Abstract

In unconventional natural gas development, the true triaxial stress field and anisotropic structure plane play an important role in the deformation and permeability evolution of reservoir rocks. In this research, the true triaxial experiments of sandstone, shale and coal with their own unique structural characteristics under the identical mean principal stress (p) and generalized shear stress (q) are performed. The results show that the volumetric strain of sandstone is not affected by the stress path within the elastic range. However, that of shale is closely related to the deformation of shale bedding. The stress path of shale has a significant impact on its deformation behavior and permeability evolution. Interestingly, the sandstone permeability is strongly dependent on the shear behavior under fixed p and q, i.e., it increases with the increasing generalized shear strain. The frictional sliding between sandstone particles induced by shear effect plays an important role in promoting the permeability. For shales, the stress normal to bedding planes plays an important role in permeability evolution, which is stronger than the frictional sliding effect between shale particles. The permeability of coal evolves between sandstones and shales, which is related to its unique cleat system. To avoid neglecting the important factors affecting permeability evolution in unconventional natural gas extraction, the permeability model should be selected according to the structural characteristics of reservoir rocks.

Article highlights

• Frictional sliding between particles determines the sandstone permeability.

• Stress normal to bedding planes plays an important role in shale permeability.

• The permeability of coal is related to its unique cleat system.

中文翻译：

---

真三轴应力路径下渗透率演化对储集岩各向异性结构的力学响应

摘要

在非常规天然气开发中，真三轴应力场和各向异性结构面在储层岩石的变形和渗透率演化中起着重要作用。在这项研究中，在相同的平均主应力（p）和广义剪应力（q） 执行。结果表明，在弹性范围内，砂岩的体积应变不受应力路径的影响。但是，页岩的变形与页岩层理的变形密切相关。页岩的应力路径对其变形行为和渗透率演化具有重要影响。有趣的是，在固定的p和q下，砂岩的渗透率强烈地取决于剪切行为。即，它随着广义剪切应变的增加而增加。剪切作用引起的砂岩颗粒之间的摩擦滑动在提高渗透率方面起着重要作用。对于页岩，垂直于顺层平面的应力在渗透率演化中起着重要作用，它比页岩颗粒之间的摩擦滑动作用更强。煤的渗透性在砂岩和页岩之间演化，这与其独特的夹层系统有关。为避免忽视影响非常规天然气开采过程中渗透率演化的重要因素，应根据储集岩的构造特征选择渗透率模型。

文章重点

• 颗粒之间的摩擦滑动决定了砂岩的渗透率。

• 垂直于层理面的应力在页岩渗透率中起重要作用。

• 煤的渗透性与其独特的割理系统有关。