imbibition experiments, and complementary high-pressure mercury intrusion porosimetry (HPMI) data to examine the relationship between pore–throat structure and imbibition behavior and develop a kinetic model of imbibition-height evolution. Cumulative-frequency matching of HPMI and NMR data provided an equivalent-size calibration for one core, enabling comparison of NMR signal changes across calibrated size ranges. Based on capillary imbibition theory, the model incorporated a pore–throat size-distribution parameter, a capillary-pressure correction factor, and an empirical height-dependent pressure correction. The cores exhibited multiscale structures comprising relatively large pores connected through finer throats, with marked differences in pore connectivity. Imbibition-front advancement was generally rapid at early times and slowed thereafter, but imbibition height, water uptake, and mass-based pore-space utilization did not vary proportionally. Within the six-core dataset, final relative imbibition height and NMR signal fraction were positively associated with gas permeability. Under the adopted calibration, the small equivalent size range exhibited a faster early increase in normalized NMR signal and a final value closer to its saturated reference. The proposed model captured the overall slowing of front advancement and yielded lower temporal holdout prediction errors than the Lucas–Washburn and capillary–gravity models for all six cores, although it underestimated continued late-stage advancement in several cores. These findings distinguish front advancement from internal pore filling in the investigated tight sandstones and provide a model for quantitatively describing imbibition-height evolution.