Résultat de la recherche
Aseismic deformations perturb the stress state and trigger induced seismicity during injection experiments
Fluid injections can trigger seismicity even on faults that are not optimally oriented for reactivation, suggesting either sufficiently large fluid pressure or local stress perturbations. Understanding how stress field may be perturbed during fluid injections is crucial in assessing the risk of induced seismicity and the efficiency of deep fluid stimulation projects. Here, we focus on a series of in situ decametric experiments of fluid-induced seismicity, performed at 280 m depth in an underground gallery, while synchronously monitoring the fluid pressure and the activated fractures movements. During the injections, seismicity occurred on existing natural fractures and bedding planes that aremisoriented to slip relative to the background stress state,whichwas determined from the joint inversion of downhole fluid pressure and mechanical displacements measured at the injection.We then compare this background stress with the one estimated from the inversion of earthquake focal mechanisms. We find significant différences in the orientation of the stress tensor components, thus highlighting local perturbations. After discussing the influence of the gallery, the pore pressure variation and the geology, we show that the significant stress perturbations induced by the aseismic deformation (which represents more than 96 per cent of the total deformation) trigger the seismic reactivation of fractures with different orientations.
Mechanics of fault reactivation and its application to stress measurements
Dans ce projet de recherche, nous avons étudié la mécanique de la réactivation des failles due à l’injection de fluide et les contraintes in situ estimées à partir du débit, de la pression et du record de dislocation collecté lors des tests de réactivation des failles. L’estimation des contraintes in situ est un élément essentiel de tout projet souterrain. Cela est particulièrement vrai pour les projets difficiles comme l’exploitation de la géothermie profonde, l’élimination des déchets nucléaires et la capture et le stockage du CO2. Ces projets ont le potentiel de réactiver des failles par des perturbations de contraintes induites par l’injection de fluide ou l’excavation souterraine. L’estimation correcte des contraintes in situ et de l’activation de la mécanique des défauts due à la perturbation des contraintes est importante pour fournir une conception sûre et stable de ces projets. Dans ce travail, nous avons développé un protocole pour estimer la contrainte in situ en utilisant les données du déplacement tridimensionnel (3D) induit par l’injection de fluide. Les données de déplacement sont obtenues à l’aide de la méthode d’injection à débit progressif pour les propriétés de fracture in situ (SIMFIP), qui permet une surveillance couplée débit-pression-déplacement tridimensionnel pendant l’injection de fluide. Le protocole a été appliqué à deux expériences d’activation de failles menées dans deux types différents de roches: des schistes au laboratoire de roches du Mont Terri, en Suisse, et des carbonates au laboratoire souterrain à faible bruit de Rustrel (LSBB). Les deux expériences montrent une réponse complexe couplée débit-pression-déplacement 3D pendant l’injection. Notre protocole d’inversion de contrainte repose sur la combinaison de données d’orientation de glissement de rupture et d’analyses de pression et de débit. Une étape fondamentale de notre travail a été de vérifier la validité de l’hypothèse de Wallace-Bott sous champ de pression inhomogène. Cela nous permet d’utiliser l’orientation de glissement comme un indicateur robuste de la direction de la contrainte de cisaillement résolue sur le plan de fracture testé et ainsi d’utiliser des techniques d’inversion de contrainte pour déduire les contraintes de champ lointain. Une deuxième étape importante a été de développer un repere pour identifier l’orientation du glissement de fracture dans le signal de déplacement complexe enregistré pendant un essai. Ce signal complexe résulte de la superposition de multiples processus couplés hydromécaniques. Nous avons basé notre approche sur les analyses du taux de déplacement lors des étapes d’injection à pression constante. Enfin, nous développons une méthodologie pour intégrer systématiquement nos données et estimer la contrainte en champ lointain. Notre méthode permet d’estimer le tenseur de contrainte complet à partir d’un test d’injection unique ouvrant de nouvelles opportunités pour acquérir des profils de tenseur de contrainte complet le long de forages d’exploration. Aux deux sites où nous avons appliqué notre méthodologie, le Mont Terri et LSBB, nos estimations de stress dérivées sont en bon accord avec la caractérisation indépendante du stress des sites. Nous proposons de développer davantage la méthodologie à l’avenir en améliorant le protocole d’injection et en le testant dans d’autres emplacements et environnements géologiques.
Abstract
In this research project, we studied the mechanics of fault reactivation due to fluid injection and estimated in-situ stress from flow rate, pressure and the dislocation record collected during fault reactivation tests. In-situ stress estimation is an essential part of any underground project. This is particularly true for challenging projects as deep geothermal energy exploitation, nuclear waste disposal and CO2 capture and storage. These projects have the potential to reactivate faults through stress perturbations induced by fluid injection or underground excavation. The correct estimation of the in-situ stresses and fault mechanics activation due to stress perturbation is important to provide safe and stable design of these projects.
