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Concept

Séisme induit

Résumé

Un séisme induit est un séisme déclenché directement ou indirectement par des activités humaines. Il se produit quand l'Homme modifie le jeu des forces et contraintes visco-élastiques et poro-élastiques géologiques, par exemple via des modifications géographiques, tridimensionnelles et temporelles de poids, de tensions, de fronts de pression, de points de compression, de déplacement des seuils de rupture mécanique des roches, etc.). Il peut résulter d'une explosion intense (ex : essai nucléaire souterrain, des rééquilibrages microgéologiques faisant suite à des chantiers de grande ampleur (mines profondes à ciel ouvert, canal de Suez), ou à l’extraction, à l’injection ou au déplacement ou à l’accumulation locale de « fluides géologiques » (gaz, pétrole, eau) dans les mécanismes de rupture de la croûte superficielle. Il a généralement des effets très locaux, et le plus souvent imperceptibles pour l'homme. Mais il peut parfois agir sur des objets géologiques à des échelles de dizaines

Source officielle

https://fr.wikipedia.org/wiki/S%C3%A9isme_induit

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Publications associées (41)

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Unités associées (3)

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Concepts associés (4)

La fracturation hydraulique est la dislocation ciblée de formations géologiques peu perméables par le moyen de l'injection sous très haute pression d'un fluide destiné à fissur

An enhanced geothermal system (EGS) generates geothermal electricity without natural convective hydrothermal resources. Traditionally, geothermal power systems operated only where naturally occurrin

vignette|upright=1.5|Carte de la répartition mondiale des séismes en 2010, montrant leur distribution essentiellement le long des frontières des grandes plaques tectoniques (dorsales dans les océans,

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Cours associés (6)

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Séances de cours associées (5)

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Séances de cours associées

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Laboratory Testing on Hydraulic Fracturing of Barre Granite: Studying Visual Observations, Acoustic Emissions and Radiated Energy related to Fracture Mechanisms

Michela Casanova

Hydraulic fracturing is frequently used to increase the permeability of rock formations. This can be done by creating new fractures as usually done for hydrocarbon extraction or extending and opening fractures as usually done in Enhanced Geothermal Systems (EGS). In both cases, only indirect information such as in-situ stress, pumping records and microseisms are recorded. However, understanding the exact processes of hydraulic stimulation remains very challenging as some of the fundamental aspects such as the fracturing processes and induced/triggered seismicity are still unknown. Unique equipment developed by the MIT CEE Rock Mechanics Group allows one to visually observe hydraulic fracturing while recording Acoustic Emissions (AE) simultaneously. Important information is obtained by applying different fluid pressure in individual pre-cut flaws with distinct flow rates and external stresses. This paper focuses on the main processes inducing hydraulic fracturing, and particularly hydro-shearing, by relating the AE data to visual observations, and investigating different experimental concepts in terms of focal mechanisms and normalized radiated seismic energy. In this work we show that (1) hydro-shearing may be induced by applying high external loads i.e. induce global shear failure; (2) local failure around the flaw tips is expected to occur in tensile mode; (3) the AE hypocenter location matches well with the visual crack observations; (4) some differences of focal mechanisms based on AE data and visual observations exist; (5) cyclic pressurization results in high normalized radiated seismic energy.

