Options
Valley, Benoît
Nom
Valley, Benoît
Affiliation principale
Fonction
Professeur ordinaire
Email
benoit.valley@unine.ch
Identifiants
Résultat de la recherche
Voici les éléments 1 - 10 sur 76
- PublicationAccès libreTransient inverse analyses of overcoring data for improved stress estimationOvercoring 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.
- PublicationAccès libreCharacterization, Hydraulic Stimulation, and Fluid Circulation Experiments in the Bedretto Underground Laboratory for Geosciences and Geoenergies(: ARMA, 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, ValentinReservoir 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 - PublicationAccès libreObservation of a Repeated Step-wise Fracture Growth During Hydraulic Fracturing Experiment at the Grimsel Test Site(2021-4-19)
; ; ;Villiger, Linus ;Gischig, ValentinAmann, FlorianHydraulic 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. - PublicationAccès libreDepth-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; ;Genter, AlbertSass, IngoEngineering 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. - PublicationAccès librePlay-Fairway Analysis for Deep Geothermal Resources in Switzerland(2021-4-5)
; 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. - PublicationAccès libreCapturing non-linear stress-strain response of brittle rocks due to closure of coring-induced micro-cracks using 3D bonded block modelThe stress-strain curves of brittle rocks can be divided into five regions: 1. crack closure, 2. elastic region, 3. crack initiation, 4. crack damage, and 5. peak and post-peak region. The initial non-linear section of the stress-strain curve is known to be due to the closure of pre-existing micro-cracks. This non-linear section may or may not be present depending on the density and geometry of pre-existing micro-cracks. It is known that some of these micro-cracks may form due to the stress redistribution and tensile stresses generated inside the cores during drilling from deep and high stress grounds. The presence of such micro-cracks may affect the properties of rock specimens determined from laboratory tests. Therefore, the knowledge of the level of core damage (micro-crack density) and associated changes in the laboratory properties of brittle rocks is of paramount importance for reliable designs of deep underground excavations. In this paper, the discontinuum numerical program 3DEC and its Bonded Block Model (BBM) is used to explicitly simulate drilling-induced core damage. The laboratory test data from the well documented case of the AECL's Underground Research Laboratory (URL) is used for numerical simulation and model calibration. The numerical simulations involve: 1) calibrating a 3D BBM to the properties of undamaged Lac du Bonnet (LdB) granite under an unconfined condition, 2) simulating core drilling and associated micro-cracks in the cored specimen (i.e., BBM), and 3) uniaxially loading the damaged BBM and comparing its mechanical properties with those of damaged LdB granite. It is found that the initial region of the stress-strain curve of the damaged BBM is non-linear. This is interpreted to be due to the closure of micro-cracks generated during core drilling simulation. The results of numerical study presented in this paper demonstrate the capability of the proposed modeling approach for a realistic simulation of drilling-induced core damage and associated non-linear stress-strain response of brittle rocks
- PublicationAccès libreTemporal borehole breakout evolution and its impact on stress estimation(: American Rock Mechanics Association, 2018-6-20)
; ;Azzola, J. ;Schmittbuhl, J.Genter, A.The estimation of the in-situ stress state is required for the design and execution of deep engineering operations related to Enhanced Geothermal System (EGS). Borehole failures, often referred as borehole breakouts, which are controlled by local stress concentration around the wellbore, are recognized being a useful indicator to assess in-situ stress conditions. However, breakouts evolve with time and this may affect our ability to use them for quantifying the stress state. We use a unique data set from the deep geothermal well of Rittershoffen GRT-1 in order to verify the hypothesis concerning wellbore breakout geometrical evolution. In GRT-1 wellbore, imaging has been acquired 4 days, 348 days and 946 days after drilling completion. Thermal, hydraulic and chemical stimulations have been performed between the first and the second image acquisition. Using this data set, we were able to describe in-situ the breakout evolution with time. We show increase in the extension of breakouts along the well. Contrary to the common assumptions, we also show that breakout widen, but within the limit of the accuracy of our analysis they do not deepen. The consequences of the breakout evolution for stress characterization are significant and add up to other important uncertainties in such analyses like the estimation of strength parameters. - PublicationAccès libreA comparison of FBG- and Brillouin-strain sensing in the framework of a decameter-scale hydraulic stimulation experiment(: American Rock Mechanics Association, 2018-6-18)
