Process understanding of induced seismicity during stimulation of enhanced geothermal systems
(English) This doctoral thesis investigates the causes that induce (micro)seismicity as a result of hydraulic stimulation in fractured low-permeability rock. Understanding such phenomenon is of paramount importance to eventually forecast induced seismicity in geo-energy applications, like Enhanced G...
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| Tipo de recurso: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2023 |
| País: | España |
| Institución: | CBUC, CESCA |
| Repositorio: | TDR. Tesis Doctorales en Red |
| OAI Identifier: | oai:www.tdx.cat:10803/690449 |
| Acceso en línea: | http://hdl.handle.net/10803/690449 https://dx.doi.org/10.5821/dissertation-2117-405950 |
| Access Level: | acceso abierto |
| Palabra clave: | Àrees temàtiques de la UPC::Enginyeria civil 624 626/627 |
| Sumario: | (English) This doctoral thesis investigates the causes that induce (micro)seismicity as a result of hydraulic stimulation in fractured low-permeability rock. Understanding such phenomenon is of paramount importance to eventually forecast induced seismicity in geo-energy applications, like Enhanced Geothermal Systems (EGS). The research is driven by both scholarly and engineering considerations, addressing the intricate coupled hydro-mechanical (HM) processes that are at play and aiming to advance in the understanding of the mechanisms underlying co-seismicity during the hydraulic stimulation phase of EGS. The thesis begins with a comprehensive review of existing modeling methodologies of coupled processes in fractured rock. This exploration highlights the significant advancements these methodologies have brought to the foundational understanding of fractures, ultimately improving predictive capabilities related to coupled processes within fractured systems. The subsequent focus of the research involves an investigation into the flow regimes induced by constant flow rate water injection into a fracture surrounded by a low-permeability matrix. The study sheds light on the implications for hydraulic test interpretation and numerical simulations. The findings reveal that even in very low-permeability confining rock matrix, leakage is non-negligible due to the small fracture aperture, which maximizes pressure gradients across the fracture-matrix interface. The transition between flow regimes, often overlooked in field tests, has important implications for accurately estimating fracture transmissivity in injection tests in fractured media and modeling approaches. The thesis then proposes an innovative approach for the implicit representation of fractures surrounded by low-permeability rock matrix. This approach assimilates fractures as equivalent continua, demonstrating that a relatively thick equivalent continuum layer can accurately represent a fracture and reproduce HM behavior. The proposed method is validated through the modeling of a hydraulic stimulation carried out at the Bedretto Underground Laboratory, showcasing its ability to improve the simplicity and efficacy of continuum methods in representing fractures in fractured media. Finally, the research delves into the modeling of a highly-monitored test at the Bedretto Underground Laboratory to investigate the impact of fluid injection on permeability enhancement and induced microseismicity. Three models are examined, with the viscoplastic fracture with dilatancy and strain-weakening approach emerging as the most comprehensive in capturing the spatio-temporal coupled response of fractured rock to hydraulic stimulation. This model proves effective in estimating the extent of the stimulated fracture, permeability enhancement, and its impact on the local state of stress and pore pressure at surrounding fractures, presenting a valuable tool for the design of effective hydraulic stimulation. In summary, this doctoral thesis contributes to the understanding of micro-seismicity induced by EGS operations, offering insights into coupled processes, flow regimes, and innovative modeling approaches, ultimately advancing the field of geothermal energy research and hydraulic stimulation design. |
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