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Abstract. In this contribution, we present a review of scientific research results that address seismo-hydromechanically coupled processes relevant for the development of a sustainable heat exchanger in low-permeability crystalline rock and introduce the design of the In situ Stimulation and Circulation (ISC) experiment at the Grimsel Test Site dedicated to studying such processes under controlled conditions. The review shows that research on reservoir stimulation for deep geothermal energy exploitation has been largely based on laboratory observations, large-scale projects and numerical models. Observations of full-scale reservoir stimulations have yielded important results. However, the limited access to the reservoir and limitations in the control on the experimental conditions during deep reservoir stimulations is insufficient to resolve the details of the hydromechanical processes that would enhance process understanding in a way that aids future stimulation design. Small-scale laboratory experiments provide fundamental insights into various processes relevant for enhanced geothermal energy, but suffer from (1) difficulties and uncertainties in upscaling the results to the field scale and (2) relatively homogeneous material and stress conditions that lead to an oversimplistic fracture flow and/or hydraulic fracture propagation behavior that is not representative of a heterogeneous reservoir. Thus, there is a need for intermediate-scale hydraulic stimulation experiments with high experimental control that bridge the various scales and for which access to the target rock mass with a comprehensive monitoring system is possible. The ISC experiment is designed to address open research questions in a naturally fractured and faulted crystalline rock mass at the Grimsel Test Site (Switzerland). Two hydraulic injection phases were executed to enhance the permeability of the rock mass. During the injection phases the rock mass deformation across fractures and within intact rock, the pore pressure distribution and propagation, and the microseismic response were monitored at a high spatial and temporal resolution.

SUMMARY Digital rock physics combines modern microscopic imaging with advanced numerical simulations to analyse the physical properties of rocks. Elastic‐wave propagation modelling based on the microstructure images is used to estimate the effective elastic properties of the rock. The goal of this paper is to describe and understand how laboratory experiments compare with digital rock physics results using Berea sandstone. We experimentally measure pressure‐dependent ultrasonic velocities and the pore size distribution. The effective elastic properties resulting from numerical simulations are based on microcomputed tomography (micro‐CT) images, which are systematically stiffer than the laboratory measures. Because the tomographic images do not resolve the small‐scale pore and crack network of the sample, we hypothesize that the numerical overprediction is attributable to the smallest pores and grain‐to‐grain contacts that are missing in the images. To reconcile the difference between numerical and experimental data, we suggest to use a grain boundary reconstruction algorithm. This allows to implement and approximate so far unresolved features in the virtual rock model. As a result, we can predict pressure‐dependent effective velocity using micro‐CT images.

9The application of clumped isotopes ( 47 ) in carbonate minerals as a sensitive temperature proxy in 10 paleo-environments depends on a well-constrained clumped isotope fractionation for the necessary 11 step of phosphoric acid digestion of the carbonate mineral to produce CO 2 . Published estimates for 12 clumped isotope fractionations vary, and the effect of different carbonate mineralogies is still under 13 debate. Differences in the sample preparation design and sample digestion temperatures are 14 potential sources for varying acid fractionations and could be a source for discrepant  47 -15 temperature calibrations observed in different laboratories. To evaluate the clumped isotope acid 16 fractionation at 70 °C and simultaneously account for a potential cation effect we analyzed a set of 17 eight carbonate minerals (calcite, aragonite, dolomite and magnesite) that were driven to a 18 stochastic isotope distribution by heating them to temperatures of 1000 °C. Our study reveals 19 significant cation-and mineral-specific differences for the  47 acid fractionation of carbonate 20 minerals digested at 70 °C or 100 °C. The  47 acid fractionation at 70 °C for calcite is 0.197±0.002 ‰, 21 for aragonite 0.172±0.003 ‰, whereas dolomite has a significantly larger acid fractionation of 22 0.226±0.002 ‰. For magnesite digested at 100 °C we observed a  47 acid fractionation of 23 0.218±0.020 ‰. Projected to an acid digestion at 25 °C, our acid fractionation for calcite of 0.260 ‰ 24 is statistically indistinguishable from existing studies. We further show that the  47 of the calcite 25 standards ETH-1 and ETH-2 of 0.265 ‰ and 0.267 ‰, respectively, are in the range of the 26 determined acid fractionation projected to 25 °C suggesting that they have an identical and near 27 stochastic isotope distribution. The observed differences in the  47 acid fractionation between calcite 28 and aragonite ( 47 = -0.025 ‰) and between calcite and dolomite ( 47 = -0.029 ‰) does not 29 correlate with the phosphoric acid fractionation of oxygen isotopes, but rather depends on the radius 30 of the cation as well as on the mineral structure. Our results reveal that the acid fractionation of 31 dolomite at 70 °C is significantly distinct from the one of calcite, but at 90 °C the two are within error 32 © 2016. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/ with M being the metal cation of the corresponding carbonate mineral (e.g. calcite and aragonite 52 (CaCO 3 ), dolomite (CaMg(CO 3 ) 2 ), magnesite (MgCO 3 ), siderite (FeCO 3 ), witherite (BaCO 3 )). Having 53 attained equilibrium this exchange reaction (Eq. 1) is thermodynamically controlled and produces an 54 excess in the doubly substituted isotopologue relative to a stochastic isotope distribution (0.5 ‰ at 0 55 °C), which decreases with increasing temperature and approaches a stochastic isotope composition 56 at 1000 °C (Schauble et al., 2006). Ghosh et al. (2006a) demonstrated the feasibility...

Synchrotron radiation X-ray tomographic microscopy is a nondestructive method providing ultra-high-resolution 3D digital images of rock microstructures. We describe this method and, to demonstrate its wide applicability, we present 3D images of very different rock types: Berea sandstone, Fontainebleau sandstone, dolomite, calcitic dolomite, and three-phase magmatic glasses. For some samples, full and partial saturation scenarios are considered using oil, water, and air. The rock images precisely reveal the 3D rock microstructure, the pore space morphology, and the interfaces between fluids saturating the same pore. We provide the raw image data sets as online supplementary material, along with laboratory data describing the rock properties. By making these data sets available to other research groups, we aim to stimulate work based on digital rock images of high quality and high resolution. We also discuss and suggest possible applications and research directions that can be pursued on the basis of our data.

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