In this work, we developed a protocol to estimate the in-situ stress by using the data of three dimensional (3D) displacement induced by fluid injection. The displacement data is obtained using the Step-Rate Injection method for Fracture in- Situ properties (SIMFIP), which allows coupled flowrate-pressure-three dimensional displacement monitoring during the fluid injection. The protocol was applied to two fault activation experiments conducted in two different types of rocks – shales at the Mont Terri rock laboratory, Switzerland, and carbonates at the Rustrel Low Noise Underground Laboratory (LSBB). Both experiments show complex coupled flowrate-pressure-3D displacement response during the injection.
Our stress inversion protocol relies on the combination of fracture slip orientation data and pressure and flow rate analyses. A fundamental step in our work was to verify the validity of the Wallace-Bott hypothesis under inhomogeneous pressure field. This allows us to use the slip orientation as a robust indicator of the resolved shear stress direction on the tested fracture plane and thus to use stress inversion techniques to deduce far-field stresses. A second important step was to develop a rational to identify fracture slip orientation within the complex displacement signal recorded during a test. This complex signal results from the superposition of multiple hydromechanical coupled processes. We based our approach on the analyses of displacement rate during constant pressure injection steps. Finally, we develop a methodology to systematically integrate our data and estimate the far field stress. Our method allows to estimate the complete stress tensor from a single injection test opening future opportunities for acquiring profiles of full stress tensor along exploration boreholes. At the two sites where we applied our methodology, the Mont Terri and LSBB, our derived stress estimates are in good agreement with independent stress characterization of the sites. We propose to further develop the methodology in the future by improving the injection protocol and by testing it in other locations and geological environments.
In Situ Direct Displacement Information on Fault Reactivation During Fluid Injection
The three dimensional (3D) displacement induced by fluid injection was measured during two fault reactivation experiments conducted in carbonate rocks at the Rustrel Low Noise Underground Laboratory (LSBB URL), France, and in shale rocks at the Mont Terri Rock laboratory, Switzerland. The faults were activated by injecting high pressure fluid and using the Step-Rate Injection Method for Fracture In-Situ Properties, which allows a coupled pressure-flowrate-3D displacement monitoring in boreholes. Both experiments mainly show complex aseismic deformation of preexisting fractures that depend on (1) the fluid pressure variations related to chamber pressurization and leakage into the formation and (2) irreversible shear slip and opening of the reactivated fractures. Here we detail the processing of the 3D displacement data from both experiments to isolate slip vectors from the complex displacement signal. Firstly, we explain the test protocol and describe the in situ hydromechanical behavior of the borehole/fault system. Secondly, we define the methodology of the displacement data processing to isolate slip vectors with high displacement rates, which carry information about the key orientation of fault reactivation. Finally, we discuss which slip vectors can potentially be used to solve the stress inversion problem.
Slip perturbation during fault reactivation by a fluid injection
Slip orientation inferred from fault striae or focal mechanism datasets is commonly used in stress inversion methods based on the Wallace-Bott hypothesis. The hypothesis postulates that slip on a fault plane is collinear with the orientation of the resolved shear stress. It is valid for a single planar fault subjected to a homogeneous far-field stress. However, the experimental displacement data from an induced fault reactivation experiment, conducted in the Mont Terri rock laboratory, Switzerland, indicated multiple triggered slip orientations, thereby preventing application of the above inversion method. We present numerical and analytical results of slip on a reactivated fracture with a non-uniform fluid pressure distribution. Using these models, we evaluate the reasons for the inconsistency of our observations and the traditional Wallace-Bott hypothesis and test the physical effects of various parameters on fault slip. In the fully coupled hydromechanical numerical model (three-dimensional distinct element method), fluid pressure at a point on the fault surface is increased stepwise (assumed planar and singular) until shear reactivation of the fault is induced. We studied two different models with high and low fault plane stiffness to represent hard and soft rock masses, respectively. The model shows that high fault stiffness preserves the planarity of the fault plane, while low fault stiffness permits dilation and morphological changes of the fracture related to fluid pressure diffusion. The highest slip perturbation was observed in the low stiffness model due to the change of the fracture shape, controlled by the non-uniform pressure distribution. The Eshelby analytical solution confirmed that the more the fracture is dilated, the more the corresponding resolved shear stress is perturbed. Additionally, when compared to dilation and fault aperture, the friction angle has the most influence on the angular difference between geomechanical slip vectors and resolved shear stress.