Michael J Heap
email: heap@unistra.fr

Marlène Villeneuve

Alexandra R L Kushnir

Jamie I Farquharson

Patrick Baud

Thierry Reuschlé

Rock mass strength and elastic modulus of the Buntsandstein: an important lithostratigraphic unit for geothermal exploitation in the Upper Rhine Graben

Keywords: Buntsandstein, uniaxial compressive strength, triaxial deformation experiments, porosity, clay content, geothermal reservoir Highlights

Geothermal exploitation in the Upper Rhine Graben increasingly targets the interface between the granitic basement and the overlying Buntsandstein unit. Results from deformation experiments are combined with structural assessments to provide reservoir-scale wet and dry strength and elastic modulus profiles for the Buntsandstein at Soultz-sous-Forêts (France). Our analysis finds five zones characterised by low strength and elastic modulus. The strength and elastic modulus of "massive" zones are lower when the rock is wet (i.e. water-saturated), highlighting the importance of performing wet deformation experiments for geothermal rock mass assessments. These data and methods can be used to provide assessments of other geothermal sites within the region to assist prospection, stimulation, and optimisation strategies.

Introduction

An understanding of the strength and elastic modulus of rock within a geothermal reservoir is an important input in models that guide reservoir prospection, stimulation, and optimisation strategies.

Laboratory experiments designed to measure these parameters are typically performed on samples with dimensions shorter than the meso-or macrofracture length scale. As a result, laboratory data gathered on nominally intact rock cannot be directly applied to large-scale geotechnical practices. Although upscaling laboratory values to the reservoir-or borehole-scale is non-trivial, widely used empirical geotechnical tools exist to bridge this length scale discrepancy. Two equations from the geotechnical toolbox, which provide estimates for rock mass strength and rock mass elastic modulus, respectively, are the generalised Hoek-Brown failure criterion [START_REF] Hoek | Hoek-Brown failure criterion[END_REF] and the Hoek-Diederichs equation [START_REF] Hoek | Empirical estimation of rock mass modulus[END_REF]. Both of these approaches take an assessment of the rock mass structure (e.g., fracture density) and the surface condition of the fractures (e.g., smooth/rough, the pervasiveness of weathering or alteration, the nature of the fracture infilling) into account to determine the rock mass strength and elastic modulus, which will be often less than that of the intact material.

Geothermal energy exploitation is common in the Upper Rhine Graben, a 350 km-long and 50 km-wide Cenozoic rift valley that extends from Frankfurt (Germany) down to Basel (Switzerland), due to the presence of anomalously high thermal gradients (areas of geothermal interest typically have thermal gradients > 80 °C) attributed to hydrothermal circulation within the fractured Palaeozoic granitic basement and the overlying Permian and Triassic sedimentary rocks (e.g., [START_REF] Pribnow | Thermal tracking of upper crustal fluid flow in the Rhine Graben[END_REF]Buchmann and Connelly, 2007;[START_REF] Guillou-Frottier | Structure of hydrothermal convection in the Upper Rhine Graben as inferred from corrected temperature data and basinscale numerical models[END_REF][START_REF] Baillieux | Localization of temperature anomalies in the Upper Rhine Graben: insights from geophysics and neotectonic activity[END_REF][START_REF] Magnenet | Two-dimensional THM modelling of the large scale natural hydrothermal circulation at Soultz-sous-Forêts[END_REF][START_REF] Freymark | The deep thermal field of the Upper Rhine Graben[END_REF]. A total of 15 geothermal wells have been drilled (to depths between ~1000 and ~5000 m) in the Upper Rhine Graben since the 1980s [START_REF] Vidal | Overview of naturally permeable fractured reservoirs in the central and southern Upper Rhine Graben: Insights from geothermal wells[END_REF] and notable geothermal sites include Soultz-sous-Forêts (e.g., [START_REF] Kappelmeyer | European HDR project at Soultz-sous-Forêts: general presentation[END_REF][START_REF] Baria | European HDR research programme at Soultz-sous-Forêts (France) 1987-1996[END_REF][START_REF] Gérard | The deep EGS (Enhanced Geothermal System) project at Soultzsous-Forêts (Alsace, France)[END_REF]; Figure 1) and Rittershoffen (e.g., [START_REF] Baujard | Hydrothermal characterization of wells GRT-1 and GRT-2 in Rittershoffen, France: Implications on the understanding of natural flow systems in the Rhine graben[END_REF][START_REF] Glaas | How do the geological and geophysical signatures of permeable fractures in granitic basement evolve after long periods of natural circulation? Insights from the Rittershoffen geothermal wells (France)[END_REF]; Figure 1) in France, Brühl (e.g., [START_REF] Bauer | Predictability of properties of a fractured geothermal reservoir: the opportunities and limitations of an outcrop analogue study[END_REF], Insheim, Trebur, and Bruchsal in Germany, and Riehen in Switzerland.

A recent study [START_REF] Villeneuve | Estimating in-situ rock mass strength and elastic modulus of granite from the Soultz-sous-Forêts geothermal reservoir (France)[END_REF] provided rock mass strengths (using the generalised Hoek-Brown failure criterion; [START_REF] Hoek | Hoek-Brown failure criterion[END_REF] and elastic moduli (using the Hoek-Diederichs equation; [START_REF] Hoek | Empirical estimation of rock mass modulus[END_REF] for the fractured Palaeozoic granitic basement from exploration well EPS-1 at the Soultz-sous-Forêts geothermal site (Figure 1) using laboratory experiments on intact material and structural data from core analyses. Our goal here, using the same approach, is to provide estimates of borehole-scale rock mass strength and elastic modulus for the lowermost overlying Permian and Triassic sedimentary rocks sampled from exploration well EPS-1 (located about 1 km southwest of Soultz-sous-Forêts; Figure 1), from a depth of 1008 to 1414 m (i.e. the Buntsandstein lithostratigraphic unit). The reasons for choosing Soultz-sous-Forêts as a case study are twofold. First, the Buntsandstein was continuously cored at exploration well EPS-1 (to a core diameter of 78 mm), providing the samples required for laboratory testing (uniaxial and triaxial compression experiments). Second, there exists a wealth of structural data (fracture density and fracture fill, for example) for the borehole at EPS-1, providing the data required for the assessment of rock mass structure. Since the Buntsandstein is known to be laterally extensive [START_REF] Aichholzer | New stratigraphic interpretation of the Soultzsous-Forêts 30-year-old geothermal wells calibrated on the recent one from Rittershoffen (Upper Rhine Graben, France)[END_REF][START_REF] Vidal | Overview of naturally permeable fractured reservoirs in the central and southern Upper Rhine Graben: Insights from geothermal wells[END_REF], the results of this study can not only be used to help optimise current geothermal sites in the Upper Rhine Graben, but also to help guide and optimise future geothermal sites currently in development, such as those at Illkirch and Vendenheim (both in Alsace, France). One of the principal motivations for this study is that recent (e.g., Rittershoffen; [START_REF] Baujard | Hydrothermal characterization of wells GRT-1 and GRT-2 in Rittershoffen, France: Implications on the understanding of natural flow systems in the Rhine graben[END_REF]; Figure 1) and future geothermal exploitation in the Upper Rhine Graben has and will target the interface between the granitic basement and the overlying Permian and Triassic sedimentary rocks.

Providing estimates of rock mass strength and elastic modulus for the Buntsandstein is complicated for two principal reasons. First, the petrophysical variability of the Buntsandstein (e.g., [START_REF] Vernoux | Geological and petrophysical parameters of a deep fractured sandstone formation as applied to geothermal exploitation[END_REF][START_REF] Aichholzer | New stratigraphic interpretation of the Soultzsous-Forêts 30-year-old geothermal wells calibrated on the recent one from Rittershoffen (Upper Rhine Graben, France)[END_REF][START_REF] Heap | Microstructural and petrophysical properties of the Permo-Triassic sandstones (Buntsandstein) from the Soultz-sous-Forêts geothermal site (France)[END_REF] demands that the intact rock properties (strength and elastic modulus) are characterised for materials sampled from a number of different depths.

Although the assumption of mechanical homogeneity (i.e. similar intact strength and elastic modulus at different depths) was justified when providing rock mass properties of the porphyritic granite (monzogranite) from EPS-1 from a depth of 1414 to 2200 m [START_REF] Villeneuve | Estimating in-situ rock mass strength and elastic modulus of granite from the Soultz-sous-Forêts geothermal reservoir (France)[END_REF], the mechanical properties of the Buntsandstein vary considerably due to the variability in porosity and microstructural attributes such as pore size and grain size (Vernoux et al., 2005;[START_REF] Haffen | Determination of fluid-flow zones in a geothermal sandstone reservoir using thermal conductivity and temperature logs[END_REF][START_REF] Griffiths | Geothermal implications for fracture-filling hydrothermal precipitation[END_REF][START_REF] Heap | Microstructural and petrophysical properties of the Permo-Triassic sandstones (Buntsandstein) from the Soultz-sous-Forêts geothermal site (France)[END_REF]Kushnir et al., 2018a), parameters known to greatly influence the strength and stiffness of rocks, including sandstone (e.g., [START_REF] Palchik | Influence of porosity and elastic modulus on uniaxial compressive strength in soft brittle porous sandstones[END_REF][START_REF] Chang | Empirical relations between rock strength and physical properties in sedimentary rocks[END_REF][START_REF] Baud | Effects of porosity and crack density on the compressive strength of rocks[END_REF].

Here, therefore, we perform systematic laboratory measurements on material sampled at regular depth intervals (from 1008 to 1414 m) from the EPS-1 core at the Soultz-sous-Forêts geothermal site. Second, experimental studies have shown that clay plays an important role in the observed weakening of sandstone in the presence of water (e.g., [START_REF] Rutter | The effect of water on stress relaxation of faulted and unfaulted sandstone[END_REF][START_REF] Hawkins | Sensitivity of sandstone strength and deformability to changes in moisture content[END_REF][START_REF] Baud | Failure mode and weakening effect of water on sandstone[END_REF][START_REF] Demarco | The impact of partial water saturation on rock strength: an experimental study on sandstone [Der Einfluss einer partiellen Wassersättigung auf die mechanischen Gesteinseigenschaften: eine Fallstudie an Sandsteinen[END_REF]. For example, [START_REF] Hawkins | Sensitivity of sandstone strength and deformability to changes in moisture content[END_REF] found that the uniaxial compressive strength (UCS) of sandstone was reduced by 78% for clay-rich sandstones and by only 8% for siliceous sandstones in the presence of water. The variable, and sometimes high, clay contents of the Buntsandstein sandstones (e.g., [START_REF] Heap | Microstructural and petrophysical properties of the Permo-Triassic sandstones (Buntsandstein) from the Soultz-sous-Forêts geothermal site (France)[END_REF] thus calls into question the use of laboratory strength and elastic moduli data determined for dry rocks typically used in geotechnical rock mass assessments. Therefore, we here perform dry and wet laboratory deformation measurements to provide estimations of the dry and wet rock mass strength and elastic modulus for the Buntsandstein unit at the Soultz-sous-Forêts geothermal site.

