Hydrothermal alteration of the Soultz-sous-Forets granite (Hot Fractured Rock geothermal exchanger) into a tosudite and illite assemblage

Bartier, Danièle; Meunier, Alain; Liewig, Nicole; Morvan, Gilles; Addad, Ahmed

doi:10.1127/0935-1221/2008/0020-1787

Cited by 44 publications

(48 citation statements)

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“…The m i value for granite in the RocData table is 32 ± 3 (Rocscience 2017). Although this value is based on test data on granite, we wanted to explore the impact of different m i values on granite as a function of weathering or hydrothermal alteration (since we know that the URG granites can be intensely hydrothermally altered; Ledésert et al 1999Ledésert et al , 2010Bartier et al 2008). We conducted eight triaxial deformation experiments using a "Hoek Cell" hydrostatic pressure vessel.…”

Section: Intact Rock Failure Criterionmentioning

confidence: 99%

“…It has been characterised, for example, in terms of its fracture density, structure, alteration, thermal conductivity, and permeability (e.g. Rummel 1992; Ledésert et al 1993Ledésert et al , 1999Ledésert et al , 2010Genter and Traineau 1996;Dezayes et al 2000Dezayes et al , 2010Surma and Géraud 2003;Sausse et al 2006Sausse et al , 2010Bartier et al 2008;Géraud et al 2010;Vidal et al 2017;Meller and Ledésert 2017). However, there are few published data on the mechanical behaviour, strength and stiffness (elastic modulus) of the URG reservoir granite rock mass.…”

Section: Introductionmentioning

confidence: 99%

“…2). Second, the URG granites can be intensely hydrothermally altered (Ledésert et al 1999;Bartier et al 2008;Ledésert et al 2010). Research in volcanic and geothermal systems has shown that alteration can have a large impact on intact rock strength (e.g.…”

Section: Introductionmentioning

confidence: 99%

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Estimating in situ rock mass strength and elastic modulus of granite from the Soultz-sous-Forêts geothermal reservoir (France)

et al. 2018

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Knowledge of the strength and elastic modulus of a reservoir rock is important for the optimisation of a particular geothermal resource. The reservoir rock for many geothermal projects in the Upper Rhine Graben, such as those at Soultz-sous-Forêts and Rittershoffen (both France), is porphyritic granite. High fracture densities (up to ~ 30 fractures/m) in this reservoir rock require that we consider the strength and elastic modulus of the rock mass, rather than the intact rock. Here we use uniaxial and triaxial deformation experiments performed on intact rock coupled with Geological Strength Index assessments-using the wealth of information from core and borehole analyses-to provide rock mass strength and elastic modulus estimates for the granite reservoir at Soultz-sous-Forêts (from a depth of 1400 to 2200 m) using the generalised Hoek-Brown failure criterion. The average uniaxial compressive strength and elastic modulus of the intact granite are 140 MPa (this study) and 40 GPa (data from this study and the literature), respectively. The modelled strength of the intact granite is 360 MPa at a depth of 1400 m and increases to 455 MPa at 2200 m (using our estimate for the empirical m i term of 30, determined using triaxial and tensile strength measurements on the intact granite). Strength of the rock mass varies in accordance with the fracture density and the extent and nature of the fracture infill, reaching lows of ~ 40-50 MPa (in, for example, the densely fractured zones in EPS-1 at depths of ~ 1650 and ~ 2160 m, respectively) and highs of above 400 MPa (in, for example, the largely unfractured zone at a depth of ~ 1940-2040 m). Variations in rock mass elastic modulus are qualitatively similar (values vary from 1 to 2 GPa up to the elastic modulus of the intact rock, 40 GPa). Our study highlights that macrofractures and joints reduce rock mass strength and should be considered when assessing the rock mass for well stability and rock mass deformation due to stress redistribution in the reservoir. We present a case study to demonstrate how a simple and cost-effective engineering method can be used to provide an indication of the in situ strength and elastic modulus of reservoir rock masses, important for a wide range of modelling and stimulation strategies. We recommend that the effect of macrofractures on rock mass strength and stiffness be validated for incorporation into geomechanical characterisation for geothermal reservoirs worldwide.

