In this study, we conduct a multi-disciplinary study, including geophysics, geochemistry, and geology, in an attempt to reconstruct geothermal geological model(s) at the shallow 3 km, for a project of potential geothermal power generation at the Hongchailin site in the Ilan plain of northeastern Taiwan. Our geophysical techniques include seismic imaging from natural earthquakes as well as ambient noise. Three seismic arrays deployed at different time periods in the past decade were used. We also incorporate geophysical imaging results from previous studies, in particular a series of seismic reflection profiles. For the magnetotelluric (MT) technique, two major surveys have been conducted, including 2015-2018 AMT by National Central University and 2021-2022 MT by GERD and Academia Sinica.  Three test holes were drilled around the Hongchailin site in 2016-2019. Logging and on-site measurements were conducted at increment depths, including stratigraphy and rock types, P/T measurements, fractures analyses, geochemical analyses (e.g., hydrogen/oxygen/helium isotope, fluid inclusion, etc.), and so on.

        Based on regional geology and structures, and incorporating geophysical subsurface imaging, we reconstruct geothermal geological models in line with detailed geological cross sections at the shallow 3 km level. We interpret that there exits a shallow geothermal reservoir within the massive quartz sandstone layers (Szeleng sandstone) at 1-2 km depth with the downhole temperature of 80-100 C.  Geologically, the reservoir is located at the regional Songlo syncline and its south limb; and geophysically, it corresponds to a relatively low resistivity area. Isotope results show that the cool meteoric water came from nearby higher altitude mountain area, then flew through the Szeleng sandstone, which plunges 1-2 km depth below the Ilan plain. Hot fluid is interpreted to be derived from deeper heat source and to be upflowed along a N-S trending vertical faults system near the Hongchailin area.  In addition, two E-W trending major faults, identified by the seismic reflection profiles, seem to be acted as hydrothermal fluid barriers to confine the hot fluid within the reservoir area.

How to cite: Lee, J.-C., Ho, G.-R., Chen, C.-C., Huang, H.-H., Lin, C.-H., Song, S.-R., Lu, Y.-C., Hase, H., Chiang, C.-W., Chen, Y.-G., and Chung, S.-L.: Combining geophysical, geochemical and geological studies toward plausible geothermal models at the Hongchailin site for geothermal power generation in northeastern Taiwan, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2534, https://doi.org/10.5194/egusphere-egu23-2534, 2023.

In the general context of the development of renewable energy in the Hauts-de-France region (N France), some growing interest has been focused recently on the potential of deep geothermy. This area displays favourable conditions due to the burial of a regionally well-defined reservoir, i.e. the Dinantian karstic and brecciated limestones (Lower Carboniferous, 360-330 Ma), below the Nord-Pas-de-Calais coal-bearing Upper Carboniferous basin, developed by flexural subsidence in the foreland of the Northern Variscan frontal thrust system. The predominance of shales within the molassic basin as well as within the basal units at the floor of the thrust wedge (the Lower Devonian clastic units) are furthermore likely to form a large-scale permeability barrier potentially favouring the localization of hot waters within the underlying carbonate reservoir. The occurrence of a Dinantian regional geothermal resource has already been proven in Southern Belgium in the Hainaut coal basin area (the eastern prolongation of the Northern France coal basin) where the temperature in three geothermal wells reaches about 70°C.

To provide further constraints on such potential deep geothermal field in a structurally complex setting (a laterally segmented thrust front), the geometry of the Dinantian reservoir in northern France has been investigated through the integration and interpolation in a 3-D model of a large database including 1 128 boreholes and 532 km of reprocessed, interpreted and depth-converted seismic reflection profiles. The results of the 3-D modelling indicate that the Dinantian reservoir is present at depth over a large area covering approximately 7675 km² in northern France-southern Belgium. It extends at least 30 to 40 km south of the coal mining district area, underneath the Ardennes Allochthonous Unit of the Northern Variscan Front. The Dinantian reservoir is less than 200 m deep in the Lille metropolitan area and strongly deepens southward through a sharp flexure. It reaches 1000-3000 m depth underneath the coal basin and a maximum of 6944 m depth at the southern end of the study area. Overall, the Dinantian reservoir is structured along two main directions oriented N70-80° and N110-130°, related respectively to deep frontal Variscan thrusts and lateral-oblique ramps. The Dinantian reservoir ends west of Douai against a major complex lateral ramp system forming a first-order transfer zone within the Northern France Variscan thrust front. The latter localizes a set of strongly dipping N110-N130 faults (the Artois faults) representing second-order structures produced during subsequent deformation periods i.e. the Late Carboniferous-Permian rifting event and the Tertiary inversion related to the far-field accommodation of the Alpine-Pyrenean shortening.

