UVM Theses and Dissertations
K C, Bijay
Civil and Environmental Engineering
A significant amount of thermal energy is present in the deep hot dry bedrocks, which can be tapped to provide clean and renewable energy for future generations. Due to very low permeability, the hot dry bedrock is artificially stimulated to create a network of engineered fractures and enhance the permeability for economic energy extraction. Such systems are known as Enhanced (or Engineered) Geothermal Systems (EGS). Usually, in EGS, cold water injected through the injection well(s) exchanges heat as it passes through the engineered fracture network in the hot dry bedrock; hot water is then extracted from the production well(s), which can be used for direct heating or electricity generation via a binary or flash-steam power cycle. Injection-induced shear stimulation, commonly known as 'hydro-shearing' is often implemented in EGS to enhance reservoir permeability. Hydro-shearing enhances the reservoir permeability due to self-propping of asperities present on the fracture surface during fracture slip. However, the fracture slip can result in a seismic event, one of the major public concerns related to development of EGS. Moreover, the coupled Thermal-Hydrological-Mechanical-Chemical (THMC) processes triggered due to fluid-fracture surface interactions in EGS reservoirs, reduces the fracture permeability leading to a decrease in production over time. Proppants, solid granular materials, can be used to prop open the fracture eliminating the necessity of fracture slip and hence minimizing the risk of induced seismicity. However, due to the limited application of proppants in EGS, our knowledge about proppants behavior and their long-term performance under EGS conditions is limited. In addition, perturbation in the state-of-stress acting on a fault (or fracture) in a reservoir due to operational activities such as stimulation/re-stimulation, injection, extraction, and water impoundment can also induce seismicity in a reservoir. In this study, we experimentally investigated (i) the impact of hydro-shearing on the fracture response to coupled THMC processes, (ii) the suitability of proppants to sustain the fracture permeability under EGS conditions, and (iii) fault/fracture reactivation due to stress perturbation in tensional and compressional faulting regime. The experimental results showed permanent permeability enhancement due to hydro-shearing. Hydro-shearing also resulted in enhanced mineral dissolution and gouge particles under EGS conditions. Proppants can be used to enhance the fracture permeability; however, the proppant can crush and embed on the fracture surface leading to permeability decline over time. In addition, the faults/fractures in extensional regime have higher affinity for slipping compared to the compressional regime. The insights from the experimental results in this study may be used to improve numerical and conceptual models of the EGS reservoir. For future experiments, it is recommended to use proppants at higher concentration and geothermal brine close to chemical equilibrium with rock (and proppants) minerals and more representative of field conditions to better understand the mechanical and chemical processes (dissolution/precipitation) affecting fracture permeability of EGS reservoir in long-term.
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