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Hydrotectonics and fault criticality analysis in karstic regions from microseismic and hydrogeological observations

2024, Perrochet, Léa, Valley, Benoît

Cette étude vise à évaluer la criticalité des failles dans les régions karstiques en combinant l’analyse de données hydrogéologiques et microsismiques, à améliorer la compréhension des interactions entre l’activité sismique et la circulation des eaux souterraines et à utiliser ces connaissances pour estimer l’état de contrainte actuel de la croûte supérieure. Ces objectifs sont motivés par les préoccupations actuelles concernant la sismicité induite et les risques qui y sont associés dans des projets tels que l’exploitation de l’énergie géothermique profonde, la séquestration de CO2 ou encore l’enfouissement des déchets radioactifs. Dans le contexte de la sismicité induite, l’eau joue un rôle majeur en influençant les contraintes effectives, et donc la stabilité des failles. Si la sismicité induite par l’Homme est souvent observée dans les cas d’injection d’eaux usées ou de stimulation de réservoirs, la sismicité induite naturellement peut également être observée à la suite d’intense période de recharge, en particulier en zone karstique où l’effet de canalisation des conduits karstiques a une influence majeure sur la pression interstitielle. Ainsi, en combinant des données hydrogéologiques aux données de sismicité naturelle, il est possible d’obtenir des informations sur l’interaction de la circulation des fluides et la stabilité des failles. Pour atteindre ces objectifs, la phase initiale du projet consiste à identifier et à quantifier les mécanismes susceptibles de déclencher un séisme à la suite d’une recharge importante (événement pluvieux ou fonte des neiges) dans les régions karstiques. Les mécanismes identifiés sont 1) une augmentation de la charge verticale due à l’eau supplémentaire dans les conduits karstiques, 2) une augmentation de la pression interstitielle suite à une déformation poro-elastic résultant de la charge supplémentaire, 3) une augmentation massive de la pression interstitielle résultant de la connexion hydrogéologique directe entre l’eau infiltrée et la profondeur focale et 4) un processus de diffusion de pression. Ces mécanismes, qu’ils agissent seuls ou ensemble, influencent la pression interstitielle, mais le moment auquel ils agissent varie. La réponse de la pression suite à une augmentation de la charge hydraulique est instantanée pour les mécanismes 1 à 3, mais retardée pour le mécanisme 4. L’utilisation de données provenant de 3 études de cas ainsi que des solutions analytiques 1D permettant de calculer la variation de la pression interstitielle suite à une augmentation massive de la charge hydraulique permet d’identifier quel processus est le plus susceptible de générer un séisme à la suite de fortes précipitations. En parallèle à cette première phase du projet, un réseau de surveillance sismique et hydrogéologique est déployé dans le Jura Neuchâtelois afin de créer un catalogue sismique et de collecter les débits des principales sources karstiques de la région, utilisés comme indicateurs des variations du niveau des eaux souterraines. Ces deux ensembles de données sont ensuite combinés et un lien statistiquement significatif est trouvé entre des périodes de hautes eaux souterraines et l’activité sismique. Une analyse détaillée de l’activité sismique des différentes zones de faille étudiées ainsi que des événements induits par la pluie qui leur sont associés est réalisée. En utilisant les solutions analytiques mentionnées ci-dessus, les variations de pressions interstitielles sont calculées pour chaque zone de faille au moment de la rupture et une valeur seuil au-delà de laquelle un séisme est envisageable, est définie pour chaque faille. Bien que la majorité des failles étudiées montre une certaine activité à la suite d’intenses précipitations, les tremblements de terre ne se produisent pas de manière régulière. L’identification des évènements induits par la pluie, le seuil de détection des tremblements de terre ou encore les propriétés hydromécaniques des failles peuvent expliquer ce manque de cohérence. En intégrant ces connaissances au contexte géologique et hydrogéologique de la région, un modèle conceptuel hydromécanique est établi. Dans la dernière partie de ce travail, l’état des contraintes actuelles de la croûte superficielle est estimé à l’aide de deux méthodes distinctes et une gamme de valeurs et d’orientations, dont les résultats sont en accord avec des études antérieurs, sont présentées. En conclusion, ce travail souligne l’importance de prendre en compte le contexte hydrogéologique d’une région lors de discussion sur la sismicité induite par la pluie, car l’effet de la canalisation des conduits karstiques augmente considérablement la pression interstitielle en profondeur, et a par conséquent une influence majeure sur la valeur seuil identifiée. Cette étude contribue également à une meilleure compréhension de l’activité sismique aux abords du lac de Neuchâtel et de la manière dont le contexte géologique et hydrogéologique l’influence. ABSTRACT This study aims at assessing fault criticality in karstic regions through the analysis of hydrogeological and microseismic data, at enhancing the understanding of interactions between seismic activity and groundwater circulation and using this insight to estimate the present-day stress state of the shallow crust. These objectives are prompted by the present concerns regarding induced seismicity and its associated risks in projects involving for example deep geothermal energy exploitation, CO2 sequestration or radioactive waste disposal. In the context of induced seismicity, fluids play a major role by influencing the effective stresses and thus fault stability. While human-induced seismicity is often observed in cases of waste water injection or reservoir stimulation, naturally triggered seismicity following seasonal recharge can also be observed, especially in karstic areas where the channeling effect of karst conduits has a major influence on pore pressure. Thus, combining natural seismicity and hydrogeological data can inform on the interaction between fluid flow and fault rupture. To fulfill these goals, the initial phase involves the identification and the quantification of what mechanisms may trigger seismicity following an important recharge period (heavy precipitation or snow melt) in karstic regions. The identified mechanisms are 1) an increasing vertical load due to the additional water within the karst conduits, 2) poro-elastic deformation resulting from the additional load, 3) a massive pore pressure increase resulting from a direct hydrogeological connection between the infiltrated water and focal depth and 4) pressure diffusion process. These mechanisms, whether they act alone or together, influence the pore pressure, however the timing of their effectiveness varies. The response of the pore pressure to an increasing hydraulic head is instantaneous for mechanism 1-3 but delayed for 4. Using data from 3 test sites and 1D analytical solutions to calculate pore pressure response to an increasing hydraulic head allows to determine, which process is more likely to trigger seismicity following heavy rainfalls. In parallel to this initial phase, a seismic and hydrogeological monitoring network is deployed in the Neuchˆatel Jura to create a seismic catalog and collect continuous flow rates from major karstic springs, which is used as proxy for groundwater level variations. These two data sets are then combined and a statistically significant link is found between high groundwater conditions and seismic activity. A detailed analysis of the seismic activity of the analyzed fault zones and their associated rain-triggered events, coupled with the afore-mentioned 1D analytical solutions is then performed. For each fault zone a triggering pore pressure increase is defined and used to determine the sensitivity of the faults to hydraulic head variations. Although the majority of the investigated faults show some activity following important precipitations, events do not occur on a regular basis. The identification of rain-triggered events, the detection threshold of seismic events as well as hydromechanical properties of the faults may explain this lack of consistency. Integrating this knowledge, together with the overall seismic activity of the region and the geological and hydrogeological context, a conceptual hydromechanical model for the study area is presented. Finally, the state of stress of the shallow crust is estimated using two distinct methods and a range of values and orientations is presented. As a conclusion, this study mainly stresses the importance of considering the hydrogeological context when discussing rain-triggered seismicity, as the channeling effect of karst conduits drastically increases the pore pressure at depth, and consequently has a major influence on the magnitude of the triggering pore pressure. This study also contributes to a higher understanding of the seismic activity near Lake Neuchˆatel and how it is influenced by the geological and hydrogeological context.

