Experimental Study on Gas–Water Two-Phase Seepage Characteristics in a Low-Permeability Gas Reservoir

The relationship between two-phase flow behavior and pore microstructure in low-permeability gas reservoirs remains poorly understood, particularly during gas accumulation (gas displacing water) and subsequent development (water displacing gas). To address this, five plug cores with permeabilities ranging from 0.02 to 20.92 mD were selected from a low-permeability sandstone gas reservoir in the Tarim Oilfield. A comprehensive experimental program was carried out, including pore-throat structure characterization, wettability measurements, and both steady-state and unsteady-state gas–water relative permeability tests. These data were further integrated with capillary pressure curves derived from high-pressure mercury injection, centrifugation, and semipermeable membrane methods, enabling systematic comparison and parametric modeling. The results indicate that the reservoir is generally characterized by low permeability, fine pore throats, and moderate water-wetness, with contact angles between 49° and 60°. As permeability declines, pore-throat dimensions decrease and pore connectivity worsens: the mercury injection saturation median pressure increases from 0.752 to 16.477 MPa, while mercury withdrawal efficiency drops from 78.59% to 40.54%. These trends suggest that finer throats impose greater resistance to nonwetting phase invasion and that the “ink-bottle” effect intensifies fluid entrapment. Relative permeability measurements show that the unsteady-state method is more responsive to gas breakthrough and localized flow channeling, yielding lower endpoint gas relative permeability compared to the steady-state approach. With deteriorating reservoir quality, irreducible water saturation during gas displacement rises from 29.03% to 45.20%, residual gas saturation during water displacement increases from 17.60% to 38.90%, and the two-phase coexisting flow zone narrows markedly from 53.37% to 15.90%. The consistent trends observed between relative permeability and mercury injection data confirm that reduced pore-throat size and heightened capillary resistance not only hinder the formation of continuous nonwetting phase pathways but also promote water retention and gas trapping. Through normalization, representative Corey-type gas–water relative permeability curves were established for the study area (with Corey exponents of 2.393 and 1.534 for the gas and water phases, respectively, under gas-displacing-water conditions), along with Brooks–Corey-type capillary pressure curves (with equivalent displacement pressures ranging from 0.025 to 0.133 MPa). These results provide essential parameters for seepage characterization and numerical simulation of underground gas storage in low-permeability gas reservoirs.