In tight crystalline rocks faults are known to be substantially more hydraulically conductive than the rock matrix. However, most of our knowledge relies on static measurements, or before/after failure data sets. The spatio-temporal evolution of the permeability field during faulting remains unknown. Here, we determine at which stage of faulting permeability changes most, and the degree of permeability heterogeneity along shear faults. We conducted triaxial deformation experiments in intact Westerly granite, where faulting was stabilized by monitoring acoustic emission rate. At repeated stages during deformation and faulting we paused deformation and imposed macroscopic fluid flow to characterize the overall permeability of the material. The pore pressure distribution was measured along the prospective fault to estimate apparent hydraulic transmissivity, and propagation of the macroscopic shear fault was monitored by locating acoustic emissions. We find that average permeability increases dramatically (by two orders of magnitude) with increasing deformation up to peak stress, where the fault is not yet through-going. Post-peak stress, overall permeability increases by a factor of three. However, at this stage we observed local heterogeneities in permeability by up to factors of eight, ascribed to a partially connected fracture network. This heterogeneity decreases with fault completion at residual shear stress. With further slip on the newly formed fault, the average hydraulic transmissivity remains mostly stable. Our results show that permeability enhancement during shear rupture mostly occurs ahead of the rupture tip, and that strongly heterogeneous permeability patterns are generated in the fault cohesive zone due to partial fracture connectivity.

中文翻译：

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花岗岩断层增长和滑移过程中的渗透率发展


在致密结晶岩石中，已知断层的液压传导性比岩石基质强得多。然而，我们的大部分知识都依赖于静态测量或故障前/后数据集。断层期间渗透场的时空演变仍然未知。在这里，我们确定了断层渗透率变化最大的阶段，以及沿剪切断层的渗透率非均质程度。我们在完整的 Westerly 花岗岩中进行了三轴变形实验，通过监测声发射率来稳定断层。在变形和断层的重复阶段，我们暂停了变形并施加了宏观流体流动，以表征材料的整体渗透率。沿预期断层测量孔隙压力分布以估计表观水力导水率，并通过定位声发射来监测宏观剪切断层的传播。我们发现，平均磁导率随着变形的增加而急剧增加（两个数量级），直到断层尚未达到峰值应力。峰后应力，整体渗透率增加了三倍。然而，在这个阶段，我们观察到局部渗透率的非均质性高达 8 倍，这归因于部分连接的裂缝网络。这种异质性随着残余剪切应力下的断层完成而降低。随着新形成的断层的进一步滑移，平均水力导水率基本保持稳定。我们的结果表明，剪切破裂过程中的渗透率增强主要发生在破裂尖端之前，并且由于部分裂缝连通性，在断层粘结带中产生了强烈的非均质渗透率模式。