Effect of Pore-Throat Structure and Displacement Fluid Type on Relative Permeability in Low-Permeability Sandstone Reservoirs

Low-permeability sandstone reservoirs are characterized by fine pore-throat structures and strong heterogeneity, leading to complex multiphase flow behavior; since relative permeability curves are highly sensitive to displacement media, they serve as critical parameters for EOR performance and development optimization, yet the coupling between the pore-throat structure and the displacement fluid type remains poorly quantified. This study investigates low-permeability sandstone cores from the strike-slip fault block of the Laojunmiao Oilfield. Based on a petrophysical analysis and a high-pressure mercury intrusion, the cores are classified into four types (X1: medium pores, high permeability; X2: micropores, low permeability; X3: fine pores, relatively homogeneous; X4: fine pores with locally dominant channels), and unsteady-state relative permeability experiments are conducted under water, polymer, and nitrogen flooding. A comprehensive structural index (SI) is proposed to characterize the pore-throat structure and its correlation with the width of the two-phase flow zone. The results show that the pore-throat structure fundamentally governs the water flooding behavior, with Type X2 cores prone to preferential channeling; polymer flooding improves the mobility ratio most prominently in Type X1 and X3 cores; and nitrogen flooding performs best in Type X2 cores by retarding gas fingering. The two-phase flow zone width correlates linearly with the structural parameters, with polymer flooding showing the greatest sensitivity to structural variation.