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In the international energy transition, geothermal energy plays a role as a renewable energy which almost does not emit CO2 and allows the reduction of dependency on fossil fuels. Geothermal energy in a crystalline basement is part of the different deep geothermal sources which can be used for heating or electricity production. In 2018, the heating generated from deep geothermal plants was up to 2.6 Twh in France and is planned to increase. For the upcoming 2030, the objective in France is to have 38% of renewable energy in the final heat consumption, where geothermal energy has its place (French energy transition law for green growth n°2015-992). Geothermal production in crystalline basement relies on the presence of fractures and sometimes stimulation techniques. In France, such exploited sites are located in Alsace (Upper Rhine Graben, URG) where a hot brine is harvested in the granitic basement to produce electricity (Soultz-sous-Forêts) or heat (Rittershoffen). However, recent unexpected seismic events occurred during the stimulation of new sites and lead to stop deep geothermal exploration in Alsace. For the resource development of geothermal reservoirs, the understanding of the properties of fractures and fracture networks has to be investigated. Uniaxial compression strength, triaxial compression strength and brazilian disk tests measurements were performed on samples collected along a fault zone from Schauinsland mine in the Black Forest. These samples are considered as analogues of the deep geothermal reservoir of Soultz-sous-Forêts in the URG. A decrease of the tensile strength and elastic modulus was measured along the fault zone while the cohesion value is much higher than expected in the core zone. The Poisson ratio is the highest in the damage zone which indicates that the samples are heterogeneous and need to be investigated at a smaller scale.

Hydrothermal alteration is a common process in active geothermal systems and can significantly change the physiochemical properties of rocks. To improve reservoir assessment and modeling of high-temperature geothermal resources linked to active volcanic settings, a detailed understanding of the reservoir is needed. The Los Humeros Volcanic Complex, hosting the third largest exploited geothermal field in Mexico, represents a natural laboratory to investigate the impact of hydrothermal processes on the rock properties through andesitic reservoir cores and outcropping analogs. Complementary petrographic and chemical analyses were used to characterize the intensities and facies of hydrothermal alteration. The alteration varies from argillic and propylitic facies characterized by no significant changes of the REE budget indicating an inert behavior to silicic facies and skarn instead showing highly variable REE contents. Unaltered outcrop samples predominantly feature low matrix permeabilities ( 1.67 W m(-1) K-1; > 0.91 10(-6) m(2) s(-1)), but a significant loss of magnetic susceptibility (10(-3)-10(-6) SI). In particular, this latter characteristic appears to be a suitable indicator during geophysical survey for the identification of hydrothermalized domains and possible pathways for fluids. The lack of clear trends between alteration facies, alteration intensity, and chemical indices in the studied samples is interpreted as the response to multiple and/or repeated hydrothermal events. Finally, the proposed integrated field-based approach shows the capability to unravel the complexity of geothermal reservoir rocks in active volcanic settings.

Fracture network is a crucial element to address in any model of the thermo-hydro-mechanical behaviour of a reservoir rock. This study aims to provide quantified datasets and a further understanding of the critical parameters of the fracture network pattern in crystalline rocks. In the Northern Upper Rhine Graben, such rock units are targeted for multiple energy applications, from deep geothermal heat extraction to heat storage. Eleven outcrops were investigated with a combined LiDAR and 2D profiles analysis to extract faults and fracture network geometrical parameters, including length distribution, orientation, connectivity, and topology. These properties are used to decipher the structural architecture and estimate the flow properties of crystalline units. Fracture networks show a multi-scale power-law behaviour for length distribution. Fracture topology and orientation are mainly driven by both fault networks and lithology. Fracture apertures and permeability tensors were then calculated for two application case studies, including the stress field effect on aperture. Obtained permeabilities are in the range of those observed in the sub-surface in currently exploited reservoirs. The dataset provided in this study is thus suitable to be implemented in the modelling during the exploration stage of industrial applications involving fractured crystalline reservoirs.