Methods and materials

Intact failure criteria

The strength of intact rock, including sandstone, in the brittle regime increases with increasing confining pressure or depth (e.g., [START_REF] Tf | The transition from brittle faulting to cataclastic flow in porous sandstones: Mechanical deformation[END_REF][START_REF] Baud | Failure mode and weakening effect of water on sandstone[END_REF][START_REF] Bésuelle | Failure mode and spatial distribution of damage in Rothbach sandstone in the brittle-ductile transition[END_REF]. The Hoek-Brown failure criterion for intact rock is an empirical failure criterion that provides strength estimates for intact rock at depth using the uniaxial compressive strength, 𝐶 " , (i.e., the strength of the rock at ambient pressure) and a constant, 𝑚 $ :

𝜎 & ' = 𝜎 ) ' + 𝐶 " +𝑚 $ 𝜎 ) ' 𝐶 " + 1- ..0 (1)
where 𝜎 & ' and 𝜎 ) ' are the effective maximum and minimum principal stresses, respectively. The unitless constant 𝑚 $ describes the shape of the failure envelope on a graph of 𝜎 & ' as a function of 𝜎 ) ' , and is therefore related to the microstructural and mineralogical attributes of the rock (mineral content, grain size, grain shape, pore size, and pore shape, amongst many others) [START_REF] Eberhardt | The Hoek-Brown failure criterion[END_REF]. The most reliable method to determine 𝑚 $ is to run a series of triaxial deformation experiments on the rock of interest [START_REF] Hoek | Empirical Strength Criterion for Rock Masses[END_REF]; however values of 𝑚 $ are often estimated using charts that offer values for common lithologies (e.g., [START_REF] Hoek | Practical estimates of rock mass strength[END_REF].

In this study, 𝐶 " is determined for samples of Buntsandstein sandstone taken from regular depth intervals (every 40 to 50 m) from the continuous core available at the EPS-1 exploration well (Figure 1). Values of empirical constant 𝑚 $ are often taken from tables of "standard" values for common lithologies; the recommended value of 𝑚 $ for sandstone is 19 [START_REF] Hoek | Practical estimates of rock mass strength[END_REF]. However, in our study, we chose instead to perform a suite of triaxial experiments on samples from one of the sampled depth intervals in order to better constrain 𝑚 $ for the Buntsandstein (see Section 2.6). Uniaxial experiments were performed under both dry and wet conditions to provide the dry intact uniaxial strength, 𝐶 "_5 , and the wet intact uniaxial strength, 𝐶 "_6 (see Section 2.5). Similarly, wet and dry triaxial experiments provide estimates for 𝑚 $ under dry conditions, 𝑚 $_5 , and under wet conditions, 𝑚 $_6 (see Section 2.6). Therefore, Equation (1) can be modified to provide the intact rock strength with depth for dry (Equation 2) and wet rock (Equation 3), respectively:

𝜎 & ' = 𝜎 ) ' + 𝐶 "_5 +𝑚 $_5 𝜎 ) ' 𝐶 " + 1- ..0 (2) 𝜎 & ' = 𝜎 ) ' + 𝐶 "_6 +𝑚 $_6 𝜎 ) ' 𝐶 " + 1- ..0
(3)

Rock mass strength failure criteria

Rock mass strength can be determined using the generalised Hoek-Brown failure criterion:

𝜎 & ' = 𝜎 ) ' + 𝐶 " +𝑚 9 𝜎 ) ' 𝐶 " + 𝑠- ; ( 4
)
where mb, s, and a are unitless fitting parameters for a fractured rock mass and are defined as follows [START_REF] Hoek | Hoek-Brown failure criterion[END_REF]:

𝑚 9 = 𝑚 $ 𝑒 > ?@AB&.. CDB&EF G
(5) 𝑠 = 𝑒 > ?@AB&..

IB)F G (6) 𝑎 = 1 2 + 1 6 L𝑒 B ?@A &0 + 𝑒 BC.
) M ( 7)

where D is a unitless disturbance factor related to blasting damage in large excavations (since well drilling does not use explosives, D = 0 in our case study; we provide the full equation here for completeness) and GSI is the Geological Strength Index, a unitless value that describes the rock mass characteristics [START_REF] Marinos | The geological strength index: applications and limitations[END_REF]. Values of GSI range from 0 to 100, where high values indicate intact or massive rock masses with fresh surfaces and low values indicate blocky/disintegrated/laminated rock masses with highly weathered surfaces and clay coatings or fillings [START_REF] Marinos | The geological strength index: applications and limitations[END_REF]. GSI is typically obtained from rock outcrop exposures, but for this study values of GSI were determined using structural data collected on the EPS-1 core material (see Section 2.7), as described in [START_REF] Villeneuve | Estimating in-situ rock mass strength and elastic modulus of granite from the Soultz-sous-Forêts geothermal reservoir (France)[END_REF]. Equation ( 4) can be recast to provide the rock mass strength with depth for dry (Equation 8) and wet rock (Equation 9), respectively:

𝜎 & ' = 𝜎 ) ' + 𝐶 "_5 +𝑚 9_5 𝜎 ) ' 𝐶 "_5 + 𝑠- ; (8) 𝜎 & ' = 𝜎 ) ' + 𝐶 "_6 +𝑚 9_6 𝜎 ) ' 𝐶 "_6 + 𝑠- ; ( 9
)
where 𝑚 9_5 and 𝑚 9_6 are given as, respectively:

𝑚 9_5 = 𝑚 $_5 𝑒 > ?@AB&.. CDB&EF G (10) 𝑚 9_6 = 𝑚 $_6 𝑒 > ?@AB&.. CDB&EF G (11)

Intact and rock mass elastic modulus

The elastic modulus of intact rock, 𝐸 $ , can be determined from the elastic portion of the stressstrain curve of a rock deforming in compression. The rock mass elastic modulus, 𝐸 ST , can be determined using the Hoek-Diederichs equation [START_REF] Hoek | Empirical estimation of rock mass modulus[END_REF]:

𝐸 ST = 𝐸 $ U0.02 + 1 - 𝐷 2 1 + 𝑒 X(Y.Z&0FB?@A)/&&\ ] (12)
As for the rock strength equations in Section 2.2, the disturbance factor 𝐷 is equal to zero for our application. As mentioned in Section 2.2, values of GSI were determined using structural data collected on the EPS-1 core material (see Section 2.7). Equation ( 12) can be modified to provide values for the elastic modulus of a dry (Equation 13) and wet rock mass (Equation 14), respectively:

𝐸 ST_5 = 𝐸 $_5 U0.02 + 1 - 𝐷 2 1 + 𝑒 X(Y.Z&0FB?@A)/&&\ ] (13) 𝐸 ST_6 = 𝐸 $_6 U0.02 + 1 - 𝐷 2 1 + 𝑒 X(Y.Z&0FB?@A)/&&\ ] (14)
where 𝐸 $_5 and 𝐸 $_6 are the intact elastic moduli of dry and wet rock, respectively.

Experimental materials and sample preparation

The Buntsandstein lithostratigraphic unit was sampled at regular (~40-50 m) depth intervals (between 1008 to 1414 m; Figure 1b) from the EPS-1 exploration well from the Soultz-sous-Forêts geothermal site (Figures 1 and2). This Permian and Triassic sedimentary unit (described in, for example, Vernoux et al., 2005;[START_REF] Haffen | Determination of fluid-flow zones in a geothermal sandstone reservoir using thermal conductivity and temperature logs[END_REF][START_REF] Vidal | How do permeable fractures in the Triassic sediments of Northern Alsace characterize the top of hydrothermal convective cells? Evidence from Soultz geothermal boreholes (France)[END_REF][START_REF] Aichholzer | New stratigraphic interpretation of the Soultzsous-Forêts 30-year-old geothermal wells calibrated on the recent one from Rittershoffen (Upper Rhine Graben, France)[END_REF][START_REF] Griffiths | Geothermal implications for fracture-filling hydrothermal precipitation[END_REF][START_REF] Heap | Microstructural and petrophysical properties of the Permo-Triassic sandstones (Buntsandstein) from the Soultz-sous-Forêts geothermal site (France)[END_REF]Kushnir et al., 2018a) directly overlies the fractured Palaeozoic granitic reservoir (described in, for example, [START_REF] Genter | Analysis of macroscopic fractures in granite in the HDR geothermal well EPS-1, Soultz-sous-Forêts, France[END_REF][START_REF] Genter | Comparative analysis of direct (core) and indirect (borehole imaging tools) collection of fracture data in the Hot Dry Rock Soultz reservoir (France)[END_REF][START_REF] Sausse | Permeability and alteration within the Soultz granite inferred from geophysical and flow log analysis[END_REF][START_REF] Dezayes | Structure of the low permeable naturally fractured geothermal reservoir at Soultz[END_REF][START_REF] Genter | Contribution of the exploration of deep crystalline fractured reservoir of Soultz to the knowledge of enhanced geothermal systems (EGS)[END_REF]. We collected 1 m-long pieces of the 78 mm-diameter core at each of the sampled depths. In total, twelve depths were sampled (one sandstone from the Voltzia unit, one from the Couches Intermédiaires unit, three from the Karlstal unit, two from the Rehberg unit, two from the Trifels unit, two from the Annweiler unit, and one from the Anté-Annweiler unit; Figure 1b).

Four or five cylindrical samples, 20, 18.75, or 12 mm in diameter (depending on the strength of the sandstone), were cored from each of the twelve cores collected and precision-ground to a nominal length of 40, 37.5, or 24 mm, respectively. These samples were cored so that their axes were parallel to the EPS-1 borehole (i.e. perpendicular to bedding; Figure 2). We complement these borehole samples with samples prepared from blocks (all from the Buntsandstein) acquired from local quarries (from Rothbach, Adamswiller, and Bust, all in France; see Figure 1c for quarry locations). Samples from the quarry blocks were cored perpendicular to bedding to a diameter of 20 mm and precision-ground to a nominal length of 40 mm. All of the samples were washed using tapwater and then dried in a vacuum oven at 40 °C for at least 48 hours.

The twelve samples collected from exploration well EPS-1 are feldspathic (>10 wt.% feldspar) or quartz-rich (<10 wt.% feldspar) sandstones that contain variable amounts of pore-filling clay (muscovite/illite-smectite) (from 2 to 13.1 wt.%) (Figure 3; Table 1; [START_REF] Heap | Microstructural and petrophysical properties of the Permo-Triassic sandstones (Buntsandstein) from the Soultz-sous-Forêts geothermal site (France)[END_REF]. Most of the sandstones are homogeneous at the sample scale, although a few (in particular samples 299, 497, and 540) contain obvious bedding-parallel laminations (Figure 2). The average grain diameter of these samples varies from 142 to 424 µm (Figure 3; Table 2; [START_REF] Heap | Microstructural and petrophysical properties of the Permo-Triassic sandstones (Buntsandstein) from the Soultz-sous-Forêts geothermal site (France)[END_REF]. The microstructural and textural features of these samples are summarised in Table 2, and are described at length in [START_REF] Heap | Microstructural and petrophysical properties of the Permo-Triassic sandstones (Buntsandstein) from the Soultz-sous-Forêts geothermal site (France)[END_REF]. The three quarry rocks are all feldspathic sandstones (Table 1). The sandstones from Bust and Adamswiller do not contain any obvious laminations/bedding, although the Adamswiller sandstone is known to exhibit a mechanical anisotropy [START_REF] Baud | Effects of bedding and foliation on mechanical anisotropy, damage evolution and failure mode[END_REF]. Rothbach sandstone, a rock that also shows significant mechanical anisotropy [START_REF] Louis | Microstructural inhomogeneity and mechanical anisotropy associated with bedding in Rothbach sandstone[END_REF], however, contains alternating layers that show differences in grain size and porosity [START_REF] Louis | Microstructural control on the anisotropy of elastic and transport properties in undeformed sandstones[END_REF].

