Valley, Benoît

2007, Valley, Benoît

2020-6, Bahrani, N., Valley, Benoît

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

2011-10-16, Valley, Benoît, 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.

2018-9, Krietsch, H., Gischig, V., Evans, K. F., Doetsch, J., Dutler, Nathan, Valley, Benoît, Amann, F.

An 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.

2012-12-1, Saleh, Ramin, Jeyaraj, R., Milkereit, Bernd, Liu, Q., Valley, Benoît

In an active underground mine there are many seismic activities taking place, such as seismic noises, blasts, tremors and microseismic events. In between the activities, the microseismic events are mainly used for monitoring purposes. The frequency content of microseismic events can be up to few KHz, which can result in wavelengths on the order of a few meters in hard rock environment. In an underground mine, considering the presence of both small wavelength and strong elastic contrasts, the simulation of seismic wave propagation is a challenge. With the recent availability of detailed 3D rock property models of mines, in addition to the development of efficient numerical techniques (such as Spectral Element Method (SEM)), and parallel computation facilities, a solution for such a problem is achievable. Most seismic wave scattering studies focus on large scales (>1 km) and weak elastic contrasts (velocity perturbations less than 10%). However, scattering in the presence of small-scale heterogeneities and large elastic contrasts is an area of ongoing research. In a mine environment, the presence of strong contrast discontinuities such as massive ore bodies, tunnels and infrastructure lead to discontinuities of displacement and/or stress tensor components, and have significant impact on the propagation of seismic waves. In order to obtain an accurate image of wave propagation in such a complex media, it is necessary to consider the presence of these discontinuities in numerical models. In this study, the effects of such a contrast are illustrated with 2D/3D modeling and compared with real broadband 3-component seismic data. The real broadband 3-component seismic data will be obtained in one of the Canadian underground mines in Ontario. One of the possible scenarios investigated in this study that may explain the observed complexity in seismic wavefield pattern in hard rock environments is the effect of near field displacements rather than far field. Considering the distribution of seismic sensors in a mine and the presence of seismic events within a mine, the recorded wavefield may represent a near-field displacement, which is not the case for most of seismic studies. The role of receiver characterization on the recorded event near the surface or around fault zones is also investigated. Using 2D/3D modeling, the effects of Vp/Vs variation on vertical and horizontal components of recorded amplitude has been shown.

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.

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

2015-11-26, Valley, Benoît, Evans, Keith F.

2015-12-20, Katsaga, T, Riahi, A, DeGagne, DO, Valley, Benoît, Damjanac, B

2014, Valley, Benoît, Evans, Keith, Hirschberg, Stefan, Wiemer, Stefan, Burgherr, Peter