Magnetic Susceptibility of Rock Samples of the Northern Upper Rhine Graben Region and Adjacent Areas

We present here a comprehensive dataset on the magnetic susceptibility of the main geologic units in and adjacent to the northern Upper Rhine Graben. It represents a complement to the PetroPhysical Property Database (P³) of Bär et al. (2020) and thus follows the database structure as described there. The magnetic susceptibility is a unitless physical quantity that describes how magnetizable a material (e.g. rocks) is. It is the ratio between the magnetic field strength H and induced magnetization M. Besides the remanence, the susceptibility of geological formations decisively influences the amplitude of crustal magnetic anomalies. This parameter depends mainly on the proportion of iron-rich minerals such as magnetite and hematite in the total rock volume. Therefore, mafic rocks usually exhibit the highest susceptibilities, but variations of several orders of magnitude within the same lithology are possible. The northern URG region is characterized by several distinct elongated magnetic anomalies associated with the high susceptibility of magmatic complexes in the crystalline basement and in particular in the Mid-German Crystalline High. The basement represents an important reservoir for geothermal projects due to the very high temperatures. Detailed geological modelling of this horizon is thus being performed within the BMWi funded Hessen 3D 2.0 project (Bär et al. 2016) and the Interreg NWE project DGE-ROLLOUT (www.nweurope.eu/DGE-Rollout). Since very deep boreholes and seismic profiles are sparse, magnetic anomalies provide important insights about the structure and composition of the basement. For realistic modelling of these anomalies, an extensive investigation of the magnetic susceptibility in the model units was required. The analyzed rock samples, mostly derived from abandoned and active quarries as well as natural and other artificial outcrops, were provided by the Institut für Steinkonservierung Mainz e.V. The majority of the in total 430 samples were collected in the Odenwald. Additional sample locations are in the Saar-Nahe Basin, the Palatinate, the Vogelsberg and the Rhenish Massif. Apart from outcrop samples, the susceptibility was measured on 24 core plugs from the three wells Weiterstadt 1, Stockstadt 33R and Worms 3. The measurements were carried out at about 20 °C with the SM-30 handheld device from ZH instruments and were usually repeated three times for each sample, but the number may vary depending on the heterogeneity and size of the samples. The results of the susceptibility measurements are shown in Fig. 1. The values range from about -2.4e-5 to 0.07 [SI]. The susceptibility of the post-Variscan units is generally very small, meaning that their effect on the crustal magnetic field can be neglected. This excludes the Permo-Carboniferous and Tertiary volcanic and volcano-sedimentary horizons, which are likely sources of stronger anomalies in the Saar-Nahe Basin, the northernmost URG and the Sprendlinger Horst. The highest susceptibility values were measured in gabbros of the Frankenstein complex. The dataset is a supplement to another publication (Frey et al. 2020, subm.) that presents the results of the joint inversion of gravity and magnetic data in the northern Upper Rhine Graben. The measured magnetic susceptibilities were a key input for this study.V1.

Interdisciplinary fracture network characterization in the crystalline basement: a case study from the Southern Odenwald, SW Germany

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

Lab-Scale Permeability Enhancement by Chemical Treatment in Fractured Granite (Cornubian Batholith) for the United Downs Deep Geothermal Power Project, Cornwall (UK)

A hydrothermal doublet system was drilled in a fault-related granitic reservoir in Cornwall. It targets the Porthtowan Fault Zone (PTF), which transects the Carnmenellis granite, one of the onshore plutons of the Cornubian Batholith in SW England. At 5058 m depth (TVD, 5275 m MD) up to 190 °C were reached in the dedicated production well. The injection well is aligned vertically above the production well and reaches a depth of 2393 m MD. As part of the design process for potential chemical stimulation of the open-hole sections of the hydrothermal doublet, lab-scale acidification experiments were performed on outcrop analogue samples from the Cornubian Batholith, which include mineralised veins. The experimental setup comprised autoclave experiments on sample powder and plugs, and core flooding tests on sample plugs to investigate to what degree the permeability of natural and artificial (saw-cut) fractures can be enhanced. All samples were petrologically and petrophysically analysed before and after the acidification experiments to track all changes resulting from the acidification. Based on the comparison of the mineralogical composition of the OAS samples with the drill cuttings from the production well, the results can be transferred to the hydrothermally altered zones around the faults and fractures of the PTF. Core Flooding Tests and Autoclave Experiments result in permeability enhancement factors of 4 to >20 and 0.1 to 40, respectively. Mineral reprecipitation can be avoided in the stimulated samples by sufficient post-flushing

Hydro-Thermal Modeling for Geothermal Energy Extraction from Soultz-sous-Forêts, France

The deep geothermal energy project at Soultz-sous-Forêts is located in the Upper Rhine Graben, France. As part of the Multidisciplinary and multi-contact demonstration of EGS exploration and Exploitation Techniques and potentials (MEET) project, this study aimed to evaluate the possibility of extracting higher amounts of energy from the existing industrial infrastructure. To achieve this objective, the effect of reinjecting fluid at lower temperature than the current fluid injection temperature of 70 °C was modeled and the drop in the production wellhead temperature for 100 years of operation was quantified. Two injection-production rate scenarios were considered and compared for their effect on overall production wellhead temperature. For each scenario, reinjection temperatures of 40, 50, and 60 °C were chosen and compared with the 70 °C injection case. For the lower production rate scenario, the results show that the production wellhead temperature is approximately 1–1.5 °C higher than for the higher production rate scenario after 100 years of operation. In conclusion, no significant thermal breakthrough was observed with the applied flow rates and lowered injection temperatures even after 100 years of operation

Multi-scale structural dataset of a crystalline reservoir analogue (Northern Odenwald)