Before the samples were deformed, their connected porosity was determined using the connected (skeletal) volume measured by a helium pycnometer (Micromeritics AccuPyc II 1340) and the bulk volume calculated using the sample dimensions.

2.5 Determining 𝐶 " (intact uniaxial compressive strength) and 𝐸 $ (intact elastic modulus)

The prepared cylindrical samples were deformed uniaxially at a strain rate of 1.0 × 10 -6 s -1 until macroscopic failure. Half of the samples were deformed "dry" (dried at 40 °C in a vacuum-oven for at least 48 hours prior to deformation) and half were deformed "wet" (vacuum-saturated in deionised water and deformed in a water bath) (see Heap at al. (2014) for a schematic of the experimental device). A lubricating wax was applied to the end-faces of the dry samples to avoid problems associated with the friction between the sample and the pistons. We chose to deform the samples at a relatively low strain rate of 1.0 × 10 -6 s -1 to ensure drainage in the low-porosity, low-permeability samples deformed under saturated conditions. During deformation, axial displacement and axial load were measured using a linear variable differential transducer (LVDT) and a load cell, respectively. These measurements were converted to axial strain and axial stress using the sample dimensions. 𝐶 "_5 and 𝐶 "_6 were taken as the peak stress obtained in these uniaxial compressive strength tests under dry and wet conditions, respectively. Values of 𝐸 $_5 and 𝐸 $_6 were determined using the slope of the linear elastic portion of the dry and wet uniaxial stress-strain curves, respectively [START_REF] Ulusay | The Complete ISRM Suggested Methods for Rock Characterization, Testing and Monitoring: 1974-2006[END_REF].

Determining 𝑚 $

The unitless parameter 𝑚 $ describes the shape of the failure envelope in a graph of 𝜎 & ' as a function of 𝜎 ) ' (i.e. the principal stress space). We determined 𝑚 $ using triaxial experiments conducted on dry and wet cylindrical cores (20 mm in diameter and 40 mm in length) prepared from the core sampled from a depth of 1239 m (from the Rehberg unit; porosity 0.18). Constant strain rate (1.0 × 10 -6 s -1 ) triaxial experiments were performed at constant effective pressures (defined here as the confining pressure minus the pore fluid pressure) of 2.5, 5, 7.5, 10, and 15 MPa. Samples deformed under dry conditions were therefore deformed at constant confining pressures of 2.5, 5, 7.5, 10, and 15 MPa (pore pressure = 0 MPa). A lubricating wax was applied to the end-faces of the dry samples to avoid problems associated with the friction between the sample and the pistons. Samples deformed under wet conditions were deformed using a constant pore fluid (deionised water) pressure of 10 MPa, and confining pressures of 12.5, 15, 17.5, 20, and 25 MPa. The confining and pore pressures were maintained constant during deformation using a confining and pore fluid pressure intensifier, respectively. During deformation, axial displacement and axial load were measured using an LVDT and a load cell, respectively. These measurements were converted to axial strain and axial stress using the sample dimensions. We determined 𝑚 $_5 and 𝑚 $_6 using the compressive strengths measured for these experiments using the data fitting function (modified cuckoo fit algorithm, basic error summation, and absolute error type) in RocData (Rocscience, 2017).

Determining the Geological Strength Index (GSI)

The GSI is a unitless value that describes the rock mass characteristics using an assessment of the rock mass structure (e.g., fracture density) and the surface condition of the fractures (e.g., smooth/rough, the pervasiveness of weathering or alteration, the nature and thickness of the fracture infilling) [START_REF] Marinos | The geological strength index: applications and limitations[END_REF]. The logging data from EPS-1 in the Bundsandstein contains the type, location (depth), orientation, infilling type, and infilling thickness of all discontinuities mapped along the core. The locations (depths) were used to derive a fracture frequency. The discontinuity type, infilling type, and infilling thickness were used to assess the joint condition. The rock mass structure and surface condition were assigned broad descriptors, such as "blocky" and "poor", respectively, which were then used to determine a GSI value using a visual look-up chart [START_REF] Marinos | The geological strength index: applications and limitations[END_REF]. Due to the available logging data and core photographs, we consider that our GSI estimates lack the subjectivity often associated with estimates of GSI.

The determination of the surface quality is relatively straightforward in this case study because of the high quality of the core logs in the Bundsandstein. Despite not having descriptions of joint roughness, we feel confident that our assessments using discontinuity type, infilling type, and infilling thickness provide a suitable basis for assessing the joint condition. For example, discontinuities identified as faults can be assumed to have smooth to slickensided surfaces, resulting in a condition assessment of fair if they have no infilling, or very poor if they have thick (> 2 mm), clayey infilling.

Discontinuities identified as joints can be assumed to have rough to smooth surfaces, resulting in a condition assessment of very good if they have no infilling, or very poor if they have thick (> 2 mm), clayey infilling.

The determination of the rock mass structure requires the selection of the scale at which the rock mass assessment is conducted. For example, [START_REF] Villeneuve | Estimating in-situ rock mass strength and elastic modulus of granite from the Soultz-sous-Forêts geothermal reservoir (France)[END_REF] assessed the rock mass strength and elastic modulus of the granite underlying the Bundsandstein at the borehole scale. These authors demonstrated that the rock mass is so densely fractured in some intervals that the intact rock parameters are not appropriate. [START_REF] Villeneuve | Estimating in-situ rock mass strength and elastic modulus of granite from the Soultz-sous-Forêts geothermal reservoir (France)[END_REF] suggested a range of 7-10 fractures per metre as the cut-off fracture density to decide between using the intact rock failure criterion (Equation 1) and using the rock mass failure criterion (Equation 4). In the present case study we derive the rock mass parameters at the reservoir scale (tens of metres). Based on the recommendations of [START_REF] Schultz | Relative scale and the strength and deformability of rock masses[END_REF], we use a block size to reservoir scale ratio of 5-10 as the cutoff for using rock mass strength and elastic modulus. We use fracture densities of 1 fracture per metre and 2 fractures per metre to differentiate between massive and blocky, and blocky and very blocky, respectively. In accordance with the reasoning detailed in [START_REF] Villeneuve | Estimating in-situ rock mass strength and elastic modulus of granite from the Soultz-sous-Forêts geothermal reservoir (France)[END_REF], the cut-off between intact and rock mass failure criteria corresponds to approximately 2 fractures per metre.

3 Laboratory testing

Uniaxial compressive strength experiments

Representative uniaxial stress-strain curves for the dry Buntsandstein samples are shown in Figure 4a. These stress-strain curves (Figure 4a) are typical of those for brittle rock in compression (e.g., [START_REF] Hoek | Brittle fracture propagation in rock under compression[END_REF][START_REF] Brace | Dilatancy in the fracture of crystalline rocks[END_REF][START_REF] Scholz | Microfracturing and the inelastic deformation of rock in compression[END_REF]. The maximum axial stress obtained during the experiment is the uniaxial compressive strength, 𝐶 " (as labelled for one of the dry curves in Figure 4a). Representative uniaxial stress-strain curves for a dry and wet sample of sample 100 are shown in Figure 4b. These curves show that the uniaxial compressive strength is lower when the sample is wet. In the example shown in Figure 4b, the dry strength, 𝐶 "_5 , was 150.8 MPa and the wet strength, 𝐶 "_6 , was 89.9 MPa. These representative curves also show that the slope of the stress-strain curve in the elastic region (i.e. the elastic modulus) and the axial strain at macroscopic failure are lower when the sample is wet (Figure 4b).

Dry (white circles) and wet (blue squares) uniaxial compressive strengths (𝐶 "_5 and 𝐶 "_6 , respectively) and elastic moduli (𝐸 $_5 and 𝐸 $_6 , respectively) are plotted as a function of connected porosity in Figures 5a and5b, respectively (data available in Table 3). These data show that the strength (Figure 5a) and elastic modulus (Figure 5b) of the Buntsandstein samples studied here both decrease as the connected porosity is increased. For example, the dry strength and dry elastic modulus of the Buntsandstein samples tested herein range from ~50 MPa and ~10 GPa, respectively, at a porosity of ~0.25 up to ~250 MPa and ~40 GPa, respectively, at a porosity of ~0.04 (Figures 5a and5b). These data also show that the wet strength and wet elastic modulus of the sandstones is systematically lower than the dry strength and elastic modulus across the entire tested porosity range (Figures 5a and5b).

The ratios of wet to dry strength are plotted as a function of connected porosity and clay content in Figures 6a and6b, respectively. These data show that the ratio of wet to dry strength varies from ~0.55 to ~0.84 for the rocks tested herein (Figure 6). Figure 6a shows that the observed water-weakening does not appear to depend on the connected porosity. However, although there is some scatter in the data, there is a trend of increasing water-weakening as clay content is increased (Figure 6b). Indeed, the sample that contains the most clay (13.1 wt.%; sample 540) has the lowest ratio of wet to dry strength, and the sample containing the least clay (1.8 wt.%; Rothbach sandstone) has the highest ratio of wet to dry strength (Figure 6b). The ratios of wet to dry elastic modulus are plotted as a function of connected porosity and clay content in Figures 7a and7b, respectively. These data show that the ratio of wet to dry elastic modulus varies from ~0.74 to ~1.00 for the rocks tested herein (Figure 7). The ratio of wet to dry elastic modulus does not appear to depend on either connected porosity (Figure 7a) or clay content (Figure 7b).

Triaxial compressive strength experiments

The stress-strain curves for the dry and wet triaxial experiments (on samples from sample 347) are shown in Figures 8a and8b, respectively (experiments unique to this study). The sandstone is brittle over the tested pressure range and failure (marked by a stress drop in the stress-strain data; Figure 8) was manifest as a through-going shear fracture. These data show that the dry (Figure 8a) and wet (Figure 8b) strength increases as the effective pressure is increased, in accordance with many experimental studies on sandstones in the brittle field (e.g., [START_REF] Tf | The transition from brittle faulting to cataclastic flow in porous sandstones: Mechanical deformation[END_REF][START_REF] Baud | Failure mode and weakening effect of water on sandstone[END_REF][START_REF] Bésuelle | Failure mode and spatial distribution of damage in Rothbach sandstone in the brittle-ductile transition[END_REF].

For example, the dry strength was increased from ~58 MPa at an effective pressure of 0 MPa to ~141

MPa at an effective pressure of 15 MPa (Figure 8a) (data available in Table 4). Our triaxial experiments also show that sample 347 is weaker when wet over the entire tested range of effective pressure (i.e. up to 15 MPa) (Figure 8).

The triaxial data plotted in principal stress space ( 1 as a function of 3; Figure 9) show that the failure envelope for the wet samples (blue symbols in Figure 9) is lower than the failure envelope for the dry samples (white symbols in Figure 9). The fits to these data were provided by the Modified Cuckoo data fitting algorithm using basic (vertical) error summation, and relative (divided by the yvalue of the curve fit) error type) in RocData (Rocscience, 2017). These fits provide values of 19 (relative residuals 0.008) and 23 (relative residuals 0.012) for 𝑚 $_5 and 𝑚 $_6 , respectively. We note that the value for dry sandstone is within the range of 𝑚 $ values typically associated with sandstone (17 ± 4, as given in Hoek ( 2007)), whereas the value for wet sandstone is slightly higher than this range.

Geological Strength Index (GSI)

The rock mass descriptions, based on fracture frequency, infilling type, and infilling thickness are summarised in Table 5, along with their associated rock mass descriptions at the reservoir scale.

