Éditeur(s) : HAL CCSD
Résumé : Today, one of the main challenges in the oil industry is the exploitation of new resources in naturally fractured reservoirs often located in structurally complex areas such as plate boundaries, mountain ranges or near salt structures. While numerical models of rock deformation based on continuum mechanics are becoming industry standard in providing efficient means for modelling natural fractures in reservoirs, the knowledge of past perturbed stress fields, which is referred as the type (normal, wrench or reverse), orientation and magnitude of the regional or local tectonic stresses that needs to be applied as boundary condition in numerical simulations, is often unknown or approximated.The essence of this thesis is to improve the recovery of the paleostress field needed to constrain numerical simulations. I first described and discuss through a comprehensive mechanically-based multi-parametric study how far we can rely on the commonly used paleostress inversion methods based on fault slip data and the Wallace and Bott assumptions. I then compared this method with a new generation technique based on geomechanics using field observations. Since slip markers on faults are hardly observed in core or image log, I used observed natural fracture data as main drivers for the inversion of the paleostress using geomechanically-based method. I demonstrated, through various outcrop and subsurface examples, how this can efficiently be done. Finally, I aimed at addressing the problem of polyphase fracture data sets with unknown mechanical type (joints, faults, stylolites ...) and expanded the mechanical stress inversion to the separation of tectonic phases.
Aujourd'hui, l'un des principaux défis dans l'industrie pétrolière est l'exploitation de nouvelles ressources dans les réservoirs naturellement fracturés souvent situés dans des zones structurellement complexes tels que les frontières de plaques, les chaînes de montagnes ou les structures de proches des dômes de sel. Alors que les modèles numériques de déformation des roches basée sur la mécanique des milieux continus fournissant des moyens efficaces pour la modélisation de fractures naturelles dans les réservoirs, sont devenus la norme dans l'industrie, les paléo-contraintes régionales ou locales appliquées comme condition aux limites dans les simulations numériques, définies par le régime (normal, décrochant ou inverse), l'orientation, et la magnitude des contraintes tectoniques, sont souvent inconnues ou approximées.L'essence de cette thèse est d'améliorer la prédiction du champ de paléo-contrainte nécessaire pour contraindre les simulations numériques. J’ai décrit dans un premier temps à travers une étude comparative multi-paramétrique complète basée sur la mécanique, dans quelle mesure nous pouvions compter sur les méthodes d'inversion des paléo-contraintes basées sur les hypothèses de Wallace et Bott et les données de glissement sur les failles. J’ai ensuite comparé ces méthodes avec une technique d’inversion de nouvelle génération basée sur la géomécanique en utilisant les observations de terrain. Puisque les marqueurs de glissement sur les failles sont rarement observés le long du forage, j’ai utilisé l’inversion géomécanique des contraintes pour les données de fractures naturelles observées. J’ai alors démontré, l’efficacité de cette technique à travers divers exemples d'affleurements et de subsurface. Enfin, j’ai proposé une solution afin de résoudre le problème des données de fractures polyphasées à type cinématique inconnu (joints, failles, stylolites ...) et étendu l’inversion de contrainte basée sur la géomécanique à la séparation des phases tectoniques.
https://hal.archives-ouvertes.fr/tel-01324110

tel-01324110
https://hal.archives-ouvertes.fr/tel-01324110
https://hal.archives-ouvertes.fr/tel-01324110/document
https://hal.archives-ouvertes.fr/tel-01324110/file/theselejri.pdf