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A systematic methodology to calibrate wellbore failure models, estimate the in-situ stress tensor and evaluate wellbore cross-sectional geometry

2022-1-1, Dahrabou, Asmae, Valley, Benoît, Meier, Peter, Brunner, Philip, Alcolea, Andres

Deep geothermal boreholes, often drilled to the crystalline basement, suffer from borehole breakouts that compromise borehole stability and/or lead to low drilling performance. These issues increase the cost of deep geothermal projects and lead to irregular cross-sectional geometries that may entangle well completion (e.g., packer isolation for zonal stimulation, cementing, etc.). Thus, the proper knowledge of rock strength, state of stress and their interactions at the closest vicinity of the borehole is key to the success of deep geothermal drilling. Typically, the magnitudes of the vertical and minimum horizontal principal stresses, 𝑆𝑣 and 𝑆ℎ𝑚𝑖𝑛, respectively, can be estimated while 𝑆𝐻𝑚𝑎𝑥 is difficult to constrain. This paper presents a systematic methodology to jointly evaluate the heterogeneous distributions of the stress tensor principal components and orientations, and the rock strength properties (e.g. cohesion, friction). Model parameters are estimated from measurements available during or shortly after drilling, i.e., breakout width, breakout extent/depth of penetration, breakout orientation and drilling induced tensile fractures. Additionally, measurements of estimated parameters or transformations of them can be considered in the calibration in a generic manner (e.g., 𝑆ℎ𝑚𝑖𝑛 interpreted from XLOT). For illustration purposes, the methodology is applied to the extensive borehole data set along the crystalline section of the borehole BS-1, in Basel (Switzerland). The methodology allows us (1) to derive plausible sets of stress and strength parameters reproducing the complex distribution of breakouts along BS-1, and (2) to unveil the paradox of having no borehole breakouts at sections with high density of natural fractures.