Figure 10 shows how the look-up chart from [START_REF] Marinos | The geological strength index: applications and limitations[END_REF] was used to assess the GSI for two example intervals. For the depth interval 1365-1381 m the fracture density is 2.1 fractures per metre, and is therefore assessed as a "very blocky" (see Figure 10) rock mass structure. The fractures are infilled with barite and minor calcite (both hard minerals that tend to seal fractures) with an infilling thickness averaging 2 mm. The surface condition of the fractures is therefore assessed as "good" (see Figure 10) because barite and calcite are precipitated mineral infillings and will therefore contribute cohesive and frictional strength to the fracture. The combination of the two assessments results in a GSI range of 45-60. Since our analysis requires a single value of GSI, we use GSI = 55 for this interval (1365-1381 m) (Table 5). Similarly, the depth interval 1012-1020 m has been assessed a "very blocky" (see Figure 10) structure. Because of the presence of four faults within an 8 m thick interval, and an infilling thickness averaging 9 mm, the surface condition is assessed as "poor" (see Figure 10). The combination results in a range of GSI of 25-40. Again, since our analysis requires a single value of GSI, we use GSI = 35 for this interval (1012-1020 m) (Table 5).

The majority of the rock mass in the Bundsandstein has a GSI of 100 (Table 5) because of its low fracture density (< 2 fractures per metre), with the resulting assessment of "massive" for the rock structure and the use of the intact Hoek-Brown failure criterion (Equation 1) to determine rock strength over these intervals. The intervals with GSI less than 100, of which there are five, are associated with highly fractured zones (e.g., the interval between depths 1151 and 1156 m has a fracture density of 2 discontinuities per metre) (Table 5). The five zones that are characterised by low values of GSI are located at depths of ~1012-1020, ~1151-1156, ~1205-1216, ~1347-1350, and ~1365-1381 m (Table 5).

The zones with the lowest GSI (GSI = 35) are associated with highly fractured zones that contain fractures with a very thick average infilling (e.g., the interval between depths 1205 and 1216 m has an average infilling thickness of 11 mm; Table 5). The three zones with the lowest GSI (Table 5) were previously identified as the three main fractured zones within the EPS-1 core (Vernoux et al., 2005).

4 Reservoir-scale strength and elastic modulus for the Buntsandstein

The dry and wet intact and rock mass strength and elastic modulus, calculated using the reservoir scale GSI and Equations 1, 2, 3, 8, and 9 (for the strength) and Equations 13 and 14 (for the elastic modulus) are given in Table 5. The fracture density with depth (Figure 11a) is plotted alongside the dry and wet strength and elastic modulus in Figures 11b and11c, respectively. Figures 11b and11c clearly show the five zones for which the rock mass strength and rock mass elastic modulus is applicable (at depths of ~1012-1020, ~1151-1156, ~1205-1216, ~1347-1350, and ~1365-1381 m; indicated by the black arrows in Figure 11), as opposed to those where the intact values of either property are appropriate.

These zones are characterised by low values of strength (Figure 11b) and elastic modulus (Figure 11c).

Apart from these five zones, the strength and elastic modulus of the rock mass is given by the intact strength and the intact elastic modulus (GSI = 100; Table 5). Excluding the rock at the interface between the granite and the sedimentary rocks (the Anté-Annweiler), which has relatively high values of strength and elastic modulus, the overall strength (Figure 11b) and elastic modulus (Figure 11c) of the Buntsandstein appears to decrease with depth; this is especially true for the elastic modulus (Figure 11c).

We also note that, in the five zones where the strength and elastic modulus are described by the rock mass strength and elastic modulus, respectively, there is essentially no difference between the dry and wet values (Figures 11b and11c). In these zones, the strength and elastic modulus of the rock mass very much depends on the fractures present within the rock mass, rather than the condition (wet or dry) of the rock mass. However, in the depths characterised by the intact strength and elastic modulus, a large difference can be observed between the wet and dry values (Figures 11b and11c).

Discussion

Water-weakening in sandstones

Water-weakening in rocks can be the result of both mechanical and chemical processes.

Mechanically, pressurised pore fluids typically weaken and embrittle rocks (e.g. [START_REF] Paterson | Experimental rock deformation-the brittle field[END_REF]. Chemically, rock can be weakened by subcritical crack growth processes (e.g., [START_REF] Atkinson | Subcritical crack growth in geological materials[END_REF] and the reduction of surface free energy (e.g., [START_REF] Parks | Surface and interfacial free energies of quartz[END_REF]. The mechanical effect can be expressed in terms of the effective pressure law, where the effective pressure equals the confining pressure minus the pore fluid pressure multiplied by a poroelastic constant. The study of [START_REF] Baud | Mechanical compaction and strain localization in Bleurswiller sandstone[END_REF] shows that this poroelastic constant is very close to unity for porous sandstone. As a result, we do not consider herein the mechanical influence of pore fluids on the strength of our sandstone samples.

Our deformation experiments have shown that sandstone is weaker in the presence of water (Figures 4,5,6,7,and 8), in accordance with many published studies (e.g., [START_REF] Rutter | The effect of water on stress relaxation of faulted and unfaulted sandstone[END_REF][START_REF] Bell | Petrographical factors relating to porosity and permeability in the Fell Sandstone[END_REF][START_REF] Hadizadeh | Water-weakening of sandstone and quartzite deformed at various stress and strain rates[END_REF][START_REF] Dyke | Evaluating the strength and deformability of sandstones[END_REF][START_REF] Hawkins | Sensitivity of sandstone strength and deformability to changes in moisture content[END_REF][START_REF] Kasim | An investigation of the relationship between uniaxial compressive strength and degradation for selected rock types[END_REF][START_REF] Zang | Acoustic emission, microstructure, and damage model of dry and wet sandstone stressed to failure[END_REF][START_REF] Bell | Petrographic and engineering properties of sandstones from the Sneinton Formation, Nottinghamshire, England[END_REF][START_REF] Baud | Failure mode and weakening effect of water on sandstone[END_REF][START_REF] Cuss | The application of critical state soil mechanics to the mechanical behaviour of porous sandstones[END_REF][START_REF] Lin | Wetting weakening of tertiary sandstones-microscopic mechanism[END_REF][START_REF] Vásárhelyi | Influence of water content on the strength of rock[END_REF][START_REF] Demarco | The impact of partial water saturation on rock strength: an experimental study on sandstone [Der Einfluss einer partiellen Wassersättigung auf die mechanischen Gesteinseigenschaften: eine Fallstudie an Sandsteinen[END_REF][START_REF] Shakoor | Relationship between unconfined compressive strength and degree of saturation for selected sandstones[END_REF]Nespereia et al., 2010;[START_REF] Siedel | Sandstone weathering in tropical climate: results of low-destructive investigations at the temple of Angkor Wat, Cambodia[END_REF][START_REF] Wasantha | Water-weakening behavior of Hawkesbury sandstone in brittle regime[END_REF]. A handful of these studies highlight that the observed water-weakening in sandstones is related to their clay content. For example, [START_REF] Hawkins | Sensitivity of sandstone strength and deformability to changes in moisture content[END_REF] found that the uniaxial compressive strength (UCS) of sandstone was reduced by 78% for clay-rich sandstones and by only 8% for siliceous sandstones in the presence of water.

Although our data suggest that the water-weakening in sandstone does not depend on connected porosity (Figure 6a), they do highlight that water-weakening may increase as clay content is increased (Figure 6b). To better understand the role of connected porosity and clay content on water-weakening in sandstone, we plot ratios of wet to dry strength from numerous studies (data available in Table 6) alongside our new data (Table 3) as a function of porosity and clay content in Figures 12a and12b, respectively. Figure 12a shows that there is no discernible trend between water-weakening in sandstone and porosity. However, Figure 12b shows that, although there is some scatter in the data, waterweakening increases as clay content is increased. Indeed, the sample containing the highest clay content (~70 %) has the lowest ratio of wet to dry strength (Figure 12b).

To explore the reason for the reduction in strength in the presence of water we use the micromechanical model of [START_REF] Sammis | The failure of brittle porous solids under compressive stress states[END_REF]. This model, based on the classic work of Griffiths, has been previously used to describe the brittle failure of porous sandstones (e.g., [START_REF] Baud | Effects of porosity and crack density on the compressive strength of rocks[END_REF]. The micromechanical model of [START_REF] Sammis | The failure of brittle porous solids under compressive stress states[END_REF] is an inclusion model comprising a two-dimensional elastic medium populated by circular holes of uniform radius, 𝑟. As the stress on the medium increases, cracks nucleate and propagate from the circular holes (parallel to the direction of the applied stress) when the stress intensity at the tip of a small crack on the circular surface reaches a critical value (𝐾 A`, the fracture toughness). The cracks interact once they are long enough, thus increasing the local tensile stress intensity. Eventually, the cracks coalesce and conspire to induce the macroscopic failure of the elastic medium. In the case of uniaxial compression, [START_REF] Zhu | Micromechanics of cataclastic pore collapse in limestone[END_REF] Assuming that Equation ( 15) can capture the behaviour of the sandstones of Table 6, there are three variables that influence uniaxial compressive strength: the porosity, 𝜙, the fracture toughness, 𝐾 A`, and the pore radius, 𝑟. If we assume that the porosity and pore radius are the same for a given sandstone then, according to Equation (15), the observed water-weakening (Figure 12) must be due to a decrease in 𝐾 A`. Figure 13 shows strength as a function of porosity using Equation (15) for a fixed pore radius (125 µm) and different values of 𝐾 A` (𝐾 A` = 0.1, 0.3, 0.5, and 0.8 MPa m -1/2 ) alongside the dry (white circles) and wet (blue squares) data for the compiled sandstones. These modelled curves aptly demonstrate that decreasing the 𝐾 A` at a given porosity and pore size lowers the strength (Figure 13).

The reduction in uniaxial compressive strength observed in wet rocks has been attributed to a reduction in fracture surface free energy due to the adsorption of water on the fracture surface (e.g., [START_REF] Parks | Surface and interfacial free energies of quartz[END_REF][START_REF] Baud | Failure mode and weakening effect of water on sandstone[END_REF]. We conclude here, in line with the conclusions of [START_REF] Baud | Failure mode and weakening effect of water on sandstone[END_REF], that a lower fracture surface free energy in the presence of water reduces 𝐾 A` and therefore strength. Additionally, we highlight that water adsorption on clay minerals is extremely efficient due to their large surface areas.

To emphasise, the specific surface areas of samples 157 (clay content = 2 wt.%) and 540 (clay content = 13.1 wt.%) are 204 and 6170 m 2 /kg, respectively [START_REF] Heap | Microstructural and petrophysical properties of the Permo-Triassic sandstones (Buntsandstein) from the Soultz-sous-Forêts geothermal site (France)[END_REF]; Tables 1 and2). Therefore, a higher clay content should ensure that more water is adsorbed on the surface of the fracture, perhaps explaining the correlation between clay content and water-weakening (Figure 12b). The scatter in the data of Figure 12b suggests that clay content does not govern water-weakening in sandstones alone. One factor likely to play a key role is whether the clays are pore-coating or pore-filling, or whether the clays are present within the rock matrix. For water adsorption on clay minerals to influence the fracture surface free energy, the clays must be present within the material through which the fracture propagates (i.e. the matrix). By contrast, clay present within pores may not exert a significant influence on the waterweakening process.

