In engineering, it is considerably important to understand the evolution process of sandstone deformation and permeability under complex stress conditions. Although numerous scholars have reported on sandstone permeability, many of their studies are only based on uniaxial or conventional triaxial stress conditions. In practical engineering, in situ stress evidently indicates three-dimensional anisotropy. In recent years, many scholars have performed experimental studies on rock permeability under true triaxial stress. There are, however, few investigations related to the influence of intermediate principal stress and intermediate principal stress coefficient on sandstone permeability; moreover, the mechanism is unclear. In this study, the permeability of sandstone under true triaxial stress conditions is measured using a true triaxial apparatus. The results show that the intermediate principal stress has significant effects on permeability and deformation of sandstone sample. Under different principal stress conditions, the permeability is observed to decrease with the increase of the principal stress. With increasing intermediate principal stress coefficient b (from 0 to 1), the major principal strain and the minor principal strain always compressed and finally decreased by 27.31 and 36.96%, while the intermediate principal strain finally changes from expansion to compression (from − 0.045 to 0.255%). Also the normalized permeability loss increased from 9.57 to 16.59%. In addition, by fitting the experimental data with several permeability models, it was found that a true triaxial permeability model can well describe the changes in sandstone permeability. Based on the fitting parameters, the influence of intermediate principal stress and its coefficient on the stress sensitivity of sandstone was analyzed.

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中间主应力对砂岩渗透率影响的试验研究

在工程中，了解复杂应力条件下砂岩变形和渗透率的演化过程具有十分重要的意义。虽然众多学者对砂岩渗透率进行了报道，但他们的许多研究仅基于单轴或常规三轴应力条件。在实际工程中，地应力明显表现出三维各向异性。近年来，许多学者对真三轴应力下的岩石渗透率进行了实验研究。但关于中主应力和中主应力系数对砂岩渗透率影响的研究较少；此外，机制尚不清楚。在这项研究中，砂岩在真三轴应力条件下的渗透率是使用真三轴仪测量的。结果表明，中间主应力对砂岩样品的渗透率和变形有显着影响。在不同的主应力条件下，观察到渗透率随着主应力的增加而降低。随着中间主应力系数 b 的增加（从 0 到 1），大主应变和小主应变总是压缩，最后分别减小 27.31 和 36.96%，而中间主应变最终从膨胀变为压缩（从 - 0.045 到0.255%)。归一化渗透率损失也从 9.57% 增加到 16.59%。此外，通过将实验数据与几种渗透率模型拟合，发现真三轴渗透率模型可以很好地描述砂岩渗透率的变化。根据拟合参数，