Two types of experiments were conducted under different boundary conditions to characterize the anisotropic deformation and directional permeability of the coal sample. Gas sorption-induced strains in the three principal directions were studied to examine the anisotropic deformation. The results show that the coal sample can be treated as isotropic in those planes parallel to the bedding direction. However, the strong strain anisotropy of coal indicates that the gas sorption-induced matrix strain in the direction parallel to the bedding is smaller than that which is perpendicular to the bedding, under different hydrostatic pressures for unconstrained conditions. Permeability measurements were conducted under different effective stresses to determine the magnitude and orientation of directional permeability. It was found that cleat distribution plays a dominant role in the magnitude of the measured permeability. After the two principal permeabilities parallel to the bedding planes are calculated, Mohr’s circle of permeability can be then determined and used to estimate permeability in other directions of the bedding. The experimental results show that the principal permeability directions vary under different effective stresses, illustrating that coal cleat system would be reoriented during gas production. It was also found that cleat reorientation is attributed to three factors: stress contrast, the difference of Biot’s coefficient, and the difference in the sorption-induced stresses in the direction perpendicular to and parallel to the bedding planes. However, cleat reorientation had little influence on changes in the magnitude of coal permeability. Permeability anisotropy degree between the three principal directions varies dynamically under different effective stresses, and then possibly changes direction of gas flow. This study demonstrates that stress-dependent coal anisotropy should be considered for simulating gas flow behavior and predicting coalbed methane production.

中文翻译：

---

煤层气藏各向异性变形和应力相关定向渗透率研究

在不同边界条件下进行了两种类型的实验，以表征煤样的各向异性变形和定向渗透率。研究了三个主要方向上的气体吸附诱导应变，以检查各向异性变形。结果表明，煤样在平行于层理方向的那些平面内可以被视为各向同性。然而，煤的强应变各向异性表明，在无约束条件下，在不同的静水压力下，平行于层理方向的气体吸附诱导基体应变小于垂直于层理的方向。在不同的有效应力下进行渗透率测量，以确定定向渗透率的大小和方向。发现割理分布在测量渗透率的大小中起主导作用。在计算出平行于层理平面的两个主要渗透率之后，就可以确定莫尔渗透率圆并用于估计层理其他方向的渗透率。实验结果表明，不同有效应力下主渗透率方向不同，说明煤割理系统在采气过程中会发生重新定向。还发现割理重新定向归因于三个因素：应力对比、Biot 系数的差异以及垂直和平行于层理平面的方向上的吸附诱导应力的差异。然而，割理重新定向对煤渗透率大小的变化几乎没有影响。三个主方向之间的渗透率各向异性程度在不同的有效应力下动态变化，进而可能改变气流方向。这项研究表明，在模拟气体流动行为和预测煤层气产量时，应考虑与应力相关的煤的各向异性。