2020

Chargement

The stress state of the subsurface has been shown to have an influence on a number of key processes. For example, the criticality of the stress state indicates how large stress changes need to be before a fault begins to slip, the mean effective stress controls compaction and permeability loss in hydrocarbon reservoirs, the differential stress has been seen in seismology to have a strong influence on the size distribution of earthquake catalogues, with stress state further influencing the affinity of a fault to slip either seismically or aseismically, and the stress state strongly influences the propagation of a hydraulic fracture, with stress jumps being capable of completely halting a fracture's propagation. For reasons such as these, it has long been recognized that the state of stress is an important parameter for subsurface industrial operations such as hydrocarbon production, Enhanced Geothermal System (EGS) stimulation, carbon storage, and hydraulic fracturing. Generally, however, the state of stress does not remain constant during many of these operations. Decreasing effective stresses have led to induced seismicity during injection, fluid production has led to total stress changes which have induced seismicity, even in areas where pore pressure has decreased, temperature-induced stress changes have led to shear stimulation, and changes in total stress have led to so-called "frac-hits" where hydraulic fractures propagate towards depleted zones. These examples demonstrate an operator's ability to alter the state of stress, yet deliberate attempts to alter the stress state for the benefit of future operations have only been suggested a handful of times. Here, this idea of altering the state of stress is extended through a series of investigations. It is shown that, because production-induced seismicity is caused by total stress changes associated with the gradient of the produced fluid pressure, a hydraulic fracture which reduces the gradient required for production will also reduce the resulting seismicity. Conversely, compaction which leads to permeability loss will result in higher pore pressure gradients and therefore more induced seismicity. Both of these studies, which also have implications for optimal horizontal wellbore orientation, are investigated through the use of a poroelastic reservoir simulator developed during the thesis. In a following study, it is suggested that high stress path reservoirs are attractive targets for fluid injection, as total stress changes will result in an increased stability despite increasing pore pressure. Further, the concept of stress preconditioning is introduced, such that operators alter the stress state of an EGS reservoir to promote more favorable earthquake distributions. These concepts, which make up the first three chapters of the thesis, all are forms of reservoir management that reduce the risk associated with induced seismicity. Following this, in a numerical study, the idea of stress preconditioning is extended to allow for directed stimulation treatments in EGS's. Finally, the idea of inducing stress jumps through production such that hydraulic fractures do not propagate vertically is evaluated by means of scaling analyses and numerical simulation, with implications for carbon storage. More than the individual propositions, this thesis promotes the mindset that, where appropriate, attempts should be made to alter the state of stress for the benefit of future operations.

EPFL2020

Chargement

How does seismic anisotropy evolve as a function of mineralogical and textural changes across ductile shear zones? – an experimental and modelling approach

Henri Joseph François Gilles Leclère

Strain localization and the development of ductile shear zones in the middle and lower crust play major roles in lithosphere dynamics. Geophysical imaging of ductile shear zones is an issue for ore geology, for the understanding of the lithosphere rheology and of the faults behavior. Indeed, shear zones constitute the ductile roots of the faults of the upper crust; studying these structures can thus help to better understand the behavior of the seismogenic faults (e.g. Sibson, 1977). In the absence of significant seismicity, methods using active source illumination cannot be used to image shear zones. However, by the way of metamorphic and metasomatic reactions, ductile strain localization is accompanied by mineralogical, textural and microstructural changes, that can produce seismic anisotropy. The propagation of teleseismic waves from distant earthquakes is sensitive to this anisotropy, and can be used to image the anisotropic crustal structures through which they propagate. Deep crustal seismic anisotropy has thus the potential to fundamentally advance our understanding of lithospheric structure and deformation processes. Intrinsically anisotropic minerals, the development of preferential crystallographic orientations and layered textures, can produce seismic anisotropy within shear zones (e.g. Mainprice and Nicolas, 1989). However, these intrinsic properties of shear zones change from their margins to their mylonitic cores, that may imply changes in seismic anisotropy. Furthermore, developed shear zones acquire a more complex geometry than these "ideal" tabular strain zones. They form anastomosed networks, with a mylonitic matrix delimiting weakly deformed lenses showing more localized deformation, and in which preexisting textures can be eventually preserved. Thus, there are multiple potential contributions for the shear zones "final" anisotropy. Although many geophysical studies aim to better image crustal anisotropic structures, it is also crucial to study outcropping fossil shear zones, in order to identify and estimate the different contributions to the final anisotropy. This study aims to bring a geological contribution to the understanding of geophysical signals. This approach begins with the study of the simplest case of an "ideal" shear zone, to determine how evolves the seismic anisotropy along a strain gradient. The present study is based on natural samples from eclogitic shear zones developed in the Monte Mucrone metagranodiorite (Sesia zone, Western Alps). We consider homogeneously deformed samples, corresponding to various stages of deformation: undeformed protolith (with potential inherited magmatic textures), orthogneiss, mylonite and ultramylonite. Besides, a composite sample contains a cm-scale shear zone, representing the combination of the different contributions estimated with the homogeneous samples. The seismic anisotropy of both homogeneous and composite samples will be (1) experimentally estimated, by P-and S-waves velocity measurements and (2) modelled using the program MTEX (Mainprice et al., 2011), and a microstructure-sensitive finite element orientation averaging using the ESP toolbox (Vel et al., 2016). The so calculated anisotropies are compared with the petrological features of each sample. The objective is to test the hypothesis that shear zones seismic anisotropy may reflect the lower strained margins of the zones (e.g. Almqvist et al., 2013; Tatham et al., 2008; Schulte-Pelkum and Mahan, 2011).

2019

Chargement

EPFL2020

How does seismic anisotropy evolve as a function of mineralogical and textural changes across ductile shear zones? – an experimental and modelling approach

Henri Joseph François Gilles Leclère

2019