We further highlight that the micromechanics of low-porosity rocks is often described using the wing-crack model of Ashby and Sammis (1990), although we note that no or few microcracks were found in the intact microstructure of the studied materials (Figure 3). If the wing-crack model was considered appropriate for the low-porosity materials studied herein, then there is an additional micromechanical parameter-the coefficient of friction-could also be influenced by the presence of water.

Implications for geothermal energy exploitation

We have provided estimates of the rock mass strength (Figure 11b) and elastic modulus (Figure 11c) for the Bundsandstein sedimentary sequence (from the Soultz-sous-Forêts geothermal site) that directly overlies the fractured granitic basement used throughout the Upper Rhine Graben as a geothermal resource. One of the main motivations for this study is that recent (e.g., Rittershoffen; [START_REF] Baujard | Hydrothermal characterization of wells GRT-1 and GRT-2 in Rittershoffen, France: Implications on the understanding of natural flow systems in the Rhine graben[END_REF] and future geothermal exploitation in the Upper Rhine Graben has and will target rock at or close to this interface. Since the Buntsandstein is known to be a laterally extensive unit (e.g. [START_REF] Aichholzer | New stratigraphic interpretation of the Soultzsous-Forêts 30-year-old geothermal wells calibrated on the recent one from Rittershoffen (Upper Rhine Graben, France)[END_REF][START_REF] Vidal | Overview of naturally permeable fractured reservoirs in the central and southern Upper Rhine Graben: Insights from geothermal wells[END_REF], these new reservoir-scale strength and elastic modulus estimates could assist prospection, stimulation, and optimisation strategies (for instance, the Buntsandstein was stimulated at the Rittershoffen site in 2013; [START_REF] Vidal | Pre-and post-stimulation characterization of geothermal well GRT-1, Rittershoffen, France: insights from acoustic image logs of hard fractured rock[END_REF][START_REF] Baujard | Hydrothermal characterization of wells GRT-1 and GRT-2 in Rittershoffen, France: Implications on the understanding of natural flow systems in the Rhine graben[END_REF] at geothermal sites within the Upper Rhine Graben. We can combine our strength and elastic modulus estimates for the Buntsandstein with those for the granite reservoir provided in [START_REF] Villeneuve | Estimating in-situ rock mass strength and elastic modulus of granite from the Soultz-sous-Forêts geothermal reservoir (France)[END_REF] to give strength (Figure 14b) and elastic modulus (Figure 15b) estimates for the EPS-1 borehole from a depth of ~1000 to ~2200 m. These data highlight, for example, that the rock mass strength of the granitic reservoir can be lower than that for the Buntsandstein, due to the high fracture density of the granite (Figure 14).

However, since the rock mass strength and elastic modulus estimations (Figures 11b,11c,14b,and 15b) are very much dependent on the fracture density (and their surface condition), the estimations provided herein may not be directly applicable to the Buntsandstein sedimentary sequence found at other geothermal sites, which will likely be characterised by different fracture densities and intersect regional scale faults at different depths. For example, the Buntsandstein is much deeper at the Rittershoffen site (from ~1800 to ~2200 m; [START_REF] Aichholzer | New stratigraphic interpretation of the Soultzsous-Forêts 30-year-old geothermal wells calibrated on the recent one from Rittershoffen (Upper Rhine Graben, France)[END_REF] than at Soultz-sous-Forêts and major fracture zones are found at depths of ~1975, ~1986, and ~2140 m [START_REF] Vidal | Permeable fracture zones in the hard rocks of the geothermal reservoir at Rittershoffen, France[END_REF]. Further, the extent of fracture infilling will also be spatially variable. For instance, [START_REF] Vidal | How do permeable fractures in the Triassic sediments of Northern Alsace characterize the top of hydrothermal convective cells? Evidence from Soultz geothermal boreholes (France)[END_REF] note that the extent of the fracture sealing can differ in the same fractured zone at different wells (e.g., GPK-2, GPK-3, and GPK-4, which are separated by no more than 1.5 km) at Soultz-sous-Forêts. We further note that the sedimentary units overlying the granitic reservoir can be appreciably different from those at the Soultz-sous-Forêts site at a distance of several tens of kilometres (Kushnir et al., 2018b). However, we highlight that our laboratory data can be re-used to provide upscaled strength and elastic modulus profiles for other sites, using the methods outlined herein, as long as site-specific rock mass descriptions are performed. We highlight that direct core data will provide a more complete characterisation of the fracture network than borehole imaging tools [START_REF] Genter | Comparative analysis of direct (core) and indirect (borehole imaging tools) collection of fracture data in the Hot Dry Rock Soultz reservoir (France)[END_REF]. A recent study by [START_REF] Bauer | Predictability of properties of a fractured geothermal reservoir: the opportunities and limitations of an outcrop analogue study[END_REF] also highlights that, when core materials are not available, care should be taken when selecting outcrop analogues for geothermal reservoir rock.

Our laboratory data have shown that the Buntsandstein sandstones are weaker when wet (Figure 5). In zones characterised by the intact rock strength and elastic modulus (i.e. all zones except the five highly fractured zones) (Figures 11b and11c), the reservoir-scale strength and the elastic modulus are lower when the rock is wet. Differences between the dry and wet strength and elastic modulus within the five fractured zones are small: the strength and elastic modulus in these zones is primarily dictated by the rock mass structure (fractures) (Figures 11b and11c). Since the pore space of rocks within a geothermal reservoir will be saturated with liquid water/brine in-situ, our measurements and analysis therefore highlight the importance of performing wet deformation experiments to provide upscaled strength and elastic modulus assessments for geothermal applications. Indeed, not only are the wet and dry strengths and elastic moduli different (Figure 5), but we also measured a difference between the dry and wet 𝑚 $ term (Figure 9). We recommend that future studies that adopt this technique also perform wet deformation experiments, particularly because water-weakening has been reported in a wide range of rock types (e.g., sandstone: [START_REF] Hawkins | Sensitivity of sandstone strength and deformability to changes in moisture content[END_REF]McConnell, 1992, tuff: Zhu et al., 2011;[START_REF] Heap | The influence of water on the strength of Neapolitan Yellow Tuff, the most widely used building stone in Naples (Italy)[END_REF], basalt: Zhu et al., 2016, carbonates: Baud et al., 2016).

Although we performed wet deformation measurements to provide more accurate strength and elastic modulus assessments, we highlight that our experiments were performed at room temperature and used deionised water. First, the temperature of the Buntsandstein rocks at Soultz-sous-Forêts, for example, is between 120 and 140 °C (e.g., [START_REF] Genter | Contribution of the exploration of deep crystalline fractured reservoir of Soultz to the knowledge of enhanced geothermal systems (EGS)[END_REF]. The intact strength and stiffness of sandstone measured in the laboratory may be different at high-temperature, although there is a paucity of data. For example, [START_REF] Ranjith | Transformation plasticity and the effect of temperature on the mechanical behaviour of Hawkesbury sandstone at atmospheric pressure[END_REF] found that the uniaxial compressive strength and stiffness of Hawkesbury sandstone (containing 20% clay) increased from 36.5 to 42.5 MPa and from 2.8 to 3.3 GPa, respectively, as the testing temperature was increased from 25 to 200 °C. [START_REF] Rao | Experimental study of mechanical properties of sandstone at high temperature[END_REF] also observed an increase in uniaxial compressive strength and stiffness for sandstone as temperature was increased from 25 to 150 °C. Therefore, based on these few data, it would appear that the rock mass strength and elastic modulus assessments provided herein (measured at room temperature) may underestimate the strength and elastic modulus of the rock in-situ. However, firm conclusions cannot be drawn without further experimental investigations. For example, Heap et al. (2009a) showed that the compressive strength of three sandstones under triaxial testing conditions (an effective pressure of 30 MPa) was slightly lower at 75 °C than at room temperature. At present it is unclear as to the influence of, for example, mineral content, porosity, and grain size on the compressive strength of sandstone at hightemperature. Second, the rocks within the reservoir are saturated with hydrothermal brines with a pH between 5.2 and 5.5 (e.g., Scheiber et al., 2002). A lower pH should reduce the fracture surface free energy (e.g., [START_REF] Parks | Surface and interfacial free energies of quartz[END_REF] and therefore reduce intact strength. Indeed, [START_REF] Singh | Effect of acidic water on physicomechanical behaviour of rock[END_REF] measured a reduction in uniaxial compressive strength in sandstone from ~65 to ~40 MPa as the pH was reduced from 7 to 2. The uniaxial compressive strength of sandstone samples was also found to decrease with increasing salinity [START_REF] Shukla | Mechanical behaviour of reservoir rock under brine saturation[END_REF]Rathanweera et al., 2014). However, recent triaxial experiments showed that the triaxial compressive strength of sandstone increased with increasing NaCl concentration [START_REF] Huang | The Effects of NaCl Concentration and Confining Pressure on Mechanical and Acoustic Behaviors of Brine-Saturated Sandstone[END_REF]. It is therefore difficult to draw firm conclusions as to the influence of the pore fluid composition and pH on the strength and stiffness of sandstone without further laboratory testing.

We highlight that the reservoir-scale strength and elastic modulus estimates provided here are representative of the short-term strength. These rock properties, and empirical term 𝑚 $ , were all determined at a laboratory strain rate of 1 × 10 -6 s -1 . Although this strain rate is an order of magnitude lower than typically used in laboratory testing, we highlight that the strength of rock, including sandstone, decreases as strain rate is decreased due to time-dependent subcritical crack growth processes [START_REF] Atkinson | Subcritical crack growth in geological materials[END_REF][START_REF] Brantut | Time-dependent cracking and brittle creep in crustal rocks: A review[END_REF]. There are many experimental studies on time-dependent deformation in sandstone (e.g., [START_REF] Ngwenya | A constitutive law for lowtemperature creep of water-saturated sandstones[END_REF][START_REF] Baud | Damage accumulation during triaxial creep of Darley Dale sandstone from pore volumometry and acoustic emission[END_REF]Heap et al., 2009a;2009b;[START_REF] Yang | Triaxial mechanical creep behavior of sandstone[END_REF][START_REF] Brantut | Rate-and strain-dependent brittle deformation of rocks[END_REF][START_REF] Tang | The Influence of Water Saturation on the Shortand Long-Term Mechanical Behavior of Red Sandstone[END_REF]. For example, Heap et al. (2009b) showed that sandstone fails at 80% of its short-term failure stress in about 60 hours. Further, and pertinent to the geothermal reservoir case study, the deformation rate during a brittle creep test can increase by up to three orders of magnitude as the temperature is increased from 20 to 75 °C (Heap et al., 2009a). Based on the data provided in these studies, the long-term strength of the Buntsandstein sedimentary sequence may be lower than the short-term strength estimates provided herein.