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Characterization, Hydraulic Stimulation, and Fluid Circulation Experiments in the Bedretto Underground Laboratory for Geosciences and Geoenergies

2021-6-18, Hertrich, Marian, Brixel, Bernard, Broeker, Kai, Driesner, Thomas, Gholizadeh, Nima, Giardini, Domenico, Jordan, D., Krietsch, Hannes, Loew, Simon, Ma, Xiadong, Maurer, Hansruedi, Nejati, M., Plenkers, K., Rast, M., Saar, Martin O., Shakas, A., van Limborgh, R., Villiger, Linus, Wenning, Q. C., Ciardo, F., Kaestli, P., Obermann, A., Rinaldi, P., Wiemer, Stefan, Zappone, Alba, Bethmann, Falco, Christe, Fabien, Castilla, Raymi, Dyer, Ben, Karvounis, Dimitrios, Meier, Peter, Serbeto, Francisco, Amann, Florian, Gischig, Valentin, Valley, Benoît

Reservoir stimulation and hydraulic fracturing in oil-and-gas reservoirs has become common practice and the techniques are continuously improved. However, directly applying the same techniques to extract geothermal energy from low permeability crystalline rocks (i.e., Enhanced Geothermal Systems, EGS) continues to present operational challenges. The research community and industry have shown great interest in addressing the unresolved problems using down-scaled in-situ hydraulic stimulation experiments. Focus has been on the 1–10 m field scale, but in comparison to a realistic EGS operations (1000s m) the scale is two orders too small, the depth and associate stress field differ, and the hydraulic conditions are not perfectly representative. To study the processes in-situ and to bridge the scale between in-situ labs and actual EGS projects, the Bedretto Underground Laboratory for Geosciences and Geoenergies (BULGG) was built in a tunnel in the Swiss Alps so that hydraulic stimulation experiments could be performed with dense monitoring systems at the 100 m scale. This effort enables process-oriented research and testing of field scale techniques at conditions that are closer to target reservoir depths and scale. This study gives in-sight on the initial geologic, hydraulic, and stress characterization of the BULGG related to on-going stimulation and circulation experiments

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Play-Fairway Analysis for Deep Geothermal Resources in Switzerland

2021-4-5, Valley, Benoît, Miller, Stephen Andrew

Switzerland initiated an energy transition plan for a massive development of renewable energy sources in response to climate change challenges and the decision to shut down nuclear power plants. Geothermal energy represents one component of this transition, with energy scenarios planning for more than 5% of the Swiss electricity demand produced from geothermal energy by 2050. Geothermal energy potentially provides base-load electricity supply, while also contributing to direct and indirect heat supply for replacing fossil fuels, and thus reducing greenhouse gas emissions. In response to this initiative, the Swiss Geological Survey (swisstopo) compiles information of the subsurface relevant for deep geothermal energy, including well data, seismic data interpretation of major stratigraphic horizons and faults, heat flux maps, thermal models of the underground, and geothermal potential studies. Access to this database provides opportunities for reviewing the geothermal potential of Switzerland using a quantitative play-fairway approach. In this contribution, we first review the available data sets and propose conceptual classifications of geological and structural settings favorable for deep-seated fluid circulation in Switzerland. We use the available data to determine best-estimate stress models, which are then used to compute slip and dilation tendency on the main faults identified in the database. We also combine all available information to provide quantitative mapping of the fairway score (favorability maps) for geothermal exploration. Model resolution does not yet capture local effects relevant for specific project development, but does identify general trends at the scale of Switzerland. Currently, this approach provides best estimate models for the currently available data, and will be refined and better-resolved with the acquisition and implementation of future data.