How to cite: Averbuch, O., Laurent, A., Beccaletto, L., Graveleau, F., Lacquement, F., Caritg, S., Marc, S., and Capar, L.: 3-D geometrical modelling of the Dinantian carbonate reservoir in Northern France: new constraints for the regional development of deep geothermy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6370, https://doi.org/10.5194/egusphere-egu23-6370, 2023.

The so-called Green Transition EU strategy encourages oil businesses to move towards a circular economy and create green energy. The data already collected in the past during hydrocarbon exploration offer enormous possibilities for geothermal reutilization. Revealing this led to the joint project with MOL Plc. to evaluate the geothermal potential for Zala county, Hungary. Although the preliminary geothermal potential of the Zala region (SW Hungary) is assessed to be good, sustainable thermal water use is critical due to the need for reinjection, with only one operating doublet. The study focuses on the geothermal assessment of the siliciclastic Neogene formations.

Due to the need for reinjection wells for sustainable thermal water production, the evaluation has to handle the potential injectivity for the same siliciclastic reservoir. However, several injection-related problems are associated with the Neogene (so-called Pannonian) siliciclastic reservoirs in Hungary. Predicting the issues makes it possible to mitigate them, and it contributes to a “reinjection potential” assessment which can be part of the geothermal potential estimation.

Based on methodological contribution from previous studies (e.g., Markó et al., 2021), this research considers a variety of problem scales, including regional hydraulics, reservoir scale properties, and local clogging processes. As a next step, in the recent study, we investigate the extension of the reservoir sandstone bodies of the deltaic aquifer. This is done by interpreting 3D seismic volumes by amplitude extraction to detect sand-prone volumes combined with well-logs to delineate the reservoir. The analysis helps to predict where to drill the next reinjection well as well as to evaluate the potential of the existing hydrocarbon wells.

The first author was supported, and the research was financed through the KDP-2021 Cooperative Doctoral Programme of the Ministry of Innovation and Technology (Hungary) from the source of the National Research, Development and Innovation Fund, grant number: KDP_2021_ELTE_C1789026. The study was funded by the National Multidisciplinary Laboratory for Climate Change, RRF-2.3.1-21- 2022-00014 project.

How to cite: Markó, Á., Tóth, M., Brehme, M., and Mádl-Szőnyi, J.: Determining the „geothermal reinjection potential” into sedimentary formations using datasets of hydrocarbon exploration, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6992, https://doi.org/10.5194/egusphere-egu23-6992, 2023.

In 2018, the company St1 Oy performed an Enhanced Geothermal System (EGS) experiment using near-real-time seismic monitoring as feedback for the pumping procedure and thus allowing the control of the induced seismicity. Between 4 June and 22 July (49 days), ~18,000 cubic meters of water was injected at around 6 km depth beneath the University campus in Otaniemi, Espoo, Finland (Kwiatek el al. 2019). The hydraulic stimulation and post-stimulation stages were monitored by a temporary ~100 three-components geophone network installed by the Institute of Seismology, University of Finland (Hillers et al. 2020). If the requirements for induced seismicity have been successfully met, the medium changes produced by the hydraulic stimulation remain unclear. Especially with regard to the activation or opening of cracks. Recent application of the distortion matrix concept (Badon et al. 2020) in seismology (Touma et al. 2021) allows us to consider resolving some of these questions by imaging the distribution of the scatterers in the medium during the different injection stages. The distortion matrix operator makes it possible to correct a large part of the aberrations present in images obtained by focusing in depth a reflection matrix recorded at the surface. The phase distortions of the seismic wavefield due to complex velocity distributions can be recovered for any virtual source in the medium by comparing the recorded reflection matrix to the ideal geometric wave-front corresponding to this virtual source. By taking advantage of this powerful approach this study present 3D images of the volume surrounding the injection well of the EGS experiment.

