Effect of the Triaxial State of Stress in the Connectivity of Hydraulically-created Fractures in Crystalline Rocks

在增强型地热系统(EGS)和人工地下水补给(AGR)应用中，利用水力压裂来提高结晶岩层的渗透率具有巨大的潜力。EGS有可能在未来50年为美国提供超过100,000兆瓦的水；AGR是一种增强较浅深度井的补给的方法，这可能对增加干旱地区人口的清洁水供应特别有用。在这两种应用中，岩石都会被水力压裂，以便形成一个相互连接的裂缝网络，从而增加模拟地层的渗透率。水力刺激后裂缝连通性的缺乏可能会导致水力压裂岩层渗透率的低或微不足道的增加，这反过来又可能导致经济损失，并最终导致EGS和AGR项目在没有充分发挥其潜力的情况下被放弃。因此，了解现场变量的影响，如本项目中将研究的三轴应力状态，对这些应用的成功具有至关重要的作用。该项目还将就应用于EGS和AGR的水力压裂的基本原理对本科生、研究生和高中生进行培训。该项目将1)评估三轴应力状态对结晶岩中水力产生的裂缝和现有裂缝之间连通性的影响，以及2)识别水力压裂引起的岩石破坏的视觉和声发射(AE)前兆。为了做到这一点，将重新设计首席调查员开发并以前使用的在垂直载荷下对棱柱状花岗岩试件进行水力压裂的仪器，以允许在棱柱状岩石试件中施加三轴应力状态。在这些试件中预制的缺陷将用水加压，从而导致水力裂缝的扩展。整个压裂过程将使用高分辨率和高速摄像机，并通过监测新裂缝发展产生的声发射来目视观察。通过将可视化与破裂过程的声发射监测相结合，将有可能评估三轴应力状态在断裂连通性模式中的影响，并识别可见裂缝发展的前兆。确定这些前兆可能会使人们能够在这些现象发生之前预测岩石破坏和水力裂缝的预期连通模式。这项研究的实验观测将为进一步开发和验证用于模拟结晶岩石水力压裂的流体力学耦合模型奠定基础。

There is great potential in the use of hydraulic fracturing to increase the permeability of crystalline rock formations in Enhanced Geothermal Systems (EGS) and Artificial Groundwater Recharge (AGR) applications. EGS has the potential of providing more than 100,000 MWe for the next 50 years in the United States; AGR is a method that enhances the recharge of wells at shallower depths, which may be particularly useful to increase the supply of clean water to populations in dry regions. In both applications, the rocks are hydraulically fractured in order to create a network of interconnected fractures that will increase the permeability of the stimulated formations. The lack of fracture connectivity after the hydraulic stimulations may lead to low, or insignificant, increases in the permeability of the hydraulically-fractured rock formations which, in turn, may result in financial losses and ultimately cause the abandonment of EGS and AGR projects without taking advantage of their full potential. Therefore, understanding the effect of field variables, such as the triaxial state of stress which will be investigated in this project, in fracture connectivity is of adamant importance to the success of these applications. This project will also engage and educate undergraduate and graduate as well as high-school students, on the fundamentals of hydraulic fracturing applied to EGS and AGR.This project will 1) evaluate the effect of the triaxial state of stress in the connectivity between hydraulically-created and existing fractures in crystalline rocks, and 2) identify visual and acoustic emission (AE) precursors of rock failure due to hydraulic fracturing. In order to do so, an apparatus developed and previously used by the Principal Investigator in the hydraulic fracturing of prismatic granite specimens subject to a vertical load will be redesigned to allow one to apply a triaxial state of stress in prismatic rock specimens. Flaws pre-fabricated in these specimens will be pressurized with water leading to the propagation of hydraulic fractures. The entire fracturing process will be observed visually, using High-Resolution and High-Speed Video cameras, and by monitoring the acoustic emissions produced by the development of new fractures. By integrating the visualization with the AE-monitoring of the fracturing processes, it will be possible to evaluate the effect of the triaxial state of stress in the mode of fracture connectivity and to identify precursors of visible crack development. Determining these precursors will potentially allow one to predict rock failure and the expected mode of connectivity of hydraulic fractures before these phenomena take place. The experimental observations of this research will be the basis for further developments and validation of coupled hydro-mechanical models used to simulate the hydraulic fracturing of crystalline rocks.