Concluding remarks

Recent geothermal exploitation in the Upper Rhine Graben has targeted the interface between the fractured granitic basement and the overlying Permian and Triassic sedimentary rocks (the Buntsandstein), and geothermal projects being developed in the region will continue to do so. Our study provides wet and dry strength and elastic modulus estimates for the Buntsandstein sedimentary sequence (from 1008 to 1414 m) at the Soultz-sous-Forêts geothermal site in France. To provide such estimates we performed 64 uniaxial (32 dry and 32 wet) and 10 triaxial deformation experiments (5 dry and 5 wet). These data were coupled with Geological Strength Index (GSI) assessments to provide down-well wet and dry strength and elastic modulus profiles using the Hoek-Brown failure criterion and the Hoek-Diederichs equation, respectively. Our analysis finds five zones characterised by low values of GSI (zones with high fracture densities and infilling thicknesses) that have low values of rock mass strength and elastic modulus. The zones with low fracture densities (i.e. a "massive" rock structure) are characterised by the intact rock properties (i.e. a GSI value of 100) and therefore have higher values of strength and elastic modulus. Additionally, we find that the strength and elastic modulus of these massive zones are lower when the rock is wet, highlighting the importance of performing wet deformation experiments for rock mass assessments of rock that is typically saturated with a liquid phase, such as in a geothermal reservoir. We combine these new data with those for the underlying fractured granite reservoir to produce down-well strength and elastic modulus profiles from a depth of ~1000 to ~2200 m for the Soultz-sous-Forêts geothermal site. Although this type of analysis is somewhat site specific, the methods used and data provided herein can be easily adapted to provide strength and elastic modulus estimates for other geothermal sites within the Upper Rhine Graben to assist reservoir prospection, stimulation, and optimisation strategies. 3). The dry uniaxial compressive strength, 𝐶 ._5 , is labelled for one of the curves. (b)

Representative dry (black curve) and wet (blue curve) uniaxial stress-strain curves for samples taken from box number 100 (see Table 3). The dry uniaxial compressive strength, 𝐶 ._5 , and the wet uniaxial compressive strength, 𝐶 ._6 , are labelled on the corresponding curves. 3) and data from published studies (Table 6) as a function of connected porosity. (a) The ratio of wet to dry uniaxial compressive strength for the data unique to this study (Table 3) and data from published studies (Table 6) as a function of clay content. 3) and published data (Table 6)) as a function of porosity. Modelled curves, using Equation ( 15 Table 1. X-ray powder diffraction (XRPD) analysis showing quantitative bulk mineralogical 912 composition for the twelve sandstones sampled from exploration well EPS-1 at the Soultz-sous-Forêts 913 geothermal site and the three quarry rocks. Values in wt.%. Data from [START_REF] Heap | Microstructural and petrophysical properties of the Permo-Triassic sandstones (Buntsandstein) from the Soultz-sous-Forêts geothermal site (France)[END_REF]. 914 Table 2. Average connected porosity, average grain diameter, specific surface area, average dry and wet 918 P-wave velocity, and average permeability for each of the twelve sandstones sampled from exploration 919 well EPS-1 at the Soultz-sous-Forêts geothermal site and the three quarry rocks. Physical property data 920 taken from [START_REF] Heap | Microstructural and petrophysical properties of the Permo-Triassic sandstones (Buntsandstein) from the Soultz-sous-Forêts geothermal site (France)[END_REF]. Elastic wave velocities were measured at room pressure and 921 permeabilities were measured under a confining pressure of 1 MPa. Grain size data for Rothbach and 922

Adamswiller sandstone from [START_REF] Louis | Microstructural control on the anisotropy of elastic and transport properties in undeformed sandstones[END_REF]2009) and [START_REF] David | Laboratory measurement of compaction-induced permeability change in porous rocks: Implications for the generation and maintenance of pore pressure excess in the crust[END_REF] twelve sandstones sampled from exploration well EPS-1 at the Soultz-sous-Forêts geothermal site and 927 the three quarry rocks. Also included are the connected porosity, average grain diameter, and clay 928 content (average grain diameter and clay content taken from [START_REF] Heap | Microstructural and petrophysical properties of the Permo-Triassic sandstones (Buntsandstein) from the Soultz-sous-Forêts geothermal site (France)[END_REF] See Excel spreadsheet for Table 5.

FiguresFigure 1 .

 1 Figures

  Figure 1. (a) Map of France showing the location of the Bas-Rhin (Lower Rhine) department of Alsace

Figure 2 .

 2 Figure 2. Photographs of each of the twelve sandstones (20 mm diameter core samples nominally 40

Figure 3 .

 3 Figure 3. Backscattered scanning electron microscope (BSE) images for each of the twelve sandstones

Figure 4 .

 4 Figure 4. (a) Representative dry uniaxial stress-strain curves for each of the twelve sandstones sampled

Figure 5 .

 5 Figure 5. (a) Dry (white circles) and wet (blue squares) uniaxial compressive strength for the twelve

Figure 6 .

 6 Figure 6. (a) The ratio of wet to dry uniaxial compressive strength for the twelve sandstones sampled

Figure 7 .

 7 Figure 7. (a) The ratio of wet to dry elastic modulus for the twelve sandstones sampled from exploration

Figure 8 .

 8 Figure 8. (a) Dry triaxial stress-strain curves for samples taken from box number 347 (from the Rehberg

Figure 9 .

 9 Figure 9. Triaxial data for wet and dry Rehberg unit (depth = 1239 m) plotted in principal stress space

Figure 10 .

 10 Figure 10. Geological Strength Index (GSI) lookup chart (modified from Marinos et al., 2005) showing

Figure 11 .

 11 Figure 11. (a) Stratigraphic column showing the units within the Buntsandstein from exploration well

Figure 12 .

 12 Figure 12. (a) The ratio of wet to dry uniaxial compressive strength for the data unique to this study

Figure 13 .

 13 Figure 13. Dry (white circles) and wet (blue squares) uniaxial compressive strength data (data unique

  ), show curves of uniaxial compressive strength as a function of porosity for a fixed pore radius (125 µm) and different values of 𝐾 A` (𝐾 A` = 0.1, 0.3, 0.5, and 0.8 MPa m -1/2 ) (see text for details).

Figure 14 .

 14 Figure 14. (a) Stratigraphic column of exploration well EPS-1 at Soultz-sous-Forêts (from 1012 to 2200

Figure 15 .

 15 Figure 15. (a) Stratigraphic column of exploration well EPS-1 at Soultz-sous-Forêts (from 1012 to 2200

Table 3 .

 3 Summary of the dry and wet uniaxial compressive strength experiments performed on the 926

Table 4 .

 4 Summary of the dry and wet triaxial compressive strength experiments performed on samples taken from box number 347 (from the Rehberg unit).

	Sample	Connected porosity	Condition	Confining pressure (MPa)	Pore fluid pressure (MPa)	Effective pressure (MPa)	Peak differential stress (MPa)
	347_12	0.181	Dry	2.5	0	2.5	71.8
	347_8	0.179	Dry	5	0	5	97.7
	347_13	0.177	Dry	7.5	0	7.5	105.8
	347_9	0.178	Dry	10	0	10	122.5
	347_11	0.181	Dry	15	0	15	141.1
	347_23	0.179	Wet	12.5	10	2.5	62.3
	347_14	0.180	Wet	15	10	5	73.4
	347_17	0.177	Wet	17.5	10	7.5	86.3
	347_16	0.177	Wet	20	10	10	95.9
	347_18	0.179	Wet	25	10	15	111.8

Table 5 .

 5 Summary of rock mass descriptions for Geological Strength Index (GSI) assessment, and the resulting intact and rock mass strength and elastic modulus for the wet and dry sandstone units. Wet and dry intact Co and Ei are given as averages for the rock type associated with the rock mass interval. Shmin

	= minimum horizontal stress; UCS = uniaxial compressive strength; Ei = intact elastic modulus; Erm =
	rock mass elastic modulus.

Table 6 .

 6 Published wet and dry uniaxial compressive strength data for sandstones. Rate in brackets for the Pennant sandstone experiments (Hadizadeh and Law, 1991) is the experimental strain rate. H & L = Hadizadeh and Law; H & M = Hawkins and McConnell; para = parallel to bedding; perp = perpendicular to bedding.

	Sandstone	Porosity	Clay	Dry	Wet	UCSwet /	Reference
			content	UCS	UCS	UCSdry
			(%)	(MPa)	(MPa)
	Fell sandstone	0.111	-	33.2	19.1	0.58	Bell (1978)
	Fell sandstone	0.127	-	51.9	31.0	0.60	Bell (1978)
	Fell sandstone	0.115	-	73.7	43.3	0.59	Bell (1978)
	Fell sandstone	0.117	-	79.1	53.5	0.68	Bell (1978)
	Fell sandstone	0.205	-	38.1	21.6	0.57	Bell (1978)
	Fell sandstone	0.096	-	108.9	98.6	0.91	Bell (1978)
	Fell sandstone	0.095	-	88.7	70.2	0.79	Bell (1978)
	Fell sandstone	0.104	-	90.2	71.2	0.79	Bell (1978)
	Fell sandstone	0.095	-	89.9	63.4	0.71	Bell (1978)
	Fell sandstone	0.094	-	51.1	33.5	0.66	Bell (1978)
	Fell sandstone	0.099	-	59.0	38.2	0.65	Bell (1978)
	Fell sandstone	0.101	-	91.7	62.7	0.68	Bell (1978)
	Fell sandstone	0.092	-	92.4	60.9	0.66	Bell (1978)
	Fell sandstone	0.072	-	112.4	97.2	0.87	Bell (1978)
	Fell sandstone	0.071	-	53.9	29.6	0.55	Bell (1978)
	Fell sandstone	0.076	-	75.2	62.0	0.82	Bell (1978)
	Fell sandstone	0.096	-	60.2	37.3	0.62	Bell (1978)
	Fell sandstone	0.101	-	52.3	30.6	0.59	Bell (1978)
	Fell sandstone	0.091	-	77.2	43.1	0.56	Bell (1978)
	Fell sandstone	0.078	-	55.7	42.7	0.77	Bell (1978)
	Fell sandstone	0.081	-	93.1	43.9	0.47	Bell (1978)
	Fell sandstone	0.069	-	107.2	98.4	0.92	Bell (1978)
	Fell sandstone	0.065	-	95.8	64.9	0.68	Bell (1978)
	Fell sandstone	0.087	-	80.5	50.8	0.63	Bell (1978)
	Pennant sandstone (10 -2.0 s -1 )	0.025	25	257.3	143.5	0.56	H & L (1991)
	Pennant sandstone (10 -2.0 s -1 )	0.025	25	263.7	151.9	0.58	H & L (1991)
	Pennant sandstone (10 -2.2 s -1 )	0.025	25	268.3	141.3	0.53	H & L (1991)
	Pennant sandstone (10 -2.2 s -1 )	0.025	25	259.0	136.2	0.53	H & L (1991)
	Pennant sandstone (10 -2.4 s -1 )	0.025	25	263.1	162.4	0.62	H & L (1991)
	Pennant sandstone (10 -2.4 s -1 )	0.025	25	247.1	151.8	0.62	H & L (1991)
	Pennant sandstone (10 -2.4 s -1 )	0.025	25	272.9	157.3	0.58	H & L (1991)
	Pennant sandstone (10 -2.6 s -1 )	0.025	25	267.7	154.7	0.58	H & L (1991)
	Pennant sandstone (10 -2.6 s -1 )	0.025	25	274.1	161.0	0.59	H & L (1991)
	Pennant sandstone (10 -2.9 s -1 )	0.025	25	275.7	156.4	0.57	H & L (1991)

Acknowledgements

This study was funded by LABEX grant ANR-11-LABX-0050_G-EAU-THERMIE-PROFONDE (this research therefore benefited from state funding managed by the Agence National de la Recherche (ANR) as part of the "Investissements d'avenir" program) and ANR grant CANTARE (ANR-15-CE06-0014-01). We are extremely grateful to Albert Genter, who provided the fracture data from EPS-1 (we therefore also acknowledge the Bureau de Recherches Géologiques et Minières (BRGM)). We also wish to thank Chrystal Dezayes, Bertrand Renaudié, Coralie Aichholzer, and Philippe Duringer. The comments of two anonymous reviewers helped clarify certain aspects of this manuscript.