How to cite: Compaire, N., Campillo, M., Hillers, G., Touma, R., and Aubry, A.: Imaging medium changes during a hydraulic stimulation using the distortion matrix framework:  case of an Enhanced Geothermal System (EGS) in Espoo, Finland., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7884, https://doi.org/10.5194/egusphere-egu23-7884, 2023.

The Hengill volcano system and its adjacent geothermal fields are Iceland's most productive harnessed high-temperature geothermal area. The geothermal resources are powered by cooling magmatic intrusions connected to three volcanic systems, with Hengill being the youngest that erupted ~2000 years ago. This area is located at a triple junction and has, therefore, a highly heterogeneous crustal structure in addition to a fissure swarm intersecting the Hengill volcanic center. Although explorations of Hengill began more than half a century ago, outstanding questions remain, such as the geophysical signature of super-critical fluid and a more detailed understanding of the underlying geothermal resources. Our particular focus in this work is to explore how high-resolution 3D isotropic and anisotropic seismic velocity models can help to address relevant questions in Hengill.

We perform ambient noise Rayleigh and Love wave imaging by combining a 498-node dense geophone array and a 44-station seismic backbone network. The backbone netowrk has ~2.5 years of data between late 2018 and 2021, with a >3 km station spacing and up to 40 km aperture. The dense array data, targeting provisioned geothermal subfields in the northern and southern parts of Hengill, was acquired through a 1–2 months campaign in the summer of 2021, with a <500 m station spacing and an aperture of 20 km. We demonstrate that, even with the shorter duration of data, the seismic imaging capacities are greatly enhanced in the top 5 km compared to the images retrieved from the backbone network alone. In addition to the shallow structure (<2 km), the dense sampling with good azimuthal coverage provides essential constraints on the deeper structure (>2 km). We observe a primary slow velocity anomaly at ~4 km depth which we associate with solidified magmatic intrusions. The trend of the anomaly is perpendicular to the NE striking Hengill fissure swarm, coinciding with a previously found deep lying low-resistivity anomaly. We find that most of the earthquake locations are near the margin of velocity contrasts, indicating a structure or permeability change in the subsurface.

From the anisotropy model, we observe a predominant fast direction along the vertical axis in the top 2 km of crust, implying an overall vertical crack formation resulting from extensional stress with ~10 mm/yr westward deformation. From 3 to 5 km depth, the fast direction transitions to the horizontal axis, broadly in agreement with intrusions or lava deposits. Around the same depth, the southwestern Hengill geothermal field remains in the vertically-fast direction. This area resides in the junction of distinct geologic, tectonic, and geodetic manifestations. We hypothesize that the extension of  vertically-oriented formation toward depth relates to crustal thinning and increasing permeability that promote one of the powerful boreholes in Hengill.

How to cite: Wu, S.-M., Sánchez-Pastor, P., Ágústsdóttir, T., Obermann, A., Hersir, G., and Mordret, A.: Multiparapmeter Seismic Tomography Across High-Temperature Geothermal Field in Hengill (Iceland) Using a Large-N Nodal Array, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8220, https://doi.org/10.5194/egusphere-egu23-8220, 2023.

One key aim of the DIG (De-risking Ireland’s Geothermal Energy Potential) project is to determine and evaluate the potential low-enthalpy geothermal resources at reservoir scale in the Mallow warm springs area (MWSA), by performing a joint interpretation of new and existing geophysical, geochemical and petrophysical datasets together with structural geology and hydrochemistry results.

As a first step, based on the ambient detected noise sources in the study area, about 100 seismic stations (5Hz nodes) were deployed for two weeks along the railway, straddling fault structures that are thought to control hot spring fluid flow in the Mallow area. We performed seismic interferometry imaging on the recorded train-induced vibrations to map shallow subsurface (top ~2 km) structures and to extract the physical properties (e.g. seismic velocity and density) of these structures. Preliminary result shows a good correlation between S-wave velocity variation and the near-surface lateral changes of lithology, especially across the Killarney-Mallow Fault Zone (KMFZ).