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Transient inverse analyses of overcoring data for improved stress estimation

2022-6-26, Valley, Benoît

Overcoring is a common technique for measuring stresses in mining projects. Knowledge of the in-situ stress state is essential to ensure the stability of underground infrastructures as well as to assess the induced microseismic risk associated with deep mining operations. There are different types of overcoring probes. Some are bonded in the pilot hole with an epoxy resin and allow for 3D stress measurement (e.g. CSIRO-HI), and others are based solely on a mechanical coupling of the probe, but are limited to biaxial stress measurement (e.g. USBM). The need to glue the probe to obtain a 3D measurement limits the applicability of this technique to short boreholes because it is technically difficult to glue probes in deep boreholes. In any case, traditionally the data analysis is done only based on the final deformation obtained after overcoring. In this paper we propose to use the transient deformation response during overcoring to: (1) allow to evaluate the 3D stress field from a single biaxial overcoring measurement, and (2) add a quality control component by reproducing the entire overcoring response. The general principle of our approach is to simulate the transient response of overcoring by numerical elastic simulation. The principle of superposition is used to derive the responses for any set of parameters from a limited number of basic models and thus allows to limit the total number of model runs. Such approach allows for a systematic inversion procedure to be applied for determining the optimal parameter sets that best capture the transient response of the overcoring. This allows the estimation of a 3D stress tensor from biaxial measurements. It also allows to have a quality control on the measurements evaluating the quality of the fit between the model and the data. In this paper, we evaluate the robustness of the proposed approach by performing a systematic sensitivity analysis on the stress estimation from the inversion of transient overcoring data. We demonstrate the advantages of the approach but also its limitations. The preliminary results obtained in this study suggest that the transient overcoring response contains sufficient information for constraining efficiently the 3D stress tensor. The inversion must be performed using multiple starting point, and the mode of the obtained calibrated parameters is in close adequacy with expected values, while some outliers can be present in the calibrated set. Further work is required for confirming these encouraging results by testing of broader range of stress configurations and applying the method to actual field measurements.

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Aseismic deformations perturb the stress state and trigger induced seismicity during injection experiments

2021-10, Duboeuf, L., De Barros, L., Kakurina, Maria, Guglielmi, Y., Cappa, F., Valley, Benoît

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.

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Depth-Dependent Scaling of Fracture Patterns Inferred from Borehole Images in GPK3 and GPK4 Wells at Soultz-sous-Forêts Geothermal Site

2021-4-19, Moein, M. J. A., Bär, Kristian, Valley, Benoît, Genter, Albert, Sass, Ingo

Engineering an Enhanced Geothermal System (EGS) requires a proper understanding of the fracture network properties from small to large scales in order to create a reliable geological model for reservoir simulations. As deterministic identification of all fractures in a reservoir is practically impossible, stochastic approaches known as Discrete Fracture Networks (DFN) are used. This consists of parametrizing a statistical realization of fracture networks constrained by direct observations from borehole images and/or outcrop data, if available. DFN models can be used to study the thermo-hydro-mechanical (THM) properties of fractured rocks and to simulate the processes associated within: I) fluid circulation, II) flow and heat production as well as III) seismic response to hydraulic stimulations. Fractal DFNs are based on multiscale fracture network characteristics and are constrained by the scaling properties of fracture network attributes such as length (or size) and spatial distribution. The dual power-law model is a mathematical representation of fractures that parametrize fractal DFNs with two scaling exponents: 1) scaling of spatial distribution using two-point correlation dimension of fracture centers in three dimensions and 2) power-law exponent of fracture length distribution. Direct measurements of fracture length exponents from borehole images or cores are an unresolved challenge and the resolution of geophysical investigations is not sufficient to image the natural fracture networks. In contrast, the spatial distribution of fractures may be precisely characterized using borehole image logs and cores. Currently, the depth-dependence of spatial clustering of fracture patterns in the earth’s crust is not fully understood, although it may be required to anticipate deep reservoir conditions from shallower datasets. Here, we study such a depth dependency by using the two-point correlation dimension of fractures along the boreholes as a reliable estimate of the fractal dimension. We investigate the data stemming from two deep boreholes, GPK3 and GPK4, drilled into the crystalline basement rocks at the Soultz-sous-Forêts geothermal site. Recent analyses unraveled no systematic variation of fractal dimension with depth in any of the boreholes at the one standard deviation level of uncertainty. This conclusion may support the hypothesis of generating fracture network models with only a single correlation dimension using the stereological relationships in reservoirs up to 5 km depth in crystalline basements.