Garima Garg

Stéphanie Foltran

Isabelle Favier

Daniel Pla

Yaocihuatl Medina-Gonzalez
email: yaocihuatl.medinagonzalez@ensiacet.fr.

Montserrat Gómez
email: gomez@chimie.ups-tlse.fr.

Yaocihuatl Medina-González

Palladium nanoparticles stabilized by novel choline-based ionic liquids in glycerol applied in hydrogenation reactions

Keywords: palladium nanoparticles, choline-based ionic liquids, amino acid, glycerol, hydrogenation __________________________________________________________________________________

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Palladium nanoparticles (PdNPs) due to their high efficiency, selectivity and their ability to perform various kinds of catalytic reactions have emerged as an important tool in organic synthesis. PdNPs are one of the most used and efficient catalysts to build C-C bonds and to perform other chemical transformations such as carbon-heteroatom bond formation, hydrogenation, carbonylation and oxidation processes [START_REF] Favier | Metallic Nanoparticles in Ionic Liquids -Applications in Catalysis[END_REF]. Well-defined PdNPs are appropriate catalysts for hydrogen activation and hydrogen spillover [2,3]. However, the choice of the right type of stabilizer is a key parameter for the synthesis of PdNPs precluding agglomeration and leaching of metal [START_REF] Kraynov | Concepts for the Stabilization of Metal Nanoparticles in Ionic Liquids[END_REF]. For this purpose, stabilizers like surfactants, ligands, polymers and ionic liquids have been widely used [5] to tailor PdNPs size, composition and morphology, thereby tuning the reactivity and selectivity of these systems [6]. In this context, ionic liquids (ILs) have emerged as promising contenders for the stabilization of nanoparticles. As mentioned by Dupont et al., ILs exhibit dispersive forces with polar and nonpolar nano-domains and can form extended networks of hydrogen bonds due to which they are to be considered as supramolecular fluids and not homogeneous solvents [7]. Imidazolium-based ionic liquids represent the most commonly used molten salts applied in catalysis, in particular for the stabilization of metal-based nanoparticles [START_REF] Xu | [END_REF]. Choline chloride, a quaternary ammonium salt, has also emerged as a popular choice for bio-sourced ILs and its use in synthesis of nanoparticles in the form of deep eutectic solvents (DESs) or otherwise have also been reported [START_REF] Renjith | For selected contributions, see: (a)[END_REF]. In our group, PdNPs stabilized by [BMI][PF6] have been generated in previous studies leading to high catalytic activity in Suzuki cross-coupling reaction without palladium leaching up to ten cycles [10,11]. In this frame, in particular, PdNPs stabilized by thioether-phosphine ligands had been used as catalytic precursors in Heck-Mizoroki cross-coupling and C=C hydrogenation reactions [12]. However, the above-mentioned ILs have environmental concerns, which consequently led to the search of an environmental friendly medium. In this context, our team has developed the synthesis of nanoparticles in glycerol medium [13][14]. Glycerol is produced in high amounts as waste in biodiesel production. Its properties like low-cost, non-toxicity, high boiling point (290 °C), negligible vapor pressure (< 1 mmHg at 293 K), high solubilizing ability for organic (except those completely apolar) and inorganic compounds, and low miscibility with other organic solvents such as ethers and alkanes, constitute striking properties that make it especially interesting for applications in catalysis. It plays an important role as support for the immobilization of PdNPs, permitting an easy recycling of the catalytic phase [13]. Our team has synthesized PdNPs in both solution and solid state, using naturally occurring cinchonabased alkaloids in neat glycerol and successfully applied in dihydrogen-based processes, such as hydrogenation of unsaturated functional groups (alkenes, alkynes, imines, and nitro-based substrates) and hydrodehalogenation of halo-aromatic compounds [14]. In order to extend our research in sustainable chemistry, we report here the synthesis of PdNPs using choline based ionic liquids in neat glycerol; the synthesized PdNPs were applied in hydrogenation reactions. Choline based ILs are environmentally friendly with good biocompatible properties and presenting interesting solvent properties: non-flammability, negligible vapor pressure at ambient conditions and high solubility [15]. Furthermore, we also avoid the plausible reactivity of ILs (such as in case of imidazolium-based ILs) with the metal surface [16].

Experimental

Materials and methods

Unless otherwise stated, all chemical reagents were obtained from commercial suppliers and used without further purification. All manipulations were performed using Schlenk techniques under argon atmosphere. Glycerol was dried under vacuum at 80 °C for 18 h prior to use. N-tosyl-alanine was prepared following previously reported methodology [17]. High-pressure reactions were carried out in a Top Industrie Autoclave. NMR spectra were recorded on a Bruker Advance 300 spectrometer at 293 K (300 MHz for 1 H NMR, 75.5 MHz for 13 C NMR and 50.6 MHz for 15 N). GC analyses were carried out on a GC Perkin Elmer Clarus 500 with ionization flame detector, using SGE BPX5 column composed by 5% phenylmethylsiloxane, coupled to a Perkin Elmer Clarus MS560 mass detector. TEM images of PdNPs dispersed in glycerol were obtained from a JEOL JEM 1400 instrument running at 120 kV. PdNPs size distributions and average diameters were determined from TEM images applying Image-J software associated to a Microsoft Excel macro. IR spectra were recorded in the range of 4000-400 cm -1 on a Varian 640-IR FTIR Spectrometer. Powder X-ray diffraction analyses were collected on a XPert (Theta-Theta mode) Panalytical diffractometer with α(Cu Kα1, Kα2)=1.54060, 1.54443 Å. Elemental and ICP-AES analyses were carried out at the "Service d'Analyse" of Laboratoire de Chimie de Coordination (Toulouse) using a Perkin Elmer 2400 series II analyser and an iCAP 6300 ICP Spectrometer. XPS measurements were performed at room temperature with a SPECS PHOIBOS 150 hemispherical analyzer (SPECS GmbH, Berlin, Germany) in a base pressure of 5x10 -10 mbar using monochromatic Al Kα radiation (1486.74 eV) as excitation source. Mass spectroscopy was done using UPLC Xevo G2 Q TOF (Waters) by high-resolution electrospray technique. Viscosity was measured using the Rheometer AR2000Ex from TA Instruments.

Synthesis of choline N-tosylalaninate (B)

Choline hydroxide (103 mg, 0.85 mol, 46 wt.% in H2O) was stirred with an aqueous solution of Ntosyl-alanine (206 mg, 0.85 mmol) at 60 °C for 12 h. After the ion exchange reaction, water was evaporated under vacuum at 80 °C, obtaining a brownish viscous oil (294.1 mg, 99.9%). B was obtained as a racemic mixture. 1 H NMR (300 MHz, DMSO-d6) 2H),2H),2H),1H),2H),9H),2.29 (s,3H),1.20 (d,J = 7.2 Hz,3H). 13 C NMR (75 MHz, DMSO-d6) δ 172. 6, 142.4, 137.8, 129.5, 126.6, 67.0, 67.0, 66.9, 55.1, 53.2, 53.1, 53.1, 52.1 20.9, 19.6. HR-MS (ESI+) for C5H14NO + : Theoretical = 104.1075, Experimental = 104.1072; HR-MS (ESI-) for C10H12NO4S ˉ: Theoretical = 242.0487, Experimental = 242.0483. IR (ATR, cm -1 ) 3256, 3045, 2954, 2922. 2866, 2853, 2427, 1718, 1598, 1456, 1397, 1324, 1221, 1093, 958, 884, 816, 708, 662. Viscosity 3054 Pa.s at 25 °C and 2.973 Pa.s at 80 °C.

Synthesis of palladium nanoparticles stabilized by choline-based derivatives in glycerol

0.05 mmol (14.1 mg) of [PdCl2(cod)] and 0.85 mmol of choline-based derivative [117 mg for choline chloride (A); 294.5 mg for choline N-tosylalaninate (B)] were dissolved in 5 mL of glycerol and stirred under argon in a Schlenk flask at 80 °C for 18 h. Then, the resulting solution was washed with dichloromethane (3x5 mL) in order to remove any insoluble IL from glycerol phase. A black colloidal solution was then obtained.

Synthesis of palladium nanoparticles stabilized by choline tosylalaninate at solid state, PdB

After synthesis, PdNPs in glycerol were transferred to a centrifugation tube and 2 mL of ethanol were added. Centrifugation was carried out at 4500 rpm for 1 h and then the solution was separated by decantation. This process was repeated 3 times until complete removal of glycerol. The remaining black powder was then dried under vacuum at 80 °C overnight. Elementary analysis (palladium content determined by ICP-AES) for PdB: Pd 84.0%, C 7.55%, N 0.30%, H 0.40%.

General procedure for Pd-catalyzed hydrogenation in glycerol

In a Fisher-Porter bottle (working from 1 to 3 bar total pressure) or an autoclave (working from 3 to 20 bar total pressure), the appropriate substrate (1 mmol for 1 mol% of catalyst or 10 mmol for 0.1 mol% of catalyst) was added to 1 mL of preformed nanoparticles (2.85 mg of Pd) in glycerol under argon. The reaction mixture was put under vacuum and then pressurized with H2 at the convenient pressure, heated up at 80 °C and stirred for the appropriate time; then cooled down to room temperature before extraction. Organic products were extracted from glycerol by a biphasic methodology, adding dichloromethane (5 × 3 mL); organic phases were collected and solvent removed under vacuum. Conversion and yields were determined by GC using decane as internal standard. The obtained products were characterized by GC-MS data and 1 H and 13 C NMR and compared to literature reports to confirm spectral identity (see Supplementary Material).

Results and discussion

Synthesis of palladium nanoparticles stabilized by choline-based ionic liquids in glycerol

Palladium nanoparticles (PdA, PdB) were prepared by thermal decomposition of [PdCl2(cod)] (cod = 1,5-cyclooctadiene) in the presence of a choline-based ionic liquid [choline chloride (A); choline Ntosylalaninate (B)], using glycerol as solvent (Scheme 1). The ionic liquid B was prepared based on the reported methodology for the preparation of choline carboxylate ionic liquids (see above, Experimental section) [18]. In both cases, black colloidal solutions were obtained, constituted by spherical small nanoparticles (for PdA, 1.4 ± 0.6 nm; for PdB, 1.7 ± 0.6 nm), exhibiting a very well-dispersion mainly thanks to the supramolecular structure of glycerol which avoids agglomeration, as evidenced in our previous works using ligands and polymers as stabilizers [13,14,19] (Fig. 1). It is important to highlight that these reaction conditions do not trigger any deprotection of choline N-tosylalaninate [20].

Scheme 1. Synthesis of palladium nanoparticles in glycerol using choline-based ionic liquids (A, B) as stabilizers.

Control tests proved that the water present on the IL (for A, 0.25%; for B, 1.70%; determined by Karl-Fischer titration) is the responsible of the reduction of Pd(II) into Pd(0); [PdCl2(cod)] in dry glycerol under the same conditions used in the synthesis of PdNPs was stable and did not exhibit decomposition. This is in agreement with the work carried out by Hirai and coworkers, proving the unsuccessful synthesis of zero-valent rhodium nanoparticles in anhydrous alcohols [21].

