Visco-plasticity of zero-thickness interfaces with softening, and application to the study of fault reactivation
(English) In the current context of both energy and environmental needs, there has been an increase in activities related to extraction and injection of fluids in the underground. It is known that a possible consequence of these activities is the reactivation of faults and, therefore, there has rece...
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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:10803/689356 |
| Acceso en línea: | http://hdl.handle.net/10803/689356 https://dx.doi.org/10.5821/dissertation-2117-396973 |
| Access Level: | acceso abierto |
| Palabra clave: | 621 624 |
| Sumario: | (English) In the current context of both energy and environmental needs, there has been an increase in activities related to extraction and injection of fluids in the underground. It is known that a possible consequence of these activities is the reactivation of faults and, therefore, there has recently been of growing interest to understand under what conditions these events may occur and to try to avoid them or minimize their effects. In this sense, the main aim of this thesis is to develop tools that can contribute to a better understanding of the geomechanical processes that take place during fault reactivation events, either when they occur naturally or are induced by any human activity. In the present thesis a methodology is proposed to identify when an instability may be triggered and, also, quantify the amount of energy released during the unstable process. Moreover, the methodology proposed can be applied in both mechanical and hydro-mechanical cases, which implies that instability may be generated by either mechanical actions or fluid injections/extractions. The numerical model is based on the Finite Element Method in which fractures are represented by zero-thickness interface elements equipped with constitutive laws based on Fracture Mechanics principles. In particular, the constitutive model used consists of the reformulation of an existing fracture-based interface constitutive law (Normal/Shear Cracking Model) in terms of visco-plasticity with Hardening/Softening. Two instability control strategies have also been implemented: (1) a new Indirect Displacement Control method based on the visco-plastic dissipation (IDC-Wvp) and (2) an adaptation of the Visco-plastic Relaxation method (VPR). Regarding the first one, it has been found that the method can only be easily applied in purely mechanical cases, and for this reason VPR has finally been the method used to model unstable cases including mechanical and hydro-mechanical.A procedure has been developed for identifying the occurrence of mechanical instabilities such as snap-back or similar sudden events. This procedure is based on the continuous tracking of the various types of energy dissipation occurring along the discontinuity at the level of the constitutive visco-plastic model. It has been found that when a mechanical instability takes place, a jump is observed in the dissipation diagrams, and a difference emerges between two types of visco-plastic dissipation: total VP dissipation and VP dissipation based on projected stresses. This difference turns out to correspond to the Viscous energy dissipated by the interface during the transit through the instability, and therefore it can be associated to the energy released by the unstable event.Finally, the methodology proposed in this thesis was verified through academic examples which represent different fracture mechanisms that can be produced in a geomechanical scenario. Moreover, some more realistic examples are also presented where instability is induced by the effects of fluid pressure or fluid flow.Overall this thesis tries to contribute to develop numerical tools for the assessment of fault reactivation problems, not only to identify the existence of an instability, but also to quantify the energy released in these processes, energy that then may be eventually linked with the earthquake magnitude. |
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