Liquid nitrogen (LN2) can be utilized in the development of enhanced geothermal systems, as well as for deep/ultra-deep hydrocarbon reservoir stimulation, fire suppression, and other high-temperature geological projects. It is a crucial issue in the utilization of LN2 to investigate the pore structure evolution, permeability, and damage characteristics of high-temperature rocks under the influence of LN2 cooling shock. These rocks were first slowly heated to 150∼600°C and held for 2 h, followed by LN2 or natural cooling. The evolution of pore volume in high-temperature rocks affected by liquid nitrogen cooling was quantified. T2 cutoff values were determined through centrifugal tests, while the contents of irreducible and mobile fluids were estimated. Based on the aforementioned analysis as well as changes in irreducible fluid saturation, pore throat, tortuosity, and permeability, this study examines the closure and development of pores along with permeability behavior. The findings suggest that, despite a more pronounced decrease in porosity at lower heating temperatures, LN2 cooling specimens exhibit superior pore connectivity and permeability compared to those cooled naturally. LN2 stimulation not only induces crack initiation and propagation but also results in further cooling induced densification based on heating densification. 225°C is considered to be the optimal temperature for cooling contraction induced densification in this study. At higher heating temperatures, the damage to rock cooled with LN2 is more severe than that of naturally cooled. This results in a greater increase in porosity, movable fluid content and proportion, and permeability of LN2 cooled specimens compared to naturally cooled specimens. The damage mechanism can be better understood by the constructed damage model that coordinates the pore increase/decrease and mutual pore transformation from the perspective of pore evolution.

中文翻译：

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液氮冷却冲击下高温岩石孔隙结构演化及损伤特征研究

液氮（LN2）可用于增强地热系统的开发，以及深层/超深层油气藏增产、灭火和其他高温地质工程。这是液氮利用中的一个关键问题2研究液氮影响下高温岩石的孔隙结构演化、渗透率和损伤特征2冷却冲击。首先将这些岩石缓慢加热至 150∼600°C 并保持 2 小时，然后加入液氮2或自然冷却。量化了受液氮冷却影响的高温岩石中孔隙体积的演变。时间2通过离心测试确定截止值，同时估计束缚性和流动性流体的含量。基于上述分析以及束缚流体饱和度、孔喉、曲折度和渗透率的变化，本研究考察了孔隙的闭合和发育以及渗透率行为。研究结果表明，尽管在较低的加热温度下孔隙率下降更为明显，但 LN2与自然冷却的样品相比，冷却样品表现出优异的孔隙连通性和渗透性。闪电网络2刺激不仅诱发裂纹萌生和扩展，而且还导致基于加热致密化的进一步冷却诱发致密化。在本研究中，225°C 被认为是冷却收缩诱导致密化的最佳温度。在较高的加热温度下，用液氮冷却岩石的损伤2比自然冷却的情况更严重。这导致孔隙率、可动流体含量和比例以及液氮渗透率的更大增加2冷却样品与自然冷却样品的比较。从孔隙演化的角度构建协调孔隙增减和相互转化的损伤模型，可以更好地理解损伤机制。