When PdNPs were synthesized in the absence of any stabilizer using wet glycerol and different palladium precursors, Pd(0) agglomerates were formed [13(d)]. Palladium acetate was also tested as starting metal precursor with the two choline derivatives, obtaining in both cases aggregates; for choline chloride, anisotropic nano-objects were observed probably due to the presence of halides at the metal surface (see Fig. S1 in the Supplementary material) [22]. We then added polyvinylpyrrolidone (mean molecular weight = 10000 g.mol -1 ), in order to improve the dispersion, but in this case very few nanoparticles could be detected by TEM (see Fig. S2 in the Supplementary material). Curiously, when dihydrogen was used as reducing agent, an immediate precipitation of bulk palladium took place.

We envisaged the use of choline alaninate as stabilizer (see experimental part and Scheme S1 in the Supplementary Material) [23], instead of the protected anion N-tosylalaninate; however, choline alaninate became unstable under atmospheric conditions, due to its high reactivity with carbon dioxide, giving the corresponding carbamate (see Scheme S2 in the Supplementary Material) [24]. Moreover, we also studied the effect of the solvent on the synthesis of colloidal PdNPs, using water and ethanol instead of glycerol under the same conditions than those described in Scheme 1. In both cases, fast precipitation of black palladium was observed, proving that glycerol avoids the agglomeration of metalbased nanoparticles probably due to its supramolecular structure [13(a), 25, 26].

Given the better catalytic behavior of PdB (see below Table 1), this catalyst was selected for full characterization. Palladium nanoparticles at solid state were isolated by centrifugation. Powder X-ray diffraction (PXRD) analysis showed the presence of crystalline nanoparticles exhibiting face cubic center Pd(0) structure (Fig. 2). The crystallite size found from the X-ray diffraction peaks (calculated by the Scherrer equation) is ca. 3.7 nm; differences in size between TEM and XRD are frequently observed because the average crystallite size (PXRD) is not necessarily the same as the particle size (TEM) [27]. XPS analysis corroborated the absence of any oxidized palladium species (Fig. 3 and Fig. S3 in the Supplementary Material); XPS showed the presence of carbon, oxygen and nitrogen that confirms the presence of the ionic liquid even after isolation of nanoparticles; the presence of chloride probably comes from the metal precursor. It is important to highlight that PdB at solid state could be re-dispersed in glycerol without formation of aggregates (see Fig. S4 in the Supplementary Material). This is in agreement with the XPS analysis that shows the presence of IL at the surface of PdNPs in solid state.

Pd-catalyzed hydrogenation reactions

The surface reactivity of palladium nanoparticles is very well known, in particular applied in hydrogenation processes involving different kinds of functions [START_REF]For selected contributions[END_REF]. The morphology effect PdNPs (size, shape) on reactivity is crucial [START_REF] Kraynov | Concepts for the Stabilization of Metal Nanoparticles in Ionic Liquids[END_REF]. PdNPs synthesized by wet methodologies contain stabilizers (salts, ligands, polymer, surfactants etc.) [29], which modify the surface state and in consequence their catalytic behavior [30]. In ionic liquid medium, many works have been reported in the literature, mainly focused on the hydrogenation of alkenes and alkynes [31], but only few of them concerning other functional groups such as carbonyls [32].

In the present study, we focused on sustainable ionic liquids from choline, due to the stabilization of PdNPs essentially by electrostatic effect, in glycerol, which helps to the dispersion of nanoparticles. We chose the hydrogenation of 4-phenylbut-3-en-2-one as benchmark reaction. Both catalysts, PdA and PdB, behave likewise, giving exclusively 4-phenylbutan-2-one (entries 1-2, Table 1), interesting skeleton present in fragrances [33]. Both systems were also highly active at low Pd load (0.1 mol%, entries 3-4, Table 1). However, at shorter times (1 h reaction), only PdB preserved the activity (entry 5 vs 6, Table 1), probably due to the non-innocent effect of the chloride anions present in PdA, which can trigger a poison effect due to the adsorption of chloride ions at the surface [32]. PdA 55 (51) a Results from duplicated experiments. Reaction conditions: 1 mmol of 1 and 1 mL of the corresponding catalytic glycerol solution of PdNPs (10 -2 mol L -1 , 0.01 mmol of total Pd). b Determined by GC and GC/MS using decane as internal standard. c 0.1 mol% Pd load. d Reaction time 1 h.

With the aim of carrying out a scope related to functions to be reduced by PdB we firstly analyzed the behavior of PdB in the hydrogenation of alkenes and alkynes (Table 2). The terminal non-substituted alkene 1-dodecene ( 2) was converted into dodecane and internal alkenes in a ratio of 54/46 respectively under smooth conditions (3 bar H2 and 2 h of reaction; entry 1, Table 2). Under harsher conditions (20 bar H2, 18 h), (-)-β-pinene (3) was mainly isomerized, giving only 18% of hydrogenated product (entry 2, Table 2); PdB was not active enough to reduce (+)--pinene (entry 3, Table 2). However, for conjugated C=C bonds (alkene 5), full conversion was achieved under 3 bar H2 overnight (entry 4, Table 2). In agreement with this behavior, the extended conjugated substrate 6 was fully hydrogenated concerning the non-aromatic C=C bonds (entry 5, Table 2). The more sterically hindered conjugated substrate 1,2,3,4,5-pentamethylcyclopentadiene (7) allowed the selective reduction of one of the two endocyclic C=C bonds working at 20 bar of H2 pressure (entry 6, Table 2).

Internal aromatic alkynes [START_REF] Xu | [END_REF][START_REF] Renjith | For selected contributions, see: (a)[END_REF] were fully reduced obtaining the corresponding alkane derivatives (entries 7-8, Table 2). At lower H2 pressure (entry 9, Table 2), only the formation of the corresponding Z-alkene was observed, giving up to ca. 50% yield; longer reaction times led to the formation of 1,2diphenylethane. However, the internal alkyl-substituted alkyne, 5-decyne (10; entry 10, Table 2) gave mainly the corresponding internal alkene; at longer time (18 h), full hydrogenation was observed (entry 11, Table 2). We were also interested in the hydrogenation of other organic functional groups, such as nitro and carbonyl derivatives (Table 3). As expected, nitrobenzene gave aniline in quantitative yield (entry 1, Table 3). PdB was also an efficient catalyst for the reduction of carbonyl groups, coming from both aldehydes (12)(13)(14) and ketones (15)(16)(17). Concerning the formyl groups, benzaldehyde derivatives exhibiting substituents inducing different electronic effects, were fully converted into the corresponding benzyl alcohols under smooth conditions (3 bar H2; entries 2-4, Table 3). Aromatic ketones 15 and 16 were hydrogenated to give the corresponding secondary alcohols, but under higher hydrogen pressure (entries 5-6, Table 3). The non-conjugated ketone (1H) gave a moderate conversion to the corresponding secondary alcohol (1I) at 40 bar H2 pressure (entry 7, Table 3). The 1-phenylprop-2-yn-1-one ( 17) mainly gave the saturated ketone (17H) with only 15% of the corresponding secondary alcohol (17I) (entry 8, Table 3). This reactivity behavior concerning the reduction of carbonyl groups follows the same trend than that reported recently by J. Dupont and co-workers using supported palladium nanoparticles on poly (ionic liquid) materials based on pyrrolidinium salts [32(a)].

Alkyl ketones (such as 3-pentanone and 2,2,4,4-tetramethyl pent-3-one) and alkyl aldehydes (like hexanal) were not reduced (up to 20 bar H2). Similarly, benzonitrile was not hydrogenated under harsh conditions.

After catalysis, no leaching of Pd was detected by ICP-AES analyses. But, some aggregation was observed by TEM preserving the zero-valent PdNPs (see Figs. S5 and S6 in the Supplementary Material); the formation of aggregates may be the reason for the observed decrease of the catalytic activity and hence it was difficult to use the PdNPs for an efficient recycling (see Fig. S7 in the Supplementary Material).

Conclusions

To sum up, we efficiently prepared and fully characterized new palladium nanocatalysts, stabilized by environmentally friendly choline-based ionic liquids in glycerol. The counter anion of the choline derivative showed a non-innocent effect on the catalytic activity, probably due to the higher adsorption of chloride than tosyl-alalinate at the metallic surface [34], and consequently hindering the interaction of reagents with palladium.

PdB was active for the hydrogenation of C-C multiple bonds and also for C=O bond of aldehydes and ketones, being more active for conjugated substrates. For non-conjugated substrates, such as internal alkyl alkynes, PdB was highly selective to the formation of the corresponding alkene. The organic products extracted from the catalytic glycerol phase did not contain palladium according to ICP-AES analyses, proving the efficient immobilization of PdNPs in the glycerol phase.

Based on these results, the immobilization of the catalytic glycerol phase on solid supports in order to improve the recyclability is under study.

Figure 1 .

 1 Figure 1. TEM images of PdA and PdB in glycerol with the corresponding size distribution diagrams.

Figure 2 .

 2 Figure 2. PXRD diffractogram of PdB at solid state (sharp blue lines correspond to the diffraction pattern of bulk fcc Pd(0)).

Figure 3 .

 3 Figure 3. HR-XPS spectra of PdB at solid state for (a) Pd(0), (b) carbon, (c) oxygen and (d) nitrogen.

a

  Results from duplicated experiments. Reaction conditions: 1 mmol of substrate (2-10) and 1 mL of the catalytic glycerol solution of PdB (10 -2 mol L -1 , 0.01 mmol of total Pd); n.r. means no reaction; n.d. means not determined. b Determined by GC and GC/MS using decane as internal standard. c Dodecane/internal alkenes = 54/46. d Dodecane/internal alkenes = 55/45. e Ratio 3/3G/3H = 12/70/18. f Ratio 3/3G/3H = 35/43/22. g Isolated yield. h Cyclooctane as internal standard. i cis-5-decene/n-decane = 90/10.

Table 1

 1 Hydrogenation of 4-phenylbut-3-en-2-one catalyzed by PdNPs in glycerol stabilized by ChCl (A) and ChTsAla (B). a

	Entry	Catalyst	Conv. (yield) b (%)
	1	PdA	100 (96)
	2	PdB	100 (93)
	3 c	PdA	50 (49)
	4 c	PdB	100 (91)
	5 c,d	PdB	96 (92)
	6 c,d

Table 2

 2 Hydrogenation of alkenes and alkynes catalyzed by PdB in glycerol. a

	Entry	Substrate	Product	pH2 (bar)	Conv. (yield) b (%)
	1	2	2H	3 20	100 c 100 d

Table 3

 3 Hydrogenation of nitro and carbonyl groups catalyzed by PdB in glycerol. a

	Entry	R1	R2	Product	Conv. (yield) b (%)
	1 c, d	NO2 (11)	H		100 (99)
				11H
	2	CHO (12)	H		98 (95)
				12H
	3	CHO (13)	Br		99 (97)
				13H
	4	CHO (14)	C(CH3)3
				14H

a Results from duplicated experiments. Reaction conditions: 1 mmol of substrate (1H,

12-18) and

  1 mL of the catalytic glycerol solution of PdB (10 -2 mol L -1 , 0.01 mmol of total Pd). b Determined by GC and GC/MS using decane as internal standard.

c H2 pressure 10 bar. d Reaction for 2 h. e H2 pressure 20 bar. f Isolated yield. g H2 pressure 40 bar. h 17H/17I ratio.

Acknowledgements

Université Fédérale de Toulouse (program IDEX Recherche -Actions Thématiques Stratégiques) and Centre National de la Recherche Scientifique (CNRS) are gratefully acknowledged for their financial support. Authors thank C. Pradel for his contribution in TEM analyses. G. G. thanks Université Fédérale de Toulouse for PhD scholarship.