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Fracture networks linked to the brittle deformation of rocks are often hosts of fluid flows or geomechanical discontinuities which are important to model for rock mass stability analyses, reservoir assessment or storage capacity evaluation. However, the fractures can be mapped only poorly by subsurface geophysics or borehole imagery. 1D scan lines or 2D maps on outcrops analogues are thus one of the current methods used for the assessemet of the networks, but are very limited regarding the 3D distribution of the fractures. This paper shows the first tests and results for a new automated workflow of fracture properties characterization, using high resolution LiDAR and Photogrammetry data on outcrops. From the obtained 3d point clouds on different objects such as quarries, cliffs or road sides, we reconstruct the outcrop surface in a 3D meshed surface with the help of a Simple and Scalable Surface Reconstruction (SSSR). The fracture planes are automatically detected based on a region-growing segmentation approach based on the spatial distribution of the points. It allows to quickly and effectively extract fracture properties such as orientation, geometry and position in the 3d space, with a possibility for direct 3D density computation without using proxies from 1D or 2D data. This paper present the workflow methodology and the tests on 5 basement rock outcrops of different outcropping quality. We also compare the method with a manual fracture picking tool and a classical 1D scan line methodology on the field. It is a first step to get more precise, automatic, easier and faster modelling workflows on fractured rocks from outcrop analogues data.

Supplementary figure 1.

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.

Intensive graduate courses in geothermal energy have traditionally been the remit of nations with a long tradition of high temperature geothermal utilization: Iceland, Italy, Japan, and New Zealand. In addition, El Salvador has had a Spanish language Geothermal Diploma Course for Latin America since 2010, courses in Indonesia, taught in Indonesian, and African geothermal training is under development. However, increasing impacts of climate change, geopolitical conflict and related refugee movement has drawn attention to the need for clean, renewable and sustainable energy worldwide. This attention has created interest in geothermal research and utilization in many countries that do not have high temperature, tectonic margin or volcano-related, geothermal systems. This paper discusses the ongoing development of an EU-funded European Erasmus + Strategic Partnership dealing aiming at the construction of a geothermal course curriculum for international students, involving Iceland France, Germany and Croatia.

In the northern Upper Rhine Graben (URG), the crystalline basement constitutes an attractive target for deep geothermal exploitation due to the favourable reservoir temperatures and abundance of natural fractures and large-scale faults. Consequently, especially the upper, hydrothermally altered part is already successfully used for heat and power generation at several locations (Insheim, Landau, Rittershoffen and Soultz-sous-Forêts). Nevertheless, because of the small number of very deep boreholes drilled into the crystalline basement, little is known about its structure and composition. An interdisciplinary multi-scale approach was applied to gain new insights into the properties of the crystalline crust. By building on existing geological models of the URG, a detailed 3D model of the crystalline basement was developed. Additional information was provided by high-resolution gravity and magnetic data, which served as input for a stochastic joint inversion. Inverted density and susceptibility models allowed to identify the predominant rock types below the sedimentary cover. The Tromm granite in the southern Odenwald was chosen as an outcrop analogue to further analyse the hydraulic properties of the crystalline reservoir. By examining the lineaments on the regional scale and the fractures in a total of 5 outcrops, statistical parameters describing the fracture network were extracted. These were then used to create discrete fracture network (DFN) models, in order to calculate the equivalent porous media permeabilities of the bedrock at reservoir depth. In addition, gravity and radon measurements were carried out, which enabled more precise localisation of naturally permeable fault zones. The combination of structural geological and geophysical methods results in a more advanced characterisation of the crystalline basement, that can in future studies be used for more realistic potential assessments and a reduction of exploration risks for geothermal projects.

The area of the Upper Rhine Graben (URG) is known for its geothermal potential. However, the recent interest for lithium co-production from geothermal brines raises questions about the quantification and dynamics of fluid flow paths in the geothermal system at the basin scale. This study aims to better understand the impact of the fluid circulation on the temperature field evolution and on the fluid recharge by performing a 3D thermal basin modeling. The Buntsandstein group sandstones, constituted by early Triassic fluvial to playa-lake deposits are one of the targeted reservoir layers for geothermal and lithium co-production. They overlay permo-carboniferous deposits and the the crystalline variscan basement. The basement inherited structural network plays an important role on heat distribution and flow pathways on the graben shoulders and within the basement. Faults control also the lateral and vertical reservoir connections and fluid mixing, and thus need to be integrated into the burial model. In this study, the focus is made on the northern part of the URG between the cities of Haguenau (France) and Frankfurt (Germany). A new structural model and the geometry of twelve sedimentary layers are implemented in TemisFlow® software. The thermal simulation included both conductive and advective heat transfer. The model integrates the Tertiary rift event from 46 Ma to 23 Ma, by coupling the lithosphere with the depositional evolution. The influence of permeability heterogeneity in the crystalline basement, the role of the main graben border faults, and some selected internal faults on the fluid flow were also investigated. The model is calibrated with the available temperature measurement data, vitrinite reflectance data and temperature maps at different depths or horizons. As a result, the simulations show that the thermal structure of the Eastern part of the URG is mainly controlled by conductive heat transfer, and directly related to the burial. Modeling outputs also highlight the impact of the basement heterogeneity on hydrothermal circulation and the temperature field of the Western part of the URG.

