The influence of water saturation on the threshold pressure gradient in tight gas reservoirs

High water saturation and complex pore-throat structure commonly restrict gas flow in tight gas reservoirs by increasing the threshold pressure gradient. This study experimentally evaluated the effect of water saturation on the gas-phase threshold pressure gradient using the microflow displacement pressure-difference method. Matrix-type and fracture-vuggy cores were tested at 80 ℃ with 30,000 mg/L brine and nitrogen. The measured water-phase threshold pressure gradient ranged from 0.0633 to 1.2860 MPa/m and showed negative correlations with permeability and reservoir quality index, but little correlation with porosity. Fracture-vuggy cores exhibited threshold pressure gradients 58–72% lower than matrix-type cores under comparable permeability, indicating that pore connectivity has a stronger control on flow initiation than pore volume alone. As water saturation increased, the gas-phase threshold pressure gradient rose nonlinearly because water blocking and the Jamin effect intensified in narrow pore throats. The ratio of water saturation to reservoir quality index showed the strongest correlation with the gas-phase threshold pressure gradient, with R² = 0.9648. Compared with using porosity or permeability alone, this combined parameter better reflects the coupled effects of water saturation and pore-structure heterogeneity. These results provide experimental support for water-blocking damage evaluation, water-control measures, and productivity prediction in tight gas reservoirs, although further reservoir-scale validation is required.