Characterizing pore structure is one of the most fundamental tasks in reservoir characterization; it is closely related to the calculation/interpretation of other critical parameters, such as permeability and capillary pressure. High-pressure mercury injection (HPMI), a low-pressure nitrogen gas adsorption (LP-N₂GA), specific surface area (SSA) analysis, nuclear magnetic resonance (NMR), and fractal theory were used to study the pore structure characteristics and irreducible water saturation of tight reservoir samples from the Chang 7 formation in Ordos Basin, China. In this study, pores are mainly composed of mesopores and macropores. HPMI is more likely to detect macropores, while the distribution of mesopores is better characterized by LP-N₂GA and SSA analysis. The capillary curves obtained by HPMI experiments are divided into two categories. The adsorption–desorption isotherms are divided into two groups, according to the rate of change of the desorption curve when the relative pressure is 0.5. The permeability contribution rate of different pore radius was studied through different methods, and the results showed the combination of HPMI and LP-N₂GA can describe the microscopic pore structure of a reservoir more comprehensively than either method alone. The irreducible water saturation obtained by NMR test is greater than the irreducible water saturation obtained by HPMI. An irreducible water saturation model was established based on fractal theory and the capillary bundle model. The factors affecting the irreducible water saturation were fractal dimension, maximum connected pore throat radius, minimum pore throat radius, and thickness of the water film. The calculation results are closer to the experimental value when the fractal dimension is greater than 2.7. This comprehensive application of various experimental and theoretical methods gives a better understanding of the pore structure characteristics and fluid distribution in tight reservoir samples.

中文翻译：

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用实验和理论方法研究致密储层的孔隙特征和不可饱和水饱和度

表征孔隙结构是表征储层最基本的任务之一。它与其他关键参数（例如渗透率和毛细管压力）的计算/解释密切相关。高压汞注入（HPMI），低压氮气吸附（LP-N ₂ GA），比表面积（SSA）分析，核磁共振（NMR）和分形理论用于研究孔结构特征鄂尔多斯盆地长7组致密储层样品的水饱和度和不可还原性。在这项研究中，毛孔主要由中孔和大孔组成。HPMI更有可能检测到大孔，而中孔的分布可以通过LP-N ₂更好地表征GA和SSA分析。通过HPMI实验获得的毛细管曲线分为两类。根据相对压力为0.5时解吸曲线的变化率，将吸附-解吸等温线分为两组。通过不同方法研究了不同孔隙半径对渗透率的贡献率，结果表明HPMI和LP-N _2的结合与单独使用任一方法相比，GA可以更全面地描述储层的微观孔隙结构。通过NMR测试获得的不可还原水饱和度大于通过HPMI获得的不可还原水饱和度。基于分形理论和毛细管束模型建立了不可约的水饱和度模型。影响不可饱和水饱和度的因素是分形维数，最大连通孔喉半径，最小孔喉半径和水膜厚度。当分形维数大于2.7时，计算结果更接近实验值。各种实验和理论方法的综合应用使人们对致密油藏样品中的孔隙结构特征和流体分布有了更好的了解。