The crystalline basement is considered a ubiquitous and almost inexhaustible source of geothermal energy in the Upper Rhine Graben (URG) and other regions worldwide. The hydraulic properties of the basement, which are one of the key factors in the productivity of geothermal power plants, are primarily controlled by hydraulically active faults and fractures. While the most accurate in situ information about the general fracture network is obtained from image logs of deep boreholes, such data are generally sparse and costly and thus often not openly accessible. To circumvent this problem, an outcrop analogue study was conducted with interdisciplinary geoscientific methods in the Tromm Granite, located in the southern Odenwald at the northeastern margin of the URG. Using light detection and ranging (lidar) scanning, the key characteristics of the fracture network were extracted in a total of five outcrops; these were additionally complemented by lineament analysis of two different digital elevation models (DEMs). Based on this, discrete fracture network (DFN) models were developed to calculate equivalent permeability tensors under assumed reservoir conditions. The influences of different parameters, such as fracture orientation, density, aperture and mineralization, were investigated. In addition, extensive gravity and radon measurements were carried out in the study area, allowing fault zones with naturally increased porosity and permeability to be mapped. Gravity anomalies served as input data for a stochastic density inversion, through which areas of potentially increased open porosity were identified. A laterally heterogeneous fracture network characterizes the Tromm Granite, with the highest natural permeabilities expected at the pluton margin, due to the influence of large shear and fault zones.

Porosity and permeability alteration due to the thermo-poro-elastic stress field disturbance from the cold fluid injection is a deciding factor for longer, more economic, and safer heat extraction from an enhanced geothermal system (EGS). In the Soultz-sous-Forêts geothermal system, faulted zones are the main flow paths, and the resulting porosity–permeability development over time due to stress reorientation is more sensitive in comparison with the regions without faulted zones. Available operational and field data are combined through a validated numerical simulation model to examine the mechanical impact on the pressure and temperature evolution. Results shows that near the injection wellbore zones, permeability and porosity values are strongly affected by stress field changes, and that permeability changes will affect the overall temperature and pressure of the system, demonstrating a fully coupled phenomenon. In some regions inside the faulted zones and close to injection wellbores, porosity doubles, whereas permeability may be enhanced up to 30 times. A sensitivity analysis is performed using two parameters which are not well discussed in the literature the for mechanical aspect, but the results in this study show that one of them impacts significantly on the porosity–permeability changes. Further experimental and field works on this parameter will help to model the heat extraction more precisely than before.

An experimental investigation was implemented to explore the mechanical parameters of the Odenwald reservoir granitoids. The specimen within this research project was the Odenwald Granodiorite (ODG) which was extracted from the quarry on Bergstrasse in Heppenheim, Germany. The study enfolds fundamental mechanical features from intact cylindrical rock specimens. The ODG was extracted from an active quarry and the extracted blocks were drilled into cylindrical samples. Thereafter, the samples were operated on the Mechanical Testing System (MTS) from the GFZ in Potsdam. This offers the possibility to test the rock strength, including tensile and compressive strength values, of different kind of rock materials. Fundamental mechanical analysis including Brazilian Disk Test (BD), Uniaxial Compressive Strength Test (UCS) and Triaxial Compression Strength Test (Triax) were part of this experimental investigation. Firstly, the BD was executed to achieve the tensile strength (T0) at different loading rates (0.001 mm/s and 0.0003 mm/s) as well as the fracture toughness mode I (KIC) after Guo from 1993. These samples have a size of 50 mm in diameter and 25 mm in length. Secondly, the UCS resulted in the Poisson’s ratio (ѵ), the static Young’s modulus (E) and the maximum uniaxial compressive strength (Co). Thirdly, the Triax was done under saturated and dry conditions. Hereby, the maximum compressive strength (σ1), the compressibility (C) and the Biot coefficient were calculated from raw data. The Triax experiments have four different setups. Three different confining pressure were executed at different specimens, 20 MPa, 40 MPa and 60 MPa. Additionally, a multiple failure test was executed including all three confining pressure steps. Both experimental setups, UCS and Triax, have samples of the size of 50 mm in diameter and 100 mm in length. All these samples were dried at 60°C for at least 24h and only some Triax experiments were done under saturated conditions. The data publication includes the raw data files from the experiment as well as files with calculated results for the above mentioned different mechanical parameters. These files are either given as DAT files or Microsoft Excel sheets summarized in a Zip folder. For further details, the full description of the data and methods is provided in the data description file. The samples used in this data publication are assigned with International Generic Sample Numbers (IGSN). These IGSN numbers can be resolved with https://igsn.org/[igsn_number] and link to the sample description in the internet.

