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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.

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.

We provide a set of grid files that collectively allow recreating a 3D geological model which covers the Upper Rhine Graben and its adjacent tectonic domains, such as portions of the Swiss Alps, the Molasse Basin, the Black Forest and Vosges Mountains, the Rhenish Massif and the Lower Rhine Graben. The data publication is a complement to the publication of Freymark et al. (2017). Accordingly, the provided structural model consists of (i) 14 sedimentary and volcanic units; (ii) a crystalline crust composed of seven upper crustal units and a lower crustal unit; and (iii) two lithospheric mantle units. The files provided here include information on the regional variation of these geological units in terms of their depth and thickness, both attributes being allocated to regularly spaced grid nodes with horizontal spacing of 1 km. The model has originally been developed to obtain a basis for numerical simulations of heat transport, to calculate the lithospheric-scale conductive thermal field and assess the related geothermal potentials, in particular for the Upper Rhine Graben (a region especially well-suited for geothermal energy exploitation). Since such simulations require the subsurface variation of physical rock properties to be defined, the 3D model differentiates units of contrasting materials, i.e. rock types. On that account, a large number of geological and geophysical data have been analysed (see Related Work) and we shortly describe here how they have been integrated into a consistent 3D model (Methods). For further information on the data usage and the characteristics of the units (e.g., lithology, density, thermal properties), the reader is referred to the original article (Freymark et al., 2017). The contents and structure of the grid files provided herewith are described in the Technical Info section.

There is consensus that conduction is the main heat transfer mechanism in the lithosphere and generally controlled by (1) the heat input from larger mantle depths, (2) the heat internally produced in the lithosphere by the decay of radioactive elements, and (3) the variations in thermal conductivity within different lithospheric layers. To quantitatively assess the interaction of these controlling factors we use validated 3D conductive thermal models that integrate observations from wells, surface geology, seismic surveys, gravity and seismology and resolve various sedimentary, crustal, and mantle units. We analyze variations of the geothermal gradient laterally and with depth for the Upper Rhine Graben rift and two magma-rich passive continental margins of the Atlantic that are characterized by a similar crustal structure but a different age, the SW African margin and the Vøring margin. In general we find that the geothermal gradient decreases with increasing depth and shows varying lateral trends and values for the young rift system and the two different margins. In the young rift the distribution of heat is mainly controlled by variations of heat input from crustal domains of different radiogenic heat production and the insulating effect of the young unconsolidated rift sediments, that partly may also be affected by advective heat transport. For the passive margins the age-controlled depth of the oceanic thermal LAB causes the main differences between the two settings and we find an ongoing process of oceanic mantle cooling at the young Norwegian margin compared with the old thermally equilibrated SW African margin.

Petrophysical properties are key to populate numerical models of subsurface process simulations and for the interpretation of many geophysical exploration methods. They are characteristic for specific rock types and may vary considerably as a response to subsurface conditions (e.g. temperature and pressure). Hence, the quality of process simulations and geophysical data interpretation critically depend on the knowledge of in-situ physical properties that have been measured for a specific rock unit. Inquiries for rock property values for a specific site might become a very time-consuming challenge given that such data are (1) spread across diverse publications and compilations, (2) heterogeneous in quality and (3) continuously being acquired in different laboratories worldwide. One important quality factor for the usability of measured petrophysical properties is the availability of corresponding metadata such as the sample location, petrography, stratigraphy, or the measuring method, conditions and authorship. The open-access database presented here aims at providing easily accessible, peer-reviewed information on physical rock properties in one single compilation. As it has been developed within the scope of the EC funded project IMAGE (Integrated Methods for Advanced Geothermal Exploration, EU grant agreement No. 608553), the database mainly contains information relevant for geothermal exploration and reservoir characterization, namely hydraulic, thermophysical and mechanical properties and, in addition, electrical resistivity and magnetic susceptibility. The uniqueness of this database emerges from its coverage and metadata structure. Each measured value is complemented by the corresponding sample location, petrographic description, chronostratigraphic age and original citation. The original stratigraphic and petrographic descriptions are transferred to standardized catalogues following a hierarchical structure ensuring intercomparability for statistical analysis. In addition, information on the experimental set-up (methods) and the measurement conditions are given for quality control. Thus, rock properties can directly be related to in-situ conditions to derive specific parameters relevant for modelling the subsurface or interpreting geophysical data.

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.