For an accurate multi-scale property modelling of fractured crystalline geothermal reservoirs, an enhanced characterisation of the geometrical features and variability of the fracture network properties is an essential prerequisite. By the combination of regional digital elevation model analysis and local outcrop investigation, detailed insight into the 3D architecture of faults and fracture networks allows the quantification of structural parameters (fracture dimension, orientation, clustering and spacing). The structural dataset presented here contains the regional DEM interpretation at two resolutions (25 m and 1 m) of the Northern Odenwald and the LiDAR and GIS structural interpretation of 5 profiles acquired in the Mainzer Berg quarry between Darmstadt and Dieburg. This quarry exhibits the fracture network affecting a granodioritic pluton. Fracture length, orientation, dip, and fracture density and intensity are calculated for each profile. On GIS 2D datasets extracted from top and side views of the profiles, a clustering and spacing analysis between digitised items is performed and compared to the orientation of artificial scanlines. Power-law parametrisation is extracted from the length distribution of 2D and 3D datasets, with a, b coefficients. This multi-scale parametrisation of the fracture network can be used to construct near-surface discrete fracture network models. The dataset is a supplement to another publication (Bossennec al. 2021) that presents the structural organisation of crystalline rocks from the analogue of Mainzer Berg in the Northern Odenwald.V1.

Multiscale Characterisation of Fracture Patterns of a Crystalline Reservoir Analogue

For an accurate multiscale property modelling of fractured crystalline geothermal reservoirs, an enhanced characterisation of the geometrical features and variability of the fracture network properties is an essential prerequisite. Combining regional digital elevation model analysis and local outcrop investigation, the study comprises the characterisation of the fracture pattern of a crystalline reservoir analogue in the Northern Odenwald, with LiDAR and GIS structural interpretation. This approach provides insights into the 3D architecture of the fault and fracture network, its clustering, and its connectivity. Mapped discontinuities show a homogeneous length distribution, which follows a power law with a −2.03 scaling factor. The connectivity of the fracture network is heterogenous, due to a fault control at the hectometric scale. Clustering is marked by long sub-vertical fractures at the outcrop scale, and strongly enhance heterogeneity around weathered fracture and fault corridors. The multi-variable dataset created within this study can be used as input data for accurate discrete fracture networks and fluid-flow modelling of reservoirs of similar type

The impact of hydrothermal alteration on the physiochemical characteristics of reservoir rocks: the case of the Los Humeros geothermal field (Mexico)

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 m2 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

Petrophysical and mechanical rock property database of the Los Humeros and Acoculco geothermal fields (Mexico)

Petrophysical and rock mechanical properties are key parameters for the characterization of the deep subsurface in different disciplines such as geothermal heat extraction, petroleum reservoir engineering or mining, are commonly used for the interpretation of geophysical data and the parameterization of numerical models and as thus are the basis for economic reservoir assessment. However, detailed information regarding these properties for each target horizon are often scarce, inconsistent or spread over multiple publications. Thus, subsurface models are often populated with generalized or assumed values resulting in high uncertainty. Furthermore, diagenetic, metamorphic and hydrothermal processes significantly affect the physiochemical and mechanical properties often leading to high variability. A sound understanding of the controlling factors is needed to identify statistical and causal relationships between the properties as basis for a profound reservoir assessment and modelling. Within the scope of the GEMex project (EU-H2020, GA Nr. 727550), which aims to develop new transferable exploration and exploitation approaches for super-hot unconventional geothermal systems, a new workflow was applied to overcome the gap of knowledge of the reservoir properties. Two caldera complexes located in the northeastern Trans Mexican Volcanic Belt - the Acoculco and Los Humeros caldera - were selected as demonstration sites. The Los Humeros geothermal system is steam dominated and has been exploited since the 1990’s with 65 wellbores (28 still producing). With temperatures above 380 °C, the system is characterized as a super-hot geothermal system. The geothermal system in Acoculco (presently consisting of two exploration wells) is characterized by temperatures of approximately 300 °C at a depth of about 2 km. It contains almost no fluids, even though a well-developed fracture network exists in the study area. Therefore, the system serves as a demonstration site for the development of an enhanced geothermal system. The workflow starts with outcrop analogue and reservoir core sample studies in order to define and characterize the properties of all key units from the basement to the cap rock as well as their mineralogy and geochemistry. This allows the identification of geological heterogeneities on different scales (outcrop analysis, representative rock samples, thin sections and chemical analysis) enabling a profound reservoir property prediction. More than 340 rock samples were taken from representative outcrops inside of the Los Humeros and Acoculco calderas, the surrounding areas and from exhumed ‘fossil systems’ in Las Minas and Zacatlán. Additionally, 66 core samples from 16 wells of the Los Humeros geothermal field were obtained. Samples were analyzed for particle and bulk density, porosity, permeability, thermal conductivity, thermal diffusivity, heat capacity, as well as ultra-sonic wave velocities, magnetic susceptibility and electric resistivity. Afterwards destructive rock mechanical tests (point load tests, uniaxial and triaxial tests) were conducted to determine tensile strength, uniaxial compressive strength, Young’s modulus, poisson ratio, bulk modulus, shear modulus, fracture toughness, cohesion and friction angle. In addition, XRD and XRF analyses were performed on 131 samples to provide information about the mineral assemblage, bulk geochemistry and the intensity of hydrothermal alteration. An extensive rock property database was created comprising 34 parameters determined on more than 2160 plugs. More than 31,000 data points were compiled covering volcanic, sedimentary, metamorphic and igneous rocks from different ages (Jurassic to Holocene), thus facilitating a wide field of applications regarding resource assessment, modeling and statistical analyses.V1.