The continuum of behavior that emerges during fracture network development in crystalline rock may be categorized into three end-member modes: fracture nucleation, isolated fracture propagation, and fracture coalescence. These different modes of fracture growth produce fracture networks with distinctive geometric attributes, such as clustering and connectivity, that exert important controls on permeability and the extent of fluid–rock interactions. To track how these modes of fracture development vary in dominance throughout loading toward failure and thus how the geometric attributes of fracture networks may vary under these conditions, we perform in situ X-ray tomography triaxial compression experiments on low-porosity crystalline rock (monzonite) under upper-crustal stress conditions. To examine the influence of pore fluid on the varying dominance of the three modes of growth, we perform two experiments under nominally dry conditions and one under water-saturated conditions with 5 MPa of pore fluid pressure. We impose a confining pressure of 20–35 MPa and then increase the differential stress in steps until the rock fails macroscopically. After each stress step of 1–5 MPa we acquire a three-dimensional (3D) X-ray adsorption coefficient field from which we extract the 3D fracture network. We develop a novel method of tracking individual fractures between subsequent tomographic scans that identifies whether fractures grow from the coalescence and linkage of several fractures or from the propagation of a single fracture. Throughout loading in all of the experiments, the volume of preexisting fractures is larger than that of nucleating fractures, indicating that the growth of preexisting fractures dominates the nucleation of new fractures. Throughout loading until close to macroscopic failure in all of the experiments, the volume of coalescing fractures is smaller than the volume of propagating fractures, indicating that fracture propagation dominates coalescence. Immediately preceding failure, however, the volume of coalescing fractures is at least double the volume of propagating fractures in the experiments performed at nominally dry conditions. In the water-saturated sample, in contrast, although the volume of coalescing fractures increases during the stage preceding failure, the volume of propagating fractures remains dominant. The influence of stress corrosion cracking associated with hydration reactions at fracture tips and/or dilatant hardening may explain the observed difference in fracture development under dry and water-saturated conditions.

中文翻译：

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干燥和水饱和的结晶岩中裂缝成核，扩展和聚结之间的竞争

结晶岩裂隙网络发育过程中出现的连续性行为可分为三种端元模式：裂隙成核，孤立的裂隙扩展和裂隙合并。这些不同的裂缝增长方式产生的裂缝网络具有独特的几何属性，例如聚类和连通性，这些裂缝网络对渗透率和流体-岩石相互作用的程度具有重要的控制作用。为了跟踪这些裂缝发展模式在整个载荷作用下直至破坏的过程中的主导地位如何变化，从而在这些条件下裂缝网络的几何属性如何变化，我们对低孔隙度晶体岩（硅铁矿）进行了原位X射线断层扫描三轴压缩实验在上地壳应力条件下。为了检查孔隙流体对三种生长方式的变化主导性的影响，我们在名义上干燥的条件下进行了两个实验，而在孔隙压力为5 MPa的水饱和条件下进行了一个实验。我们施加20-35 MPa的围压，然后逐步增加压差，直到岩石宏观破坏为止。在每个1-5 MPa的应力阶跃之后，我们获得了三维（3D）X射线吸收系数场，从中可以提取3D断裂网络。我们开发了一种在随后的断层扫描之间跟踪单个裂缝的新颖方法，该方法可以确定裂缝是从多个裂缝的合并和链接中生长还是从单个裂缝的扩展中生长。在所有实验的整个加载过程中，既有裂缝的体积大于成核裂缝的体积，表明既有裂缝的增长在新裂缝的成核中占主导地位。在所有实验中，直到整个载荷都接近宏观破坏为止，合并裂缝的体积小于正在传播的裂缝的体积，这表明裂缝的传播在合并过程中占主导地位。然而，在即将发生破坏之前，在名义上干燥的条件下进行的实验中，聚结裂缝的体积至少是传播裂缝的体积的两倍。相反，在水饱和的样品中，尽管在破裂前的阶段固结裂缝的数量增加了，但传播裂缝的数量仍然占主导地位。