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，利用水力压裂提高结晶岩层渗透率的潜力巨大。EGS有潜力在未来50年内为美国提供超过10万兆瓦的电力；AGR是一种加强较浅深度井的补给的方法，这对于增加向干旱地区人口提供清洁水可能特别有用。在这两种应用中，岩石都经过水力压裂，以形成相互连接的裂缝网络，从而增加增产地层的渗透率。水力压裂后裂缝连通性的缺乏可能导致水力压裂岩层渗透率的增加很低，甚至微不足道，这反过来可能导致经济损失，最终导致EGS和AGR项目在没有充分利用其潜力的情况下被放弃。因此，了解现场变量（如本项目将研究的三轴应力状态）对裂缝连通性的影响对这些应用的成功至关重要。该项目还将吸引和教育本科生、研究生以及高中生，让他们了解应用于EGS和AGR的水力压裂基础知识。该项目将1)评估三轴应力状态对结晶岩石中水力裂缝和现有裂缝连通性的影响，2)识别水力压裂导致岩石破坏的视觉和声发射（AE）前兆。为了做到这一点，我们将重新设计一种由首席研究员开发并先前用于垂直载荷下柱状花岗岩样品水力压裂的设备，以允许在柱状岩石样品中施加三轴应力状态。在这些试样中预制的缺陷将受到水的压力，导致水力裂缝的扩展。通过使用高分辨率和高速摄像机，以及监测新裂缝形成时产生的声发射，可以直观地观察整个压裂过程。通过将可视化与压裂过程的ae监测相结合，将有可能评估裂缝连通性模式下三轴应力状态的影响，并识别可见裂缝发育的前兆。确定这些前兆将有可能使人们在这些现象发生之前预测岩石破坏和水力裂缝的预期连接模式。本研究的实验观察结果将为进一步开发和验证用于模拟结晶岩水力压裂的耦合水力力学模型奠定基础。

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.