Wellbore stability is crucial for rock drilling. However, wellbore shrinkage is always inevitable in practice, especially for drilling in salt rock formations. The direct consequence of wellbore shrinkage is sticking, which will reduce the efficiency of drilling and bring unnecessary ramifications. In an oil and gas drilling project in Northwest China, the sticking incidents occurred frequently when passing through a super-thick compound salt layer under both high geo-stress and geo-temperature. To determine its peculiar creep characteristics, triaxial creep experiments were carried out in this study on salt rock at elevated temperatures. Our testing results show that the steady creep rate grows with the increase both in deviatoric stress and temperature, which can be expressed as a linear function and power function, respectively. The steady creep rate is more sensitive to the change of temperature under the coupled thermal-mechanical condition. The deformation generated at 26° C is negligible compared with that at higher temperatures, indicating that high temperature can weaken the bearing capacity of salt rock. The accumulative average volumetric strain also grows with increasing temperature, and the volumetric strain at 140° C was 2.19 times that at 100° C. A threshold of 10 MPa and 130° C is found in deviatoric pressure and temperature, respectively. A fractional derivative model is used to fit the creep duration curves under various conditions. The fitting results show that this model can accurately describe the creep behavior of salt rock and fully reflect the rheological properties of salt rock under high temperature and pressure conditions. The authors believe our findings could improve the understanding of the creep characteristics of rock salt and have practical implications for similar deep drilling projects across salt rock formations.

中文翻译：

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高温高压下井筒盐岩蠕变特性研究

井筒稳定性对于凿岩来说至关重要。然而，井筒收缩在实践中总是不可避免的，尤其是在盐岩地层中钻井。井眼收缩的直接后果是卡钻，会降低钻井效率，带来不必要的后果。在西北某油气钻探项目中，在高地应力和高地温下穿越超厚复合盐层时，卡钻事故频发。为了确定其特有的蠕变特性，本研究在高温下对盐岩进行了三轴蠕变实验。我们的测试结果表明，稳态蠕变速率随着偏应力和温度的增加而增加，可以分别表示为线性函数和幂函数。在热机械耦合条件下，稳态蠕变速率对温度变化更为敏感。与高温相比，26℃时产生的变形可以忽略不计，说明高温会削弱盐岩的承载能力。累积平均体积应变也随着温度的升高而增大，140℃时的体积应变是100℃时的2.19倍。偏压和温度的阈值分别为10 MPa和130℃。分数导数模型用于拟合各种条件下的蠕变持续时间曲线。拟合结果表明，该模型能够准确描述盐岩的蠕变行为，充分反映盐岩在高温高压条件下的流变特性。