;Krietsch, H. ;Gischig, V. ;Jalali, R. ;Doetsch, J.; Amann, F.In the framework of the In-situ Stimulation and Circulation (ISC) experiment Fiber-Bragg-Grating (FBG) and Brillouin strain sensing systems were installed to monitor deformation during six hydraulic shearing and six hydraulic fracturing experiments. Three boreholes were dedicated to strain monitoring. Both systems are installed in the same boreholes, offering a unique opportunity to compare these systems with respect to their applicability in hydraulic stimulation tests. A total of 60 FBG sensors with 1 m base length were installed across fractures, shear zones and intact rock. Along the entire borehole length, pre-stressed optical cables for Brillouin distributed strain (DBS) sensing were embedded in grout with two installation methods: a bare cable and a cable packed and fixed with glue every 0.65 m. The strain signals were compared as time series for a given borehole depth and as profiles along the borehole axis. The study reveals that the FBG system gives a high accuracy (0.04 µ-strain) and temporal resolution (>1s) with pointwise measurements. The bare DBS leg yield good quantitative strain data with poorer strain accuracy (>500 times poorer than FBG) and poorer temporal resolution (factor of >100). The packed DBS leg provide no meaningful information about the strain field. - PublicationAccès libreObservations of fracture propagation during decameter-scale hydraulic fracturing experiments(: American Rock Mechanics Association, 2018-6-17)
; ; ;Gischig, V. ;Jalali, M. ;Doetsch, J. ;Krietsch, H. ;Villiger, L.Amann, F.Various in-situ hydraulic fracturing experiments were carried out in the naturally fractured, crystalline rock mass of the Grimsel Test Site (GTS) in Switzerland. The purpose was to study the geometry of the newly created fractures and their interaction with the pre-existing fracture network using transient pressure and rock mass deformation observations. Under controlled conditions, six hydraulic fractures with similar injection protocols were executed in two sub-vertical injection boreholes. The rock mass is intersected by two E-W striking shear zones (S3), and two biotite-rich meta-basic dykes with a densely fractured zone in between. The S3 shear-zone intersecting the rock volume of interest acts as a high-permeability connection to the tunnel for the experiments executed south of it. Strong variation in injectivity enhancement, jacking pressure, break down pressure, instantaneous shut-in pressure and fluid flow recovery among the different injection intervals indicate different stress conditions north and south of S3. - PublicationAccès libreRelation between Induced Microseismicity and Fracture Network in the Basel Geothermal Site(: European Association of Geoscientists & Engineers, 2018-6-11)
;Afshari, M.J.; ;Tormann, T.Wiemer, S.Permeability creation during hydraulic stimulation of Enhanced Geothermal System (EGS) reservoirs is accompanied in part with induced microseismicity. A satisfactory reservoir characterization is required to mitigate the seismic risk and evaluate different development scenarios with valid hazard assessment. Creating a representative three dimensional structural model in the early stages of reservoir creation is difficult because of insufficient information on the geometry of fracture network from deep boreholes data. In this analysis, we characterized the spatial patterns of induced microseismicity in the Basel geothermal system and analyzed the rupture radius distribution. We generated and analysed synthetic data to help us with the interpretation of the correlation function of spatial patterns. Assuming a constant stress drop on every seismic event, the rupture radius distribution showed a power-law distribution. The correlation function of induced patterns showed a slope of 2 indicating the existence of a fractured zone. In addition, the existence of repeating events explains the observed drop on the local slope of the corresponding correlation function.