Petrophysical characterization of the Los Humeros geothermal field (Mexico): from outcrop to parametrization of a 3D geological model Leandra M. Weydt1* , Kristian Bär1 and Ingo Sass1,2 Abstract The Los Humeros Volcanic Complex has been characterized as a suitable target for developing a super-hot geothermal system (> 350 °C). For the interpretation of geo- physical data, the development and parametrization of numerical geological models, an extensive outcrop analogue study was performed to characterize all relevant key units from the basement to the cap rock regarding their petrophysical properties, mineralogy, and geochemistry. In total, 226 samples were collected and analyzed for petrophysical and thermophysical properties as well as sonic wave velocities and mag- netic susceptibility. An extensive rock property database was created and more than 20 lithostratigraphic units and subunits with distinct properties were defined. Thereby, the basement rocks feature low matrix porosities (< 5%) and permeabilities (< 10–17 m 2 ), but high thermal conductivities (2–5 W m−1 K−1 ) and diffusivities (≤ 4·10–6 m2 s−1 ) as well as high sonic wave velocities (≥ 5800 m s−1 ). Basaltic to dacitic lavas feature matrix porosities and permeabilities in the range of < 2–30% and 10–18 –10–14 m2 , respectively, as well as intermediate to low thermal properties and sonic wave velocities. The pyro- clastic rocks show the highest variability with respect to bulk density, matrix porosity (~ 4– > 60%) and permeability (10 –18 –10–13 m2 ), but feature overall very low thermal conductivities (< 0.5 W m−1 K−1 ) and sonic wave velocities (~ 1500–2400 m s−1 ). Specific heat capacity shows comparatively small variations throughout the dataset (~ 700–880 J kg−1 K−1 ), while magnetic susceptibility varies over more than four orders of magnitude showing formation-related trends (10–6 –10–1 SI). By applying empirical correction functions, this study provides a full physiochemical characterization of the Los Humeros geothermal field and improves the understanding of the hydraulic and thermomechanical behavior of target formations in super-hot geothermal systems related to volcanic settings, the relationships between different rock properties, and their probability, whose understanding is crucial for the parametrization of 3D geologcal models.

Deep geothermal energy represents a key element of future renewable energy production due to its base load capability and the almost inexhaustible resource base. Especially with regard to heat supply, this technology offers a huge potential for carbon saving. One of the main targets of geothermal projects in Central Europe is the Upper Rhine Graben, which exhibits elevated subsurface temperatures and reservoirs with favorable hydraulic properties. Several decades of intensive research in the region resulted in a comprehensive understanding of the geological situation. This review study summarizes the findings relevant to deep geothermal projects and thus provides a useful working and decision-making basis for stakeholders. A total of nine geological units have been identified that are suitable for deep geothermal exploitation, comprising the crystalline basement, various sandstone formations and Mesozoic carbonates. An extensive lithostratigraphic, structural, geochemical, hydraulic and petrophysical characterization is given for each of these potential reservoirs. This paper furthermore provides an overview of the available data and geological as well as temperature models.

Assessing resources of enhanced geothermal (EGS) or medium deep geothermal systems (MDGS) for direct heat use and underground thermal energy storage (UTES) is a challenging task where usually diverse data sets of multiple origin and scale have to be compiled to obtain a comprehensive conceptual model of the subsurface, its structure and its properties. Within the research project “Hessen 3D 2.0” (BMWI-FKZ: 0325944), which aims to enhance the assessment of the prospective risk (‚Fündigkeitsrisiko’) for these kinds of geothermal projects, we established a workflow to implement and analyse such broad data sets. In a first step, comprehensive datasets of physical rock-, fluid- and reservoir properties are compiled which are based on investigations on relevant reservoir analogues, hydraulic test data from boreholes and borehole geophysical logs. The second step comprises the development of 3D geological models from a combination of borehole data, geological cross sections, seismic profiles, gravity and geomagnetic anomalies and geological maps to achieve the required detail on subsurface structure. This is prerequisite to distinguish the potentially usable reservoir units both within the crystalline or metamorphic basement and the sedimentary cover. Geostatistical analysis of the acquired comprehensive geothermal database is performed in a third step of the workflow; this allows for a parametrization of the geological model, for thermohydraulic subsurface modelling, and finally for the geothermal resource assessment. Such models, which consider the variability of rock and reservoir and fluid properties provide a thorough understanding of the subsurface temperature distribution, the dominant heat transport processes and hydraulic conditions. Finally, under consideration of both technical and economic boundary conditions and the statistics for the different relevant reservoir properties of the different geological units, assessment of hydrothermal, petrothermal and UTES potentials is performed directly with the 3D model. Therefore, a multiple-criteria approach, which assesses the quality of various rock and reservoir properties and their relevance for the different geothermal utilizations is implemented. This 3D-grid based method can be used for an identification and visualization of different geopotentials using various parameters to determine each potential. Thereby, to specify the grade of each potential under technical and economic requirements, threshold values for each parameter are defined. The approach described here allows for a stochastic assessment of the geothermal resources of a particular site of interest, including the determination of the probability of success and it provides the necessary numbers to attract investors to geothermal projects.

