Dynamic Threshold Pressure Gradient in Water-Bearing Tight Gas Reservoirs: Experimental and Modeling Insights
Description
The Daniudi Gas Field in the Ordos Basin is a lithologic tight gas reservoir characterized by low permeability, high compaction, and the absence of edge or bottom water, with elastic expansion serving as its primary drive mechanism. Gas–water two-phase flow within such formations exhibits pronounced nonlinear characteristics, significantly affecting fluid transport behavior. To investigate the dynamic threshold pressure gradient (TPG)—which varies with water saturation and permeability during production—this study conducts high-pressure, high-temperature two-phase flow experiments on representative Type I and Type II tight sandstone cores using the pressure difference–flow rate method. The experimental results demonstrate that TPG is strongly influenced by both water saturation and permeability, with clear distinctions observed between the two reservoir types. Specifically, critical water saturation thresholds marking the onset of measurable TPG were identified as 28.67% for Type I and 15.15% for Type II. TPG exhibits a power-law increase with water saturation, rising sharply when saturation exceeds 40% in Type I and 30% in Type II. Additionally, permeability shows an inverse power-law relationship with TPG, with critical thresholds at 1.13 mD and 0.55 mD for Type I and Type II, respectively. These trends are attributed to differences in pore-throat geometry and capillary forces, as confirmed through CT imaging and digital core analysis. The findings offer new insights into the dynamic nonlinear flow mechanisms of gas–water systems in tight formations and establish a quantitative framework for predicting TPG under varying reservoir conditions. This study provides a theoretical basis for optimizing development strategies in water-bearing tight gas reservoirs.