Éditeur(s) : HAL CCSD
Résumé : Different applications of linear elasticity in structural geology are presented in this thesis through the development of three types of numerical computer codes. The first one uses forward modeling to study displacement and perturbed stress fields around complexly faulted regions. We show that incorporating inequality constraints, such as static Coulomb friction, enables one to explain the angle of initiation of jogs in extensional relays. Adding heterogeneous material properties and optimizations, such as parallelization on multicore architectures and complexity reduction, admits more complex models. The second type deals with inverse modeling, also called parameter estimation. Linear slip inversion on faults with complex geometry, as well as paleo-stress inversion using a geomechanical approach, are developed. The last type of numerical computer code is dedicated to restoration of complexly folded and faulted structures. It is shown that this technique enables one to check balanced cross-sections, and also to retrieve fault chronology. Finally, we show that this code allows one to smooth noisy 3D interpreted faulted and folded horizons using geomechanics.
Différentes applications de l'élasticité linéaire en géologie structurale sont présentées dans cette thèse à travers le développement de trois types de codes numériques. Le premier utilise la modélisation directe pour étudier les déplacements et champs de contraintes autour de zones faillées complexes. On montre que l'ajout de contraintes inégalitaires, telles que la friction de Coulomb, permet d'expliquer l'angle d'initiation des dominos dans les relais extensifs. L'ajout de matériaux hétérogènes et d'optimisations, telles la parallélisation sur processeurs multi-coeurs ainsi que la réduction de complexité des modèles, permettent l'étude de modèles beaucoup plus complexes. Le second type de code numérique utilise la modélisation inverse, aussi appelée estimation de paramètres. L'inversion linéaire de déplacements sur les failles ainsi que la détermination de paléo-contraintes utilisant une approche géomécanique sont développées. Le dernier type de code numérique concerne la restauration de structures complexes plissées et faillées. Il est notamment montré qu'une telle méthode permet de vérifier l'équilibre de coupes géologiques, ainsi que de retrouver la chronologie des failles. Finalement, nous montrons que ce même code permet de lisser des horizons 3D faillés, plissés et bruités en utilisant la géomécanique.
https://tel.archives-ouvertes.fr/tel-00537899

tel-00537899
https://tel.archives-ouvertes.fr/tel-00537899
https://tel.archives-ouvertes.fr/tel-00537899/document
https://tel.archives-ouvertes.fr/tel-00537899/file/thesemaerten.pdf

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Wallace (1951) and Bott (1959) were the first to introduce the idea that the slip on each fault surface has the same direction and sense as the maximum shear stress resolved on that surface. However, this simplified hypothesis is questionable since fault mechanical interactions may induce slip reorientations. Earlier numerical geomechanical models confirmed that the slickenlines (slip vectors) are not necessarily parallel to the maximum resolved shear stress but are consistent with local stress perturbations. This leads us to ask as to what extent the Wallace and Bott simplifications are reliable as a basis hypothesis for stress inversion from fault slip data. Here, a geomechanical multi-parametric study using a 3D boundary element method, covering (i) fault geometries such as intersected faults or corrugated fault surfaces, (ii) the full range of Andersonian state of stress, (iii) fault friction, (iv) fault fluid pressure, (v) half space effect and (vi), rock properties, is performed in order to understand the effect of each parameter on the misfit angle between geomechanical slip vectors and the resolved shear stresses. It is shown that significant misfit angles can be found under specific configurations invalidating the Wallace and Bott assumptions, even though fault friction tends to minimize the misfit. We therefore conclude that in such cases, stress inversions based on fault slip data should be interpreted with care.
ISSN: 0040-1951

hal-01218418
https://hal.archives-ouvertes.fr/hal-01218418
DOI : 10.1016/j.tecto.2015.06.030

Éditeur(s) : HAL CCSD Elsevier
Résumé : We present the hierarchical matrix (H-matrix) technique combined with the adaptive cross-approximation (ACA) applied to a three-dimensional (3D) elastostatic problem using the boundary element method (BEM). This is used in structural geology and geomechanics for the evaluation of the deformation and perturbed stress field associated with surfaces of displacement discontinuity. Such optimization significantly reduces (i) the time and memory needed for the resolution of the system of equations, but more importantly (ii) the time needed for the post-processing at observation points where the deformation and the perturbed stress field are evaluated. Specifically, it is shown that the H-matrix structure used with the ACA, clearly captures the kernel smoothness during the post-processing stage according to the field point positions, and optimizes the computation accordingly. Combined with the parallelization on multi-core processors, this technique allows intensive computations to be done on personal desktop and laptop computers. Numerical simulations are presented, showing the advantages of such optimizations compared to the standard method.
ISSN: 0955-7997

hal-00561032
https://hal.archives-ouvertes.fr/hal-00561032
DOI : 10.1016/j.enganabound.2009.10.016