A successful utilization of deep geothermal resources requires accurate predictions about the reservoir tem-perature distribution as well as an in depth knowledge of the hydraulic processes. The latter exert a direct in-fluence on the subsurface temperature and therefore on the potential productivity of geothermal reservoirs.The aim of this study is to investigate and quantify the influence that regional thermo-hydraulic processes exert on the geothermal configuration of potential reservoirs in the northern Upper Rhine Graben and ad-jacent areas. Specifically, we address the questions of (i) how the regional thermal and hydraulic configura-tions influence local reservoir conditions, and (ii) whether it is possible to improve subsurface predictions iteratively by relying on 3D numerical modeling techniques. Therefore, a highly detailed 3D structural model of the northern part of the Upper Rhine Graben and the adjacent areas was built. The model was then used for coupled 3D thermo-hydraulic simulations of the deep fluid flow and heat transport. By running different simulations, we systematically tested hypotheses for the regional fluid dynamics based on hydro-geochemi-cal analysis results. Springs, mineral and thermal, have been used for the validation process of the flow field and available temperature measurements for the validation of the thermal field. With this knowledge of the thermo-hydraulic mechanisms of heat transport, the next step will be to downscale this process understand-ing and implement it in predictions of geothermal potentials on a local scale for the urban area of Frankfurt am Main

A successful utilization of deep geothermal resources requires to make accurate predictions about the reservoirtemperature distribution as well as an in depth knowledge of the hydraulic processes exerting a direct influence onthe subsurface temperature distribution and therefore on the productivity of geothermal reservoirs.The aim of this study is to investigate and quantify the influence that regional thermo-hydraulic processes exert onthe geothermal configuration of potential reservoirs in the German federal state Hesse. Specifically, we addressthe question of how the regional thermal and hydraulic configuration influences the local reservoir conditionsand whether it is possible to improve subsurface predictions iteratively by relying on 3D numerical modelingtechniques. Therefore, a 3D structural model of Hesse is used as a basis for coupled 3D thermo hydraulicsimulations of the deep fluid and heat transport. To uncover the effects of process coupling, a stepwise workflow isfollowed. We first simulate the thermal and hydraulic field under steady-state conditions by means of two differentuncoupled simulations and then analyze the results of the coupled thermo-hydraulic steady-state simulations. Ina last effort, we investigate the influence of fluid viscosity and density varying with temperature and pressure intransient coupled simulations.As a result of our numerical simulations, Hesse can be differentiated into sub-areas differing in terms of the domi-nating heat transport processes. In a final attempt to quantify the robustness and reliability of the modeling results,we carry out an analysis of the modelling outcomes by comparing them to available subsurface temperature data.Modelled temperatures show different levels of fit with locally measured well temperatures. These differences inmodel fit indicate the need for either structurally refined models and/or iterative adaptions within realistic rangesof the hydraulic and thermal properties. Structural refinements can often only be handled with smaller-scalemodels, which will, in turn, benefit from the boundary conditions and improved process understanding as derivedfrom the regional modelling approach presented here.

