The competition between fracture nucleation, propagation, and coalescence in dry and water-saturated crystalline rock
The competition between fracture nucleation, propagation, and coalescence in dry and water-saturated crystalline rock
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DOI：
10.5194/se-12-375-2021
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发表时间：
2021-02
期刊：
影响因子：
3.4
通讯作者：
J. McBeck;Wen-lu Zhu;François Renard
中科院分区：
文献类型：
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作者：
J. McBeck;Wen-lu Zhu;François Renard
抽象的。在结晶岩石中裂缝网络发展过程中出现的连续行为可分为三种端元模式：裂缝成核、孤立裂缝扩展和裂缝聚结。这些不同的裂缝生长模式产生了具有独特几何属性的裂缝网络，如聚类和连通性，对渗透率和流体-岩石相互作用的程度产生重要控制。为了跟踪这些模式的裂缝发展变化的优势，在整个加载到故障，从而如何在这些条件下的裂缝网络的几何属性可能会有所不同，我们进行现场X射线断层扫描三轴压缩实验低孔隙度结晶岩（二长岩）上地壳应力条件下。为了研究孔隙流体的影响上的三种模式的增长的变化的优势，我们进行了两个实验在名义上干燥的条件下，一个在水饱和的条件下，5 MPa的孔隙流体压力。我们施加20-35 MPa的围压，然后逐步增加差应力，直到岩石宏观破坏。在1-5 MPa的每个应力步骤之后，我们获得三维（3D）X射线吸收系数场，从中我们提取3D裂缝网络。我们开发了一种新的方法，在随后的断层扫描之间跟踪单个裂缝，以确定裂缝是从多个裂缝的合并和连接中生长，还是从单个裂缝的传播中生长。在整个加载过程中，在所有的实验中，预先存在的裂缝的体积大于成核裂缝的体积，表明预先存在的裂缝的生长主导新裂缝的成核。在整个加载过程中，直到接近宏观故障在所有的实验中，聚结裂缝的体积小于传播裂缝的体积，表明裂缝传播主导聚结。然而，在失效之前，在名义上干燥条件下进行的实验中，聚结裂缝的体积至少是扩展裂缝体积的两倍。相反，在水饱和的样品中，尽管在失效前的阶段中聚结裂缝的体积增加，但扩展裂缝的体积仍然占主导地位。应力腐蚀开裂的影响与断裂尖端的水化反应和/或凝结剂硬化相关，可以解释在干燥和水饱和条件下观察到的断裂发展差异。
Abstract. 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.