Dynamic Threshold Pressure Gradient in Water-Bearing Tight Gas Reservoirs: Experimental and Modeling Insights

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.鄂尔多斯盆地大牛地气田属于岩性致密气藏，具备低渗透率、强压实且无边底水的特征，以弹性膨胀作为主要驱动机理。此类储层内的气水两相流呈现显著的非线性特征，显著影响流体渗流行为。为探究开发过程中随含水饱和度与渗透率变化的动态启动压力梯度（threshold pressure gradient，TPG），本研究采用压差-流量法，针对典型Ⅰ型与Ⅱ型致密砂岩岩心开展高温高压气水两相流实验。 实验结果表明，启动压力梯度（TPG）同时受含水饱和度与渗透率的显著影响，两类储层表现出明显差异。具体而言，可测得启动压力梯度的临界含水饱和度阈值分别为Ⅰ型28.67%、Ⅱ型15.15%。启动压力梯度随含水饱和度呈幂律增长，当Ⅰ型岩心含水饱和度超过40%、Ⅱ型超过30%时，增长趋势显著加快。此外，渗透率与启动压力梯度呈反幂律关系，两类储层的临界渗透率阈值分别为1.13 mD与0.55 mD。通过CT成像与数字岩心分析证实，上述规律源于孔喉几何形态与毛细管力的差异。 本研究结果为致密储层气水系统的动态非线性渗流机制提供了新的认识，并建立了可变储层条件下启动压力梯度的定量预测框架，为含水致密气藏的开发方案优化提供了理论依据。