Evaluation of permeability of a fracture system under true triaxial compression

在本研究中，我们对真三轴压缩条件下裂缝的渗透率进行了实验研究，并利用最大达12.8m的大型合成裂缝对裂缝的孔径和渗透率进行了理论研究。实验研究得到的主要结果可以总结如下：1。本研究建立的实验系统可以在孔隙压力小于0.3MPa、剪切位移小于50mm2的条件下，对均匀法向应力条件下裂缝渗透率的非均质性进行评价。人工裂缝试验表明，裂缝渗透率变化幅度为7.5% ~ 11.5%，水力孔径随法向应力和剪应力的增大而逐渐减小。在拉伸裂缝上的实验表明，实验系统可以在孔隙压力小于0.3MPa的情况下对渗透率进行评估，但由于空气被困在裂缝中，因此存在一个有待解决的问题。本文的主要研究结果总结如下：1。用于创建合成裂缝的计算机代码再现了孔径与表面高度之间的功率谱密度（psd）之比，测量了花岗岩中1米的拉伸裂缝。当裂缝长度超过0.2m时，常闭裂缝的尺寸效应对渗透率的影响消失，因为当裂缝尺寸超过0.2m时，尺寸效应对初始孔径标准差（SD）的影响消失。我们提出了一个与裂缝尺寸和孔径SD无关的常闭裂缝渗透率的经验公式。同样，当剪切位移小于裂缝尺寸的3.1%左右时，尺寸对剪切裂缝渗透率的影响也会消失，因为在此条件下，尺寸对初始孔径标准差（SD）的影响也会消失。剪切裂缝渗透率的各向异性随着剪切位移和闭合而增大。这种各向异性是由宏观水流两个方向孔径psd的差异引起的；垂直和平行于剪切位移。随着剪切位移的增大，形成的通道数量减少，面积增大，水流随着剪切位移的增大而更加局部化。少

In this study, we performed both experimental studies on permeability of a fracture under true triaxial compression and theoretical studies on aperture and permeability of a fracture utilizing large synthetic fractures of up to 12.8m.Main results obtained by experimental studies can be summarized as follows :1. An experimental system developed in this study can evaluate the heterogeneity in the permeability of a fracture under uniform normal stress for a pore pressure of less than 0.3MPa and shear displacement of less than 50mm.2. Experiments on an artificial fracture showed that the permeability of the fracture varied by 7.5 to 11.5% and that the hydraulic aperture gradually decreased with an increase in both normal and shear stresses.3. Experiments on a tensile fracture showed that the experimental system could evaluate the permeability for a pore pressure of less than 0.3MPa, but a problem to be solved remained since air was trapped in the fracture.Main results obtained by theoretic … More al studies can be summarized as follows :1. A computer code for creating a synthetic fracture reproduced the ratio of the power spectral densities (PSDs) between aperture and surface height, measured for a tensile fracture of 1m in granite.2. The size effect on the permeability of a normally closed fracture disappears when the fracture exceeds 0.2m since the size effect on the standard deviation (SD) of the initial aperture disappears when the fracture size exceeds 0.2m. We proposed an empirical formula for the permeability of a normally closed fracture, which is independent of the fracture size and the SD of the aperture. Similarly, the size effect on the permeability of a sheared fracture disappears when the shear displacement is less than about 3.1% of the fracture size since the size effect on the standard deviation (SD) of the initial aperture disappears under such conditions.3. The anisotropy in the permeability of a sheared fracture increases with both shear displacement and closure. This anisotropy is caused by the difference in the aperture PSDs between two directions of macroscopic water flow ; perpendicular and parallel to the shear displacement. Furthermore, water flow is more localized with the shear displacement since a fewer channels with a greater area are formed as the shear displacement increases. Less