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- PublicationAccès libreCapturing non-linear stress-strain response of brittle rocks due to closure of coring-induced micro-cracks using 3D bonded block model(: ARMA, 2020-6)
;Bahrani, N.The 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 libreObservation of a Repeated Step-wise Fracture Growth During Hydraulic Fracturing Experiment at the Grimsel Test SiteHydraulic 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 libreHydraulic fracturing operations in mining: conceptual approach and DFN modeling example(2015-12-20)
;Katsaga, T ;Riahi, A ;DeGagne, DO ;Damjanac, B
- PublicationAccès libreInfluence of confinement dependent failure processes on rock mass strength at depth(2011-10-16)
; ;Kim, B. ;Suorineni, F. ;Bahrani, N. ;Bewick, R.P.Kaiser, P.K.Changes of failure mechanism with increasing confinement, from tensile to shear dominated failure, is widely observed in the rupture of samples in laboratory and in rock masses in situ. However, common failure criteria typically consider only shear mechanisms. A hybrid criteria based on a sigmoid function is introduced to account for a transition from tensile to shear dominated failure with increasing confinement. When evaluated by fitting to an extensive laboratory database the sigmoid criteria does not provide a better fit compared to the Hoek-Brown failure envelope, but provides insight into rock strength controlling factors that have significant consequences with respect to the interpretation of laboratory test results. It also leads to a differentiated approach for design by considering two types of behaviour process: 1) in the inner shell, i.e. the direct vicinity of openings, the failure mode is dominated by tensile cracking leading to spalling and related geometric dilation processes and 2) in the outer shell, i.e. remote from excavations, where confinement promotes interlock, we suggest that rock masses could be significantly stronger than predicted by standard approaches.
- 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 libreStress Measurements for an In Situ Stimulation Experiment in Crystalline Rock: Integration of Induced Seismicity, Stress Relief and Hydraulic MethodsAn extensive campaign to characterize rock stresses on the decameter scale was carried out in three 18–24 m long boreholes drilled from a tunnel in foliated granite at the Grimsel Test Site, Switzerland. The survey combined stress relief methods with hydrofracturing (HF) tests and concomitant monitoring of induced seismicity. Hydrofracture traces at the borehole wall were visualized with impression packer tests. The microseismic clouds indicate sub-vertical south-dipping HFs. Initial inversion of the overcoring strains with an isotropic rock model yielded stress tensors that disagreed with the HF and microseismic results. The discrepancy was eliminated using a transversely isotropic rock model, parametrized by a novel method that used numerical modelling of the in situ biaxial cell data to determine the requisite five independent elastic parameters. The results show that stress is reasonably uniform in the rock volume that lies to the south of a shear zone that cuts the NNW of the study volume. Stress in this volume is considered to be unperturbed by structures, and has principal stress magnitudes of 13.1–14.4 MPa for σ1, 9.2–10.2 MPa for σ2, and 8.6–9.7 MPa for σ3 with σ1 plunging to the east at 30–40°. To the NNW of the uniform stress regime, the minimum principal stress declines and the principal axes rotate as the shear zone is approached. The stress perturbation is clearly associated with the shear zone, and may reflect the presence of more fragmented rock acting as a compliant inclusion, or remnant stresses arising from slip on the shear zone in the past.
- PublicationAccès libreDistinct element method simulation of an analogue for a highly interlocked, non-persistently jointed rockmass(2014)
;Bahrani, N ;Kaiser, P.KA grain-based distinct element model is used to reproduce the laboratory response of both intact and granulated Wombeyan marble. The term “granulated” refers to a heat treated marble where the cohesion of grain boundaries has been destroyed. The unconfined compressive strength granulated marble is less than 50% of that of intact marble, while the strength of the granulated marble increases to about 80% of that of the intact marble at higher confining stresses. An iterative calibration approach is developed to match the unconfined and confined strengths of the models to those of intact and granulated marble. The simulation test results of the models of intact and granulated marble including the transition in the failure mode, stress-strain response, and the evolution of inter- and intra-grain micro-cracks with increasing confinement are discussed. The observed rapid strengthening effect, in terms of increasing confinement, is interpreted to be due to the high degree of grain assembly geometric interlock, which arises from the tight fit geometric shape of the grains as well as the roughness of the grain boundaries. It is suggested that the granulated marble can be considered to be an analogue for a highly interlocked, non-persistently jointed rockmass. It is shown that when the generalized Hoek–Brown failure criterion and the Geological Strength Index (GSI) are used to match the strength of the granulated marble at zero confinement, the confined strength of the granulated marble is underestimated by as much as a factor of two. Therefore, the confined strength of a highly interlocked, non-persistently jointed rockmass, with strong, brittle rock blocks, could be significantly higher than that predicted by commonly adopted empirical approaches. This has practical implications for the design of highly confined pillars and abutments, which is discussed in this paper.
- PublicationAccès libreFault anatomy of the La Sarraz strike-slip fault system
- PublicationAccès libreFractal 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. ; ;Bär, KristianGenter, AlbertThe 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.
- PublicationAccès libreOn the link between stress field and small-scale hydraulic fracture growth in anisotropic rock derived from microseismicity(2018-7-1)
;Gischig, Valentin ;Doetsch, J. ;Maurer, Hansruedi ;Krietsch, Hannes ;Amann, Florian ;Evans, Keith F. ;Nejati, M. ;Jalali, Mohammadreza ; ;Obermann, A. ;Wiemer, StefanGiardini, Domenico