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Section: Intact Rock Failure Criterionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Estimating in situ rock mass strength and elastic modulus of granite from the Soultz-sous-Forêts geothermal reservoir (France)

et al. 2018

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Section: Development Of the Soultz Egsmentioning

confidence: 99%

“…K-Ar dating of illite (K-bearing clay mineral) found in a fractured and altered zone located at 2200 m in the Soultz granite (Bartier et al, 2008) indicate at least two episodic illitization at 63 Ma or slightly more for the coarsest particles and at 18 Ma or slightly less for the smallest. Other minerals precipitated at the same time (Bartier et al, 2008;Dubois et al, 2000;Ledésert et al, 1999), such as tosudite (Li-bearing mixed-layer clay mineral) and calcite (calcium carbonate). Calcite precipitated from the Ca-ions liberated by the dissolution of primary plagioclases present in rather great abundance in the granite (nearly 40% of 10%-Ca oligoclase; Ledésert et al, 1999; Table 4) during hydrothermal flow and from sedimentary brines enriched in Ca-ions during its flow within the calcareous Muschelkalk layers that penetrated into the granite (Ledésert et al, 1999).…”

Section: Development Of the Soultz Egsmentioning

confidence: 99%

The Soultz-sous-Forêts’ Enhanced Geothermal System: A Granitic Basement Used as a Heat Exchanger to Produce Electricity

Ledésert¹,

Hébert²

2012

Heat Exchangers - Basics Design Applications

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Is There a Link Between Mineralogy, Petrophysics, and the Hydraulic and Seismic Behaviors of the Soultz‐sous‐Forêts Granite During Stimulation? A Review and Reinterpretation of Petro‐Hydromechanical Data Toward a Better Understanding of Induced Seismicity and Fluid Flow

Meller

Ledésert

2017

JGR Solid Earth

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In the framework of the European Soultz‐sous‐Forêts enhanced geothermal system (EGS) in Alsace, France, 20 years of scientific and preindustrial tests had to be performed before the site began production of electricity in 2008. Stimulation tests were designed to enhance the permeability because most of the numerous natural fractures that crosscut the granite body were sealed by secondary minerals that crystallized as an effect of the circulation of local hot brines. The deep‐seated granitic reservoir is located between 4,500 and 5,000 m depths. Hydraulic stimulations were conducted in the four deep wells (GPK1, GPK2, GPK3, and GPK4) inducing different microseismic event patterns, which cannot be explained by tectonic structures alone. In the present work, we provide a review of the hydraulic tests and reinterpret them in the light of mineralogical data obtained along the boreholes. A clear relationship appears between mineralogy (mainly clay and calcite content) and the petrophysical, mechanical, and hydraulic behaviors of the rock mass. High calcite contents are correlated with an abundance of clay minerals, low Young's modulus, low magnetic susceptibility, and variation in spectral gamma ray. Microearthquakes are generated in the fresh granite zones, while clay and calcite‐rich zones, linked with hydrothermal alteration, might behave aseismically during hydraulic stimulations. These findings highlight the importance of a detailed knowledge of the petrography of a reservoir to conduct an effective stimulation while keeping the seismic hazard at a minimum.

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Hydrothermal alteration of the Soultz-sous-Forets granite (Hot Fractured Rock geothermal exchanger) into a tosudite and illite assemblage

Cited by 44 publications

References 46 publications

Estimating in situ rock mass strength and elastic modulus of granite from the Soultz-sous-Forêts geothermal reservoir (France)

Estimating in situ rock mass strength and elastic modulus of granite from the Soultz-sous-Forêts geothermal reservoir (France)

The Soultz-sous-Forêts’ Enhanced Geothermal System: A Granitic Basement Used as a Heat Exchanger to Produce Electricity

Is There a Link Between Mineralogy, Petrophysics, and the Hydraulic and Seismic Behaviors of the Soultz‐sous‐Forêts Granite During Stimulation? A Review and Reinterpretation of Petro‐Hydromechanical Data Toward a Better Understanding of Induced Seismicity and Fluid Flow

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