Die tatsächliche Nutzung petrothermaler Reservoire zur Strom- und Wärmegewinnung ist in Europa nach wie vor im Wesentlichen auf Soultz-sous-Forêts beschränkt, während einige weitere Forschungsprojekte in der Planungsphase sind. Bisherigen Potenzialermittlungen zufolge liegen jedoch mehr als 90 % des geothermischen Potenzials Deutschlands in genau diesen Reservoirsystemen. Im Verbundprojekt „Hessen 3D 2.0“ (BMWi-FKZ: 0325944) werden im Teilprojekt I petrothermale Potenziale von Hessen untersucht, quantifiziert und in einem geologischen -geothermischen 3D-Modell visualisiert. Das hierfür notwendige 3D-Strukturmodell ist mit Schichtenverzeichnissen der hessischen Bohrdatenbank des Hessischen Landesamtes für Naturschutz, Umwelt und Geologie (HLNUG) sowie geologischen Schnitten und Karten erstellt worden. Zur Definition der geothermischen Modelleinheiten sind die im hessischen Grundgebirge auftretenden Gesteinstypen mittels petrophysikalischer Analysen charakterisiert und durch eine statistische Datenanalyse klassifiziert worden. Im kristallinen Grundgebirgsteil konnten so die Modelleinheiten (1) Granit, (2) Granodiorit, (3) Gneis sowie (4) Diorit und Gabbro unterschieden werden. Diese Modelleinheiten sind im 3D-Modell als eigenstände Volumina abgebildet und mit den ermittelten petrophysikalischen Gesteinseigenschaften druck- und temperaturabhängig parametrisiert. Wo Bohrlochdaten, geologische Karten und Schnitte räumlich begrenzte Einblicke in den Untergrund bieten, stellt die Analyse des gemessenen Schwerefelds insbesondere für tiefere Krustenbereiche eine ergänzende oder auch validierende Methode dar. Mit den neu erhobenen Schwere-messungen der Hessischen Verwaltung für Bodenmanagement und Geoinformation (HVBG) liegt uns ein Datensatz vor, mit dem wir Dichteheterogenitäten mittels 3D gravimetrischer Vorwärtsmodellierung (Software IGMAS+) abbilden und hinsichtlich vorhandener Lithologie-unterschiede und Störungen interpretieren können. Mit diesem Beitrag werden methodische Abläufe und Ergebnisse der geologischen 3D-Struktur - sowie Gravimetrie-modellierung, die die Basis der Potenzialabschätzung darstellen, vorgestellt.

Die mitteltiefen und petrothermalen Potenziale für geothermische Stromerzeugung, Direktwärmenutzung und saisonale Wärmespeicherung von Hessen werden derzeit im Rahmen des Projektes “Hessen 3D 2.0” untersucht. Ziel ist die Ausweisung von Gebieten in denen eine geothermische Nutzung wirtschaftlich erfolgsversprechend ist und in denen geringe Fündigkeitsrisiken zu erwarten sind. Als Grundlage für die Potenzialbewertung werden zahlreiche bestehende Datensätze zusammengetragen, miteinander verschnitten und durch neue eigene Untersuchungen ergänzt. In einem ersten Schritt werden die petrophysikalischen, thermischen und mechanischen Eigenschaften der geothermisch relevanten Formationen sowie der Reservoirfluide ermittelt und in Datenbankanwendungen dokumentiert, vereinheitlicht und für die weitere Auswertung vorgehalten. Dies beinhaltet Labormessungen an Aufschlussanalog- und Bohrkernproben, hydraulische Bohrlochtestdaten sowie bohrlochgeophysikalische Untersuchungen. Als zweiter Schritt werden auf Basis digitalisierter Bohrdaten, geologischer Profilschnitte, interpretierter seismischer Profile sowie geologischer Kartenwerke die bestehenden 3D-Modelle verbessert und weiter untergliedert, um nutzungsbezogen geothermische Modelleinheiten, die für petrothermale Systeme, Wärmespeicherung oder Direktwärmenutzung geeignet sind, zu unterscheiden. Diese Modelleinheiten werden anhand der umfänglichen Datenbank geothermischer Gesteins- und Reservoireigenschaften attributiert, um als Basis für die Potenzialausweisung oder thermohydraulischen Modellierungen zu dienen. Die thermohydraulischen Modellierungen unter Berücksichtigung der Variabilität der Gesteins- und Gebirgseigenschaften sowie realistischer Randbedingungen sollen zur verbesserten Prognose des Untergrundtemperaturfeldes beitragen sowie ein besseres Verständnis der regional dominierenden Wärmetransportprozesse (konduktiv oder konvektiv) ermöglichen. Insbesondere in Regionen für die nur wenige Temperaturmessungen aus tiefen Bohrungen vorliegen, ist eine deutlich bessere Temperaturprognose sowie ein deutlich verbessertes Verständnis der Hydraulik im tiefen Untergrund, das entscheidend für den Betrieb offener geothermischer Systeme ist, zu erwarten. Für die 3D Potenzialausweisung werden technische und wirtschaftliche Rahmenbedingungen sowie die Kenntnis der statistischen Verteilung der geothermischen Kennwerte berücksichtigt, um stochastisch abgesicherte Prognosen als Basis für die Angabe der Fündigkeitsrisiken zu ermöglichen. Abschließend sollen die identifizierten geothermischen Potenziale mit den Wärmeüberschüssen und dem Wärmebedarf der Stadt Frankfurt a. M. verschnitten werden, um wirtschaftlich interessante Standorte für zukünftige Geothermieprojekte zu identifizieren

