Understanding and mitigating the post-injection seismicity induced by fluid injection in enhanced geothermal systems
(English) Induced seismicity due to hydraulic stimulation and circulation is a hurdle for the development of Enhanced Geothermal Systems (EGS). The monitored seismicity has occasionally reached magnitudes large enough to be felt by local populations, and in several EGS cases the largest magnitude ea...
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| Tipo de recurso: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2024 |
| País: | España |
| Institución: | CBUC, CESCA |
| Repositorio: | TDR. Tesis Doctorales en Red |
| OAI Identifier: | oai:www.tdx.cat:10803/690452 |
| Acceso en línea: | http://hdl.handle.net/10803/690452 https://dx.doi.org/10.5821/dissertation-2117-405949 |
| Access Level: | acceso abierto |
| Palabra clave: | Àrees temàtiques de la UPC::Enginyeria civil 55 624 |
| Sumario: | (English) Induced seismicity due to hydraulic stimulation and circulation is a hurdle for the development of Enhanced Geothermal Systems (EGS). The monitored seismicity has occasionally reached magnitudes large enough to be felt by local populations, and in several EGS cases the largest magnitude earthquakes occur few hours to several months after the cessation of the injection operations. This thesis aims to i) deepen the understanding of the processes that trigger such induced seismicity, ii) improve forecasting methodologies and iii) explore strategies to mitigate its occurrence. The focus is especially placed on post-injection seismicity. The well-documented case of Basel EGS, Switzerland, is adopted as verification example. Firstly, the identification of the triggering mechanisms at Basel is performed making use of a coupled hydro-mechanical model with increasing level of complexity. A simple fault-crossed elastic domain is compared with a homogeneous elastic domain to highlight the importance of structural heterogeneities on the direct and indirect (poroelastic) effects of pressure diffusion as triggering mechanisms of seismicity. Additionally, another hydro-mechanical model is implemented by a discrete faulting network, based on seismic interpretations. The model simulates the plastic reactivations of faults, and the effects of pore pressure diffusion, poroelastic stressing, shear-slip stress transfer and slip weakening are distinguished. Simulation results show that the faults located in the vicinity of the injection well fail during injection, mainly triggered by pore pressure build-up. At the stop of injection, poroelastic stress relaxation leads to the immediate rupture of faults that were stabilized during the injection. Stress redistribution is also a prominent triggering mechanisms of post-injection reactivation of distant faults. Slip-induced friction weakening on certain faults amplifies the potential of these faults to reactivate multiple time during injection and after its stop. The development of a forecasting methodology for induced seismicity is the second major achievement of this thesis. The hydro-mechanical model previously implemented is combined with a seismicity rate model, the Gutenberg-Richter law and the Epidemic Type Aftershock Sequence model to quantify the seismicity magnitude, of both mainshocks and aftershocks, associated to the stress changes due to fluid injection. The application of this hybrid methodology on the discrete-faulted hydro-mechanical model of Basel EGS gives the opportunity to explore the effects of different strategies of injection on the enhancement of fault permeability and induced seismicity rate. A constant injection protocol followed by a progressive decrease of injection rate mitigates post-injection seismicity, while enhancing the permeability of the faults in the domain. The thesis also addresses the discussion of the final cessation of injection, comparing the results of shut-in and bleed-off of the well. Results suggest that shutting-in the well can mitigate better early post-injection seismicity than bleeding-off the well, but pore pressure diffusion can destabilize critically-oriented faults in the reservoir for a longer period of time. By providing enhanced understanding, new methodologies and practical solutions, this thesis represents a substantial step-forward in the mitigation and control of induced seismicity, and in particular of post-injection seismicity, in EGS. |
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