Effect of microscopic pore–throat structure heterogeneity on waterflooding seepage characteristics of tight sandstone reservoirs

Abstract Investigations into the complex pore–throat structures (PTS) of tight sandstone reservoirs are vital for optimizing waterflooding strategies. In this study, 13 cores from the Dingbian area of the Ordos Basin were examined using casting thin sections, X-ray diffraction, high-pressure mercury injection (HPMI), and nuclear magnetic resonance (NMR). These analyses quantitatively characterized mineral composition, petrophysical properties, PTS heterogeneity, and fluid mobility. Based on these findings, high-temperature and high-pressure waterflooding experiments were performed to explore dynamic waterflooding behavior and systematic factors influencing fluid mobility and waterflooding efficiency ( E w ). Results indicate significant differences in fluid mobility across four PTS types, transitioning from uniform displacement (type I) to finger displacement (type IV), with corresponding reductions in movable fluid saturation and E w . Residual oil morphologies evolve from dispersed membranes to continuous clusters and sheet shapes. Integrated HPMI and NMR analysis accurately delineates the full pore–throat size distribution (PSD) (0.0001–10 μm) that exhibit multi-fractal characteristics, enabling differentiation of macropores, mesopores, micropores, and nanopores. Micropores, which are relatively uniform, dominate reservoir petrophysical properties and serve as primary sites for movable fluids. Elevated clay mineral content augments PTS heterogeneity, lowers permeability, and promotes nonlinear seepage phenomena (blockage and bypassing), leading to more extensive residual oil accumulation and reduced E w . Overall, PTS heterogeneity emerges as the principal factor governing fluid mobility and E w , highlighting the importance of full PSD characterization and real seepage condition simulation in reservoir engineering. This study provides key insights into optimizing water injection strategies, suggesting that maintain a stable injection pressure and injection volume multiple of less than 2 pore volumes, while also considering clay mineral sensitivity.