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  • 1.
    Figueiredo, B.
    et al.
    Portuguese Lab Civil Engn LNEC, Lisbon, Portugal..
    Cornet, F. H.
    CNRS, IPG S, Strasbourg, France..
    Lamas, L.
    Portuguese Lab Civil Engn LNEC, Lisbon, Portugal..
    Muralha, J.
    Portuguese Lab Civil Engn LNEC, Lisbon, Portugal..
    Determination of the stress field in a mountainous granite rock mass2014In: International Journal of Rock Mechanics And Mining Sciences, ISSN 1365-1609, E-ISSN 1873-4545, Vol. 72, p. 37-48Article in journal (Refereed)
    Abstract [en]

    The design of an underground hydroelectric power scheme in northern Portugal has required the characterisation of the local stress field. Nineteen hydraulic tests have been concluded in two, 500 m deep, vertical boreholes. In addition twelve overcoring tests together with twelve fiat jack tests have been performed from an existing adit located some 1.7 km away horn the location of the hydraulic tests. Results have been integrated into a stress model that takes into account both topography and tectonics effects. Most of the data are consistent with a linearly elastic, gravity loaded model, provided a very soft geomaterial is considered. This implies that the stress field in this granite rock mass is controlled by gravity alone and shear stress relaxation along faults and fractures but is unaffected by present-day tectonics. (C) 2014 Elsevier Ltd. All rights reserved,

  • 2.
    Figueiredo, Bruno
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Cornet, François
    Strasbourg University.
    Lamas, Luís
    Portuguese Laboratory for Civil Engineering.
    Muralha, José
    Portuguese Laboratory for Civil Engineering.
    Stress Field Assessment For Determining The Long-Term Rheology Of A Granite Rock Mass2016Conference paper (Refereed)
    Abstract [en]

    A case study is presented to show how stress field information can be used to assess the long-term rheological behaviour of an equivalent geo-material. The example concerns a granitic rock mass at the km3 scale where an underground hydropower scheme including a new 10 km long power conduit and a powerhouse complex located about halfway in the conduit and 500 m below ground level will be constructed. For design of the underground cavern and hydraulic pressure tunnel, several in situ stress measurements were carried out, using hydraulic, overcoring and flat jack techniques. Integration of the several in situ test results through a continuum mechanics model shows that the long-term behavior of this granite rock mass differs markedly from the short-term behaviour as defined by laboratory measurements on cores. The large scale stress field is found to depend mostly on the softer material that fills up the faults and hence results from the shear stress relaxation over a large number of pre-existing fractures and faults. The overall granite rock mass may be viewed as a combination of stiff elastic blocks separated by soft low strength material, leading to a fairly large scale homogeneous axisymmetrical stress field with vertical axis, with local strong anomalies that correspond to either high or low stress magnitudes.

  • 3.
    Figueiredo, Bruno
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Tsang, Chin-Fu
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL. Lawrence Berkeley National Laboratory, Berkeley USA.
    Niemi, Auli
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Georg, Lindgren
    Swedish Radiation Safety Authority, Stockholm, Sweden.
    Review: The state-of-art of sparse channel models and their applicability to performance assessment of radioactive waste repositories in fractured crystalline formations2016In: Hydrogeology Journal, ISSN 1431-2174, E-ISSN 1435-0157, Vol. 24, no 7, p. 1607-1622Article in journal (Other academic)
    Abstract [en]

    Laboratory and field experiments done on fractured rock show that flow and solute transport often occur along flow channels. ‘Sparse channels’ refers to the case where these channels are characterised by flow in long flow paths separated from each other by large spacings relative to the size of flow domain. A literature study is presented that brings together information useful to assess whether a sparse-channel network concept is an appropriate representation of the flow system in tight fractured rock of low transmissivity, such as that around a nuclear waste repository in deep crystalline rocks. A number of observations are made in this review. First, conventional fracture network models may lead to inaccurate results for flow and solute transport in tight fractured rocks. Secondly, a flow dimension of 1, as determined by the analysis of pressure data in well testing, may be indicative of channelised flow, but such interpretation is not unique or definitive. Thirdly, in sparse channels, the percolation may be more influenced by the fracture shape than the fracture size and orientation but further studies are needed. Fourthly, the migration of radionuclides from a waste canister in a repository to the biosphere may be strongly influenced by the type of model used (e.g. discrete fracture network, channel model). Fifthly, the determination of appropriateness of representing an in situ flow system by a sparse-channel network model needs parameters usually neglected in site characterisation, such as the density of channels or fracture intersections.