Solutions for seasonal energy storage systems are an essential component for the reliable use of fluctuating renewable energy sources and to bridge the gap between abundant heat availability from renewable sources in summer and an increased heat demand in winter. As a part of the research project ’solar crystalline borehole thermal energy storage system‘ – ‘SKEWS’, a field-scale demonstrator for a medium deep borehole thermal energy storage (BTES) system with a maximum depth of 750 m is to be built at Campus Lichtwiese of the Technische Universität Darmstadt, Germany, to demonstrate this innovative technology. In this first demonstration phase, the storage array consists of four coaxial BHEs (Borehole Heat Exchangers), which tap into a crystalline reservoir rock underneath a thin sedimentary cover. Prior to project launch a numerical model of the storage system was to be built to investigate the storages behavior under the local geological and hydrogeological conditions. In the first stage, the geological context in the surroundings of the project location was investigated using archive drilling data and groundwater measurements. The data obtained facilitated the development of a geological model concept. It suggests the assumption that the uppermost part of the intended storage domain is crosscut by a normal fault, which displaces the Permian rocks east of Darmstadt against granodioritic rocks of the Odenwald crystalline complex. The simplified geological model was implemented in a 3D-finite-element numerical model to simulate the thermal effect of the storage system operation on the surrounding subsurface. The model for the four planned BHEs did not show the formation of any significant heat plumes by groundwater flow with only a minor increase in groundwater temperature. Additionally, the numerical model was used to estimate the effect of the potentially highly permeable fault zone on the planned storage site. For this purpose, a storage operation over a time span of 30 years was simulated for different parametrizations of the fault zone and the storage system. The simulations reveal a limited but visible removal of heat from the storage region with increasing groundwater flow in the fault zone. However, since the section of the BTES system affected by the fault is very small in comparison to the system’s total depth, only a minor impairment of the storage efficiency could be observed in the worst case.

Climate change requires immediate action, and for sustainable development, and uninterrupted energy supply is necessary. Since anthropogenic emission of CO2 in the atmosphere has a major role in climate change, carbon negative energy solutions are the necessity of the time. Geothermal energy is one such renewable source that can assist in achieving an economic solution to low carbon energy. Engineered geothermal systems or enhanced geothermal systems (EGS) are more suitable from an industrial perspective and can supply uninterrupted energy supply for a long duration. In conventional EGS systems, water is the heat transfer fluid. However, the use of supercritical CO2 as the heat-carrying fluid has significant advantages over water including less chemical reactivity, low fluid viscosity, and comparatively higher thermal conductivity for shallow systems. Fluid loss is the major issue in any EGS operation. However, CO2 loss during the EGS operation could lead to carbon geosequestration, and therefore a carbon-negative energy solution is possible when using CO2 in EGS operations. A case study of Soultz-sous-Forêts geothermal site is considered in this work to investigate the feasibility of CO2 usage as the heat-carrying medium. Soultz-sous-Forêts is present in the Upper Rhine Graben, France. Geologically Soultz-sous-Forêts geothermal site comprises three layers: 1.5 km of thick quarternary and tertiary sediments, 350 m thick Buntsandstein and the basement is granite. Presently, three wells (GPK-3, GPK-4: injection wells, and GPK-2: production well) are operating at this site up to a depth of approximately 5 km. In this work, a three-dimensional Soultz-sous-Forêts site is considered with five major faults. In the present model, supercritical CO2 is injected through GPK3 and GPK4 and produced using GPK-2. This work investigates the coupled hydro-thermal processes occurring in the fractures and the rock matrix. The local thermal non-equilibrium (LTNE) approach is considered to account for the heat exchange between the rock matrix and supercritical CO2 flowing through the faults. Recent studies have reported fluid loss along the wellbore casing in all three wells. Therefore, a wellbore leakage model is also coupled along these well trajectories and its impact on final production temperature is assessed. Results obtained from different injection rate strategy at different injection temperature indicates that even 100 years of geothermal energy extraction operation will not have much impact on the production well temperature and therefore, a sustainable energy supply is feasible at the Soultz-sous-Forêts site.

