Abstract
Ultra-low-permeability sandstone reservoirs are characterized by complex pore–throat architectures that constrain the mobilization of remaining oil, yet the controls on its microscopic occurrence and the mechanisms by which chemical flooding improves its recovery remain poorly resolved. Focusing on the Shigang Oilfield in the Subei Basin, this study develops an integrated framework linking mineralogical composition, pore–throat structure, seepage behavior, and displacement dynamics through petrographic characterization, online nuclear magnetic resonance displacement, petrophysical and sensitivity measurements, core flooding, and numerical simulation. The results show that micropore–fine-throat systems dominate the reservoir, with limited seepage capacity and pronounced water and velocity sensitivities that promote clay swelling and particle migration during injection. Waterflooding preferentially advances through dominant flow channels, leaving trapped and adsorbed oil concentrated in medium- and small-sized pore throats. Its distribution is governed by the coupled effects of pore–throat heterogeneity, capillary forces, and wettability. Surfactant-assisted secondary waterflooding improves oil mobilization and displacement efficiency by reducing oil–water interfacial tension and modifying the mobility and flow distribution of the displacing phase. By integrating pore-scale oil occurrence with macroscopic displacement response, this study establishes a mechanistic framework for interpreting and optimizing synergistic chemical flooding in ultra-low-permeability sandstone reservoirs.