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Fractal characteristics of fractures in crystalline basement rocks: Insights from depth-dependent correlation analyses to 5 km depth

2022-5-27, Afshari Moein, Mohammad Javad, Evans, Keith F., Valley, Benoît, Bär, Kristian, Genter, Albert

The scaling laws describing the spatial arrangement of fractures along six deep boreholes penetrating the crystalline rocks in the Rhine Graben were derived using a correlation analysis. Five of the wells, two to 5 km depth, were located at the Soultz geothermal site and one well to 5 km depth was located at Basel, some 150 km from Soultz. Five datasets were derived from borehole imaging logs, whilst one stemmed from the analysis of 810 m of continuous core at Soultz. The two differed inasmuch as the core dataset included essentially all fractures, whereas the image log dataset had few fractures narrower than 1–3 mm. The results of the analysis for all image datasets showed that the spatial arrangement of fractures followed fractal behavior at all scales from meters to several hundred meters, the largest scale amenable to assessment, and that the fractal dimensions were confined to the narrow range 0.85–0.9. However, the core dataset showed significant deviation from fractal behavior, the best-fit fractal dimension of 0.8 being somewhat lower than values obtained from imaging logs in neighboring wells. Eliminating fractures with apertures less than 1 mm from the core dataset to improve comparability led to even lower fractal dimension estimates, indicating the discrepancy was not due to imaging log resolution. Analysis of successive depth sections of the core log suggested the discrepancy was due to the presence of a localized zone between 1750 and 2070 m where the fractal organization is disturbed or takes a lower dimension than elsewhere. Aside from this zone, no systematic variation of fractal dimension with depth was observed in any dataset, implying that a single exponent together with intensity adequately describes the arrangement of fractures along the entire length of the boreholes. The results are relevant to the parameterization of DFN models of deep rock masses.

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Evaluation of the Diametrical Core Deformation and Discing Analyses for In‑Situ Stress Estimation and Application to the 4.9 km Deep Rock Core from the Basel Geothermal Borehole, Switzerland

2021-9-14, Ziegler, Martin, Valley, Benoît

The in situ state of rock mass stresses is a key design parameter, e.g., for deep engineered geothermal systems. However, knowledge of the stress state at great depths is sparse mostly because of the lack of possible in situ tests in deep boreholes. Among different options, core-based in situ stress estimation may provide valuable stress information though core-based techniques have not yet become a standard. In this study we focus on the Diametrical Core Deformation Analysis (DCDA) technique using monzogranitic to monzonitic rock drill cores from 4.9 km depth of the Basel-1 borehole in Switzerland. With DCDA the maximum and minimum horizontal stress (SHmax and Shmin) directions, and the horizontal differential stress magnitudes (∆S) can be estimated from rock cores extracted from vertical boreholes. Our study has three goals: first, to assess photogrammetric core scanning to conduct DCDA; second, to compare DCDA results with borehole breakout and stress-induced core discing fracture (CDF) data sets; and third, to investigate the impact of rock elastic anisotropy on ∆S. Our study reveals that photogrammetric scanning can be used to extract reliable core diametrical data and CDF traces. Locally aligned core pieces showed similar SHmax orientations, conform to borehole breakout results. However, the variability of core diametrical differences was large for the Basel-1 core pieces, which leads to a large spread of ∆S. Finally, we demonstrate that core elastic anisotropy must be considered, requiring robust estimates of rock elastic moduli, to receive valuable stress information from DCDA analyses.

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Observation of a Repeated Step-wise Fracture Growth During Hydraulic Fracturing Experiment at the Grimsel Test Site

2021-4-19, Dutler, Nathan, Valley, Benoît, Villiger, Linus, Gischig, Valentin, Amann, Florian

Hydraulic fracturing (HF) experiments were conducted at the Grimsel Test Site (GTS), Switzerland, with the aim to improve our understanding of the seismo-hydro-mechanical processes associated with high-pressure fluid injection in a moderately fractured crystalline rock mass. Observations from one of these HF experiments indicate simultaneous propagation of multiple fractures during continuous fluid injection. The pressure measured in one observation interval show a cyclic response indicating repeated step-wise fracture growth. This is interpreted as a stick-split mechanism propagating fractures in an episodic manner and connecting them to the natural fracture network. In addition, transient partial closure and opening of fractures on the time-scale of seconds to minutes were observed from pressure and deformation monitoring. Our data set provides unprecedented insight in the complexity of hydraulic fracture propagation.