The Upper Rhine Graben (URG) is a target area for deep geothermal and heat storage projects, as petrophysical and hydraulic properties of the faulted crystalline basement rocks, and the temperature field comprise a high geothermal potential (Soultz-sous-Forêts, Landau, Insheim, Rittershoffen). However, there is a lack of knowledge on the multi-scale structural architecture of such rock units in fault zones. Therefore, a multi-scale structural analysis is performed on surface analogues to improve the conceptual crystalline reservoir model accuracy. The surface analogues selected are located in the Odenwald Massif, the largest outcropping section of the Mid German Crystalline High. Regional-scale lineament analysis and LIDAR and GIS interpretation of the fracture network on 21 profiles in 11 outcrops were analysed to quantify statistical parameters describing the fracture and fault network. The variability of crystalline rock lithologies (granite, granodiorite, ‘Flasergranitoid’, amphibolite and gabbro) and fault directions sampled allows for the construction of an extensive structural dataset with fracture network geometry, dimension, and connectivity features. Four significant lineament strikes dominate the structural trend of the NURG, being N000-N015°E, N050-N075°E, N100-N115°E and N150-N165°E. In the Odenwald itself, lineaments striking N100-N115°E and N055-N070°E are in a predominant proportion, compared to the N000-N015°E and N150-N165°E striking trends. Fracture length distribution follows a power law with an exponent varying from -2.2 to -1.8, depending on the background lithology. The connectivity of the fracture network is heterogeneous, with varying configurations (no fractal organisation), due to a fault control at hectometric scale and clustering marked by secondary faults. At the outcrop scale, this pattern is strongly enhanced in the vicinity of weathered fractures or fault corridors. These properties distribution can be implemented into sub-surface semi-artificial discrete fracture network models to quantify the flow properties of fractured reservoir rocks.

The Upper Rhine Graben (URG) is the target of a renewed development of the exploitation of oil and gas resources, since the discovery of the Roemerberg field in 2008. Strong improvements were made on the understanding of the petroleum system, and on the characterization of the oil and source rocks families, and migration pathways. The interactions between faults systems and fracture network, fluid flow and fracture infill, and fracture/matrix transfers, during the burial and rift opening, are the purpose of this PhD research, focused on Buntsandstein Gp. sandstone reservoir. Through the study of several outcrops on the URG shoulders, core and well logs data (Roemerberg field), the following aspects are investigated. Structural features of the fracture network in the reservoir and in the vicinity of major fault systems are characterized, with the identification of fracture families (infill and orientation), not always parallel to the major faults. Chemical and isotopic composition of matrix and fracture diagenetic phases are analysed, to determine conditions of cementation : Temperature, type of fluid, absolute dating when possible, or relative timing. The impact on petrophysical properties is assessed by investigating the influence of diagenetic processes on pore network properties, and on "bulk" petrophysical properties. These analytical results are integrated in a renewed conceptual model of fluid flow in the basin, and give new insights on the interactions between diagenesis, fault activity and fluid-flow, and their impact on petrophysical properties at the fault scale through geological times., Le Graben du Rhin est la cible d’un regain d’intérêt pour le développement de l’exploitation des ressources hydrocarbonées et géothermiques depuis une quinzaine d’années. De nombreuses améliorations ont été apportées à la compréhension de la géométrie des réservoirs et sur le système pétrolier, avec une caractérisation des huiles et des roches-mères, ainsi que sur les chemins de migration de l’huile. Cependant, il subsiste des interrogations sur les interactions entre systèmes de faille, réseau de fractures, circulations de fluides et cimentations, ainsi que sur les transferts entre matrice et fractures au sein des réservoirs, et l’évolution de ces propriétés au cours de l’histoire d’enfouissement. Cette thèse s’attache à la compréhension de ces interactions, en prenant pour objet d’étude les réservoirs gréseux du Buntsandstein Gp. Sur la base matérielle de plusieurs affleurements sur les épaules du fossé et des données de subsurface disponibles (comme celle du champs de Roemerberg), les caractéristiques du réseau de fractures au sein du réservoir et à proximité des zones de failles majeures ont été déterminées, avec l’identification de familles (remplissages, orientation) avec une variabilité de leur agencement par rapport aux failles majeures. La détermination de la composition chimique et isotopique des remplissages de fractures et des phases diagénétiques affectant la matrice a permis d’améliorer les modèles de conditions de cimentations : température, type de fluide, datation relative, ou absolue lorsque les méthodes analytiques s’y prêtent. L’impact de ces processus sur les propriétés pétrophysiques a été estimé, par le biais de la caractérisation du réseau poreux et la quantification des propriétés pétrophysiques globales ("bulk"). Ces résultats analytiques sont ensuite intégrés à un modèle conceptuel de circulation de fluides à l’échelle du bassin, répertoriant la mise en place des différentes minéralisations, apportant un nouveau regard sur le résultat à l’échelle des zones de failles des interactions entre diagénèse, tectonique, et circulations de fluides, au cours de l’histoire géologique du bassin.