Éditeur(s) : HAL CCSD Wiley
Résumé : International audience
The paper is motivated by the long term safety analysis of the CO 2 geological storage. We present a methodology for the assessment of the ge-omechanical impact of progressive rock dissolution. The method is based on the use of X-ray tomography and the numerical dissolution technique. The influence of evolution of the microstructure on the macroscopic properties of the rock is analysed by using periodic homogenization method. The numerical computations show progressive degradation of all components of the stiffness (orthotropic) tensor. Moreover, the evolution of associated mass transfer properties (as tortuosity and conductivity tensors), by using the periodic homogenization method, is also calculated. The correlation between the mechanical parameters and the transfer properties during the dissolution pro-cess is presented. The results show that the highest increase of the hydraulic conductivity (in direction Y) is not associated with the highest decrease of Young modulus in this direction. Moreover, the highest decrease of Young modulus (in the direction X) is not associated with percolation in this direction. Finally, an incremental law to calculate settlement, in case of a rock with evolving microstructure, is proposed. The solution of the macroscopic settlement problem under constant stress and drained conditions showed that the geomechanical effects of the rock dissolution are rather limited.
ISSN: 0363-9061

hal-01104204
https://hal.archives-ouvertes.fr/hal-01104204
https://hal.archives-ouvertes.fr/hal-01104204/document
https://hal.archives-ouvertes.fr/hal-01104204/file/Numerical_computations_rock_dissolution_Wojtacki_al_2014.pdf
DOI : 10.1002/nag.2316

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Four decades after their introduction, the validity of fault slip inversion methods based on Wallace (1951) and Bott (1959) hypothesis, which states that the slip on each fault surface has the same direction and sense as the maximum resolved shear stress, is still a subject of debate. According to some authors, this hypothesis is questionable since fault mechanical interactions induce slip reorientations as confirmed by geomechanical models. This leads us to ask as to what extent the Wallace-Bott simplifications are reliable as a basis hypothesis for stress inversion from fault slip data. In this paper, we compare two inversion methods; the first is based on the Wallace-Bott hypothesis, and the second relies on geomechanics and mechanical effects on fault heterogeneous slip distribution. In that context, a multi-parametric stress inversion study covering (i) the friction coefficients (μ), (ii) the full range of Andersonian state of stress and (iii) slip data sampling along the faults is performed. For each tested parameter, the results of the mechanical stress inversion and the Wallace-Bott (WB) based stress inversion for slip are compared in order to understand their respective effects. The predicted discrepancy between the solutions of both stress inversion methods (based on WB and mechanics) will then be used to explain the stress inversions results for the chimney Rock case study. It is shown that a high solution discrepancy is not always correlated with the misfit angle (ω ) and can be found under specific configurations (View the MathML sourceR−, θ, μ, geometry) invalidating the WB solutions. We conclude that in most cases the mechanical stress inversion and the WB based stress inversion are both valid and complementary depending on the fault friction. Some exceptions (i.e. low fault friction, simple fault geometry and pure regimes) that may lead to wrong WB based stress inversion solutions are highlighted.
ISSN: 0040-1951

hal-01467165
https://hal.archives-ouvertes.fr/hal-01467165
DOI : 10.1016/j.tecto.2016.11.039

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Sensitive instruments like strainmeters and tiltmeters are necessary for measuring slowly varying low amplitude Earth deformations. Nonetheless, laser and fibre interferometers are particularly suitable for interrogating such instruments due to their extreme precision and accuracy. In this paper, a practical design of a simple pendulum borehole tiltmeter based on laser fibre interferometric displacement sensors is presented. A prototype instrument has been constructed using welded borosilicate with a pendulum length of 0.85 m resulting in a main resonance frequency of 0.6 Hz. By implementing three coplanar extrinsic fibre Fabry-Perot in-terferometric probes and appropriate signal filtering, our instrument provides tilt measurements that are insensitive to parasitic deformations caused by temperature and pressure variations. This prototype has been installed in an underground facility (Rustrel, France) where results show accurate measurements of Earth strains derived from Earth and ocean tides, local hydro-logic effects, as well as local and remote earthquakes. The large dynamic range and the high sensitivity of this tiltmeter render it an invaluable tool for numerous geophysical applications such as transient fault motion, volcanic strain and reservoir monitoring.
ISSN: 0956-540X

hal-01265967
https://hal.archives-ouvertes.fr/hal-01265967
https://hal.archives-ouvertes.fr/hal-01265967/document
https://hal.archives-ouvertes.fr/hal-01265967/file/HCS1.pdf
DOI : 10.1093/gji/ggv358