During a hydraulic fracturing operation, a large quantity of salinity fracturing fluids will be retained in the shale reservoir, which may cause the problem of wellbore instability. Previous studies mainly focused on the change mechanism of clay-rich shale after brine immersion. However, the influence of NaCl concentration on mechanical behaviors of a low-clay shale is still unclear. Therefore, in this investigation, conventional triaxial compression tests were carried out on low-clay shale specimens saturated with NaCl solutions at different concentrations (0%, 5%, 15%, 25%) under different confining pressures (0 MPa, 15 MPa, 30 MPa, 45 MPa), and their mechanical properties were analyzed systematically. Optical microscopy, polarized light microscopy and scanning electron microscopy were used to understand the microscopic changes in shale samples after saturation in NaCl solutions. The brittleness indices of saturated shale specimens were evaluated based on rock strength properties and the energy evolution law. According to the experimental results, compared with the water-saturated specimens, the peak strength, elastic modulus, cohesion and internal friction angle of specimens saturated with 5%NaCl solution all showed a downward trend. However, for the specimens saturated with the NaCl solution at concentrations of 15% and 25%, higher NaCl concentration showed a greater rebound effect on the mechanical parameters compared with the 5%NaCl saturation. The microscopic observation showed that the brine could invade the interior of shale specimens from micropores, microcracks and bedding fractures, which were mainly parallel to the bedding direction. And the bedding fractures and some of micropores seemed to be filled by deposited NaCl crystals and mineral particles. The brittleness index of the brine-saturated shale specimen decreased at the low-concentration (5%) and increased at the high-concentration (15% and 25%). This investigation could provide a theoretical basis and mechanical parameters for analyzing the stability of shale wellbore affected by the salinity fracturing fluids.

中文翻译：

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NaCl浓度对低粘土页岩三轴力学行为影响的实验研究

在水力压裂作业过程中，大量盐度压裂液会滞留在页岩储层中，可能会导致井筒失稳问题。前期研究主要集中在富黏土页岩卤水浸泡后的变化机制。然而，氯化钠浓度对低粘土页岩力学行为的影响仍不清楚。因此，本次研究对饱和不同浓度（0%、5%、15%、25%）氯化钠溶液的低粘土页岩试件在不同围压（0 MPa、15 MPa、 30 MPa、45 MPa），并对它们的力学性能进行了系统分析。使用光学显微镜、偏光显微镜和扫描电子显微镜来了解页岩样品在 NaCl 溶液饱和后的微观变化。根据岩石强度特性和能量演化规律评价饱和页岩试样的脆性指数。实验结果表明，与水饱和试件相比，5%NaCl溶液饱和试件的峰值强度、弹性模量、内聚力和内摩擦角均呈下降趋势。然而，对于饱和浓度为15%和25%的氯化钠溶液的样品，与5%氯化钠饱和状态相比，较高的氯化钠浓度对力学参数的回弹效应更大。显微观察表明，盐水可以从微孔、微裂缝和层理裂缝侵入页岩标本内部，且主要平行于层理方向。层理裂隙和部分微孔似乎被沉积的氯化钠晶体和矿物颗粒所填充。饱和盐水页岩的脆性指数在低浓度（5%）时降低，在高浓度（15%和25%）时升高。该研究可为分析盐度压裂液对页岩井筒稳定性的影响提供理论依据和力学参数。