© 2015, Springer-Verlag Berlin Heidelberg.The Permocarboniferous siliciclastic formations represent the largest hydrothermal reservoir in the northern Upper Rhine Graben in SW Germany and have so far been investigated in large-scale studies only. The Cenozoic Upper Rhine Graben crosses the Permocarboniferous Saar–Nahe Basin, a Variscan intramontane molasse basin. Due to the subsidence in this graben structure, the top of the up to 2-km-thick Permocarboniferous is located at a depth of 600–2900 m and is overlain by Tertiary and Quaternary sediments. At this depth, the reservoir temperatures exceed 150 °C, which are sufficient for geothermal electricity generation with binary power plants. To further assess the potential of this geothermal reservoir, detailed information on thermophysical and hydraulic properties of the different lithostratigraphical units and their depositional environment is essential. Here, we present an integrated study of outcrop analogues and drill core material. In total, 850 outcrop samples were analyzed, measuring porosity, permeability, thermal conductivity and thermal diffusivity. Furthermore, 62 plugs were taken from drillings that encountered or intersected the Permocarboniferous at depths between 1800 and 2900 m. Petrographic analysis of 155 thin sections of outcrop samples and samples taken from reservoir depth was conducted to quantify the mineral composition, sorting and rounding of grains and the kind of cementation. Its influence on porosity, permeability, the degree of compaction and illitization was quantified. Three parameters influencing the reservoir properties of the Permocarboniferous were detected. The strongest and most destructive influence on reservoir quality is related to late diagenetic processes. An illitic and kaolinitic cementation and impregnation of bitumina document CO2- and CH4-rich acidic pore water conditions, which are interpreted as fluids that migrated along a hydraulic contact from an underlying Carboniferous hydrocarbon source rock. Migrating oil and acidic waters led to the dissolution of haematite cements in the lower Permocarboniferous formations. During the Eocene, subsidence of the Upper Rhine Graben porosities and permeabilities of the sandstones of these formations were strongly reduced to 2.5 % and 3.2 × 10−18 m2. The second important influence on reservoir quality is the distinct depositional environment and its influence on early diagenetic processes. In early stage diagenesis, the best influence on reservoir properties exhibits a haematite cementation. It typically occurs in eolian sandstones of the Kreuznach Formation (Upper Permocarboniferous) and is characterized by grain covering haematite coatings, which are interpreted to inhibit cementation, compaction and illitization of pore space during burial. Eolian sandstones taken from outcrops and reservoir depths exhibit the highest porosities (16.4; 12.3 %) and permeabilities (2.0 × 10−15; 8.4 × 10−16 m2). A third important influence on reservoir quality is the general mineral composition and the quartz content which is the highest in the Kreuznach Formation with 73.8 %. Based on the integrated study of depositional environments and diagenetic processes, reservoir properties of the different Permocarboniferous formations within the northern Upper Rhine Graben and their changes with burial depth can be predicted with satisfactory accuracy. This leads to a better understanding of the reservoir quality and enables an appropriate well design for exploration and exploitation of these geothermal resources.