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

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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.