  • 4.
    Figueiredo, Bruno
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Tsang, Chin-Fu
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Rutqvist, Jonny
    Bensabat, Jac
    Niemi, Auli
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Coupled hydro-mechanical processes and fault reactivation induced by Co-2 Injection in a three-layer storage formation2015In: International Journal of Greenhouse Gas Control, ISSN 1750-5836, E-ISSN 1878-0148, Vol. 39, p. 432-448Article in journal (Refereed)
    Abstract [en]

    The interaction between mechanical deformation and fluid flow in fault zones gives rise to a host of coupled hydro-mechanical processes fundamental to fault instability, induced seismicity, and associated fluid migration. Fault stability is studied in the context of the Heletz site which was chosen as a test site for CO2 injection experiment in the framework of the EU-MUSTANG project. The potential reservoir for CO2 storage at the Heletz site consists of three sandstone layers that are approximately one, two and nine meters in thickness, separated by impermeable shale layers of various thicknesses, and overlaid by a five-meter limestone and a thick impermeable shale, which serves as caprock. The storage formation is intersected by two pre-existing sub-vertical normal faults (F1 and F2) on two opposite sides of the injection point. A hydro-mechanical model was developed to study the interaction between mechanical deformation and fluid flow in the two faults during CO2 injection and storage. We evaluate the consequences caused by potential fault reactivation, namely, the fault slip and the CO2 leakage through the caprock. The difference in the results obtained by considering the three-layer storage formation as an equivalent single-layer storage formation is analysed. It was found that for the two cases the pore pressure evolution is similar, but the differences in the evolution of CO2 saturation are significant, which is attributed to the differences in CO2 spread in a single and three-layer storage. No fault reactivation was observed in either case. A sensitivity analysis was made to study the influence of the fault dip angle, the ratio between the horizontal and vertical stresses, the offset of the layers across fault F2, the initial permeability of the fault and the permeability of the confinement formations. Results show that reactivation of faults Fl and F2 is most sensitive to the stress ratio, the initial permeability of the faults and the permeability of the confinement formations. The offset of the layers across the fault F2 was also found to be an important parameter, mainly because an offset leads to an increase in CO2 leakage. Changes in permeability were found to be small because plastic shear strains induced by the reactivation of the faults and associated increase in volumetric strains and permeability, occur mainly in a fault section of only 10 m length, which is the approximate total thickness of the storage layers.

  • 5.
    Figueiredo, Bruno
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Tsang, Chin-Fu
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL. Univ Calif Berkeley, Lawrence Berkeley Natl Lab, Berkeley, CA 94720 USA..
    Rutqvist, Jonny
    Univ Calif Berkeley, Lawrence Berkeley Natl Lab, Berkeley, CA 94720 USA..
    Niemi, Auli
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    A study of changes in deep fractured rock permeability due to coupled hydro-mechanical effects2015In: International Journal of Rock Mechanics And Mining Sciences, ISSN 1365-1609, E-ISSN 1873-4545, Vol. 79, p. 70-85Article in journal (Refereed)
    Abstract [en]

    This paper presents a numerical study of the hydro-mechanical behaviour of a fractured rock domain at 1000 m depth below the land surface as a function of different levels of fluid pore pressure. A 2D fractured rock domain is adopted based on data obtained from outcrop mapping, displaying multiple fracture sets, fracture intersections, dead-end and curved fractures. A continuum based numerical model is used to evaluate the effects of compressive boundary stresses, cracking by tension failure in the intact rock and fractures and shear displacement along fractures on its equivalent permeability. Two in situ stress boundary conditions are considered: an isotropic case SR1 with the two horizontal boundary compressive stresses having the same magnitude, and an anisotropic case SR2 with the ratio between these compressive stress components set to be 2. In the SR2 case, changes in the local stress and stress ratio distributions due to different fluid pore pressure levels are anisotropic and more significant than in the SR1 case, because of tension failures in the intact rock forming bridges between fractures. These failure regions opened new flow connections between fractures and thereby caused important anisotropic changes in the flow paths, and significant decrease in local gradients of fluid pore pressure. The equivalent permeability increases sharply when the fluid pore pressure is approximately 90% of the magnitude of the minimum stress at the boundaries of the fractured rock domain. Results show that the equivalent permeability of the fractured rock domain is most sensitive to the fractures normal stiffness, the permeability of the tension failure regions and the power-law exponent for permeability change.

  • 6.
    Figueiredo, Bruno
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Tsang, Chin-Fu
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL. Lawrence Berkeley National Laboratory, Berkeley, California.
    Rutqvist, Jonny
    Lawrence Berkeley National Laboratory.
    Niemi, Auli
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Study of hydraulic fracturing processes in shale formations with complex geological settings2017In: Journal of Petroleum Science and Engineering, ISSN 0920-4105, E-ISSN 1873-4715, Vol. 152, p. 361-374Article in journal (Refereed)
    Abstract [en]

    Hydraulic fracturing has been applied to extract gas from shale-gas reservoirs. Complicated geological settings,such as spatial variability of the rock mass properties, local heterogeneities, complex in situ stress field, and preexistingbedding planes and faults, could make hydraulic fracturing a challenging task. In order to effectivelyand economically recover gas from such reservoirs, it is crucial to explore how hydraulic fracturing performs insuch complex geological settings. For this purpose, numerical modelling plays an important role because suchconditions cannot be reproduced by laboratory experiments. This paper focuses on the analysis of the influenceof confining formations and pre-existing bedding planes and faults on the hydraulically-induced propagation ofa vertical fracture, which will be called injection fracture, in a shale-gas reservoir. An elastic-brittle model basedon material property degradation was implemented in a 2D finite-difference scheme and used for rock elementssubjected to tension and shear failure. A base case is considered, in which the ratio SR between the magnitudesof the horizontal and vertical stresses, the permeability kc of the confining formations, the elastic modulus Epand initial permeability kp of the bedding plane and the initial fault permeability kF are fixed at reasonablevalues. In addition, the influence of multiple bedding planes, is investigated. Changes in pore pressure andpermeability due to high pressure injection lasting 2 h were analysed. Results show that in our case during theinjection period the fracture reaches the confining formations and if the permeability of those layers issignificantly larger than that of the shale, the pore pressure at the extended fracture tip decreases and fracturepropagation becomes slower. After shut-in, the pore pressure decreases more and the fracture does notpropagate any more. For bedding planes oriented perpendicular to the maximum principal stress direction andwith the same elastic properties as the shale formation, results were found not to be influenced by theirpresence. In such a scenario, the impact of multiple bedding planes on fracture propagation is negligible. On theother hand, a bedding plane softer than the surrounding shale formation leads to a fracture propagationasymmetrical vertically with respect to the centre of the injection fracture with a more limited upward fracturepropagation. A pre-existing fault leads to a decrease in fracture propagation because of fault reactivation withshear failure. This results in a smaller increase in injection fracture permeability and a slight higher injectionpressure than that observed without the fault. Overall, results of a sensitivity analysis show that fracturepropagation is influenced by the stress ratio SR, the permeability kc of the confining formations and the initialpermeability kp of the bedding plane more than the other major parameters.

    The full text will be freely available from 2019-03-07 08:00
  • 7.
    Figueiredo, Bruno
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Tsang, Chin-Fu
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL. Lawrence Berkeley Natl Lab, Berkeley, CA USA..
    Rutqvist, Jonny
    Lawrence Berkeley Natl Lab, Berkeley, CA USA..
    Niemi, Auli
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    The effects of nearby fractures on hydraulically induced fracture propagation and permeability changes2017In: Engineering Geology, ISSN 0013-7952, E-ISSN 1872-6917, Vol. 228, p. 197-213Article in journal (Refereed)
    Abstract [en]

    Fracture propagation caused by hydraulic fracturing operations can be significantly influenced by adjacent fractures. This paper presents a detailed coupled hydro-mechanical analysis to study the effects of nearby natural fractures on hydraulically induced fracture propagation and changes in fracture permeability. Two rock domains were considered in comparison: FD1, with one fracture, and FD2, with two adjacent parallel or non-parallel fractures. It was assumed that water injection occurred in a borehole that intersected the single fracture in FD1 and one of the two fractures in FD2. Simulations were made for a time period of 3 h with an injection period of 2 h followed by 1 h of shut-in. An elastic-brittle model based on the degradation of material properties was implemented in a 2D finite-difference scheme and used for elements of the intact rock subjected to tension and shear failure. The intact rock was considered to have a low but non-negligible permeability. A verification study against analytical solutions showed that the fracture propagation and stress concentrations due to differential boundary stresses could be accurately represented by our model. Next, a base case was considered, in which the stress ratio (SR) between the magnitudes of the maximum and minimum boundary stresses, the permeability k(R) of the intact rock and the initial permeability k(TF) of the tension failure regions were fixed. In FD2, the distance d(F) between the two natural fractures defined by the closest distance was also fixed. The results showed that in both rock domains, the fracture started to propagate when the pore pressure was approximately 85% of the magnitude of the minimum boundary stress. The propagation of a single fracture was significantly greater than the propagation of a double fracture system because, in the latter case, the pore pressure decreased when the two fractures connected. As a result, changes in permeability in FD2 were smaller than in FD1. At the end of injection, the maximum ratios between the final and initial permeability of the natural fractures were found to be approximately 3 and 2 for rock domains FD1 and FD2, respectively. For non-parallel fractures, the controlling factor for fracture propagation was the separation between the tips of the pressurised fracture and the neighbouring non-pressurised fracture. A sensitivity study was conducted to study the influence of the key parameters d(F), SR, k(R) and k(TF) on the simulation results. Fracture propagation showed more sensitivity to d(F) and SR than to the other parameters.

  • 8.
    Tsang, Chin-Fu
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL. Lawrence Berkeley Natl Lab, Berkeley, CA USA.
    Figueiredo, Bruno
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Niemi, Auli
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Importance of stress effects on inputs to fracture network models used for subsurface flow and transport studies2018In: International Journal of Rock Mechanics And Mining Sciences, ISSN 1365-1609, E-ISSN 1873-4545, Vol. 101, p. 13-17Article in journal (Refereed)
1 - 8 of 8
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