In the next step, 2D interactive modeling of the gravity data was performed, using physical properties determined from the previous step to constrain shallow structures. Additionally, we used the result from the receiver function method that is applied to the data recorded by four broadband stations in the study area, in order to better constrain the deeper interfaces. The 2D inversion of gravity data reveals an anomalous zone in the vicinity of the KMFZ that could be related to the possible fault conduit, associated with the Mallow warm springs area.

The project is funded by the Sustainable Energy Authority of Ireland under the SEAI Research, Development & Demonstration Funding Programme 2019 (grant number 19/RDD/522) and by the Geological Survey Ireland.

How to cite: Rezaeifar, M., Bean, C. J., Kiyan, D., O’Reilly, B., Hogg, C., Meere, P., Fullea, J., Lebedev, S., Ye, T., Chambers, E. L., Scully, A., and Tomar, G. and the DIG team: The shallow subsurface characterization in relation to geothermal resources in the Mallow area, Ireland, using passive seismic and gravity data inversion, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9144, https://doi.org/10.5194/egusphere-egu23-9144, 2023.

Sustainable and efficient use of the geothermal resource sitting below the city of Munich (Germany) is a topical issue for the local energy providers. Indeed, by 2035, Munich city expects to supply 100 % of its electricity and heat demand from renewable energies. Hence, the exploitation of the highly conductive aquifers will be intensified with the development of numerous new geothermal power plants. This is a challenging task, which requires better understanding of the coupled processes taking place underground and of their effects, especially in terms of possible induced seismicity and ground uplift or subsidence. These last two concerns are at the heart of the INSIDE research project. The present study focuses on the thermo-hydro-mechanical (THM) modelling of the deep geothermal site in Pullach im Isartal (close to Munich), which constitutes one step to reach the overall project goal.

At the Pullach site, since 2005, a deep geothermal reservoir in the Mesozoic Malm is successfully operated and supplies heat to the local grid. A finite-element (FE) reservoir model is developed consisting of eight different lithologies ranging from the crystalline basement to the Earth’s surface. Focusing on the Malm geothermal reservoir, the lithology of interest is subdivided into the three potential and permeable reservoir rocks of the lower, middle and upper Malm. The three geothermal wells are included with their real trajectories and open-hole sections as lower-dimensional elements. The FE model is used to simulate the fully-coupled THM processes taking place under continuous operation using the TIGER open-source application, which is based on the Multiphysics Object-Oriented Simulation Environment (MOOSE).

The 3D numerical model was calibrated using hydraulic tests conducted at the wells and the continuous operation of the plant, running for many years, gave the opportunity to assess the simulation results. Consequently, predictions of the long-term changes of the THM characteristics of the geothermal reservoir for the next 50 years are expected realistic. The results show that, under our assumptions and the current operational conditions, exploitation will remain sustainable in the future. Furthermore, near the injection well, a long-term increase in porosity and permeability should be expected due to thermo-poroelastic effects. At last, millimetre-scale uplift could be observed at surface above the injection well.

In addition to the present exploitation schemes, several scenarios to increase the exploitation using additional wells are investigated. A similar approach is taken, but particular attention is paid to possible interferences between existing and future neighbouring wells due to enhanced reservoir utilization.

How to cite: Gaucher, E. and Egert, R.: Change of the THM properties of a Malm geothermal reservoir under present and future exploitation schemes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11877, https://doi.org/10.5194/egusphere-egu23-11877, 2023.

What are the main challenges of exploring geothermal reservoirs with low temperatures, so-called low-enthalpy systems? A typical situation in European countries, especially Germany, is regionally different in high information density due to former intensive exploration activities for hydrocarbons. Regionally there are concepts of more or less successful exploitation for geothermal projects based on special subsurface conditions, which allow high water production at high temperatures. The development of such regions where geothermal energy plays an essential role in the energy supply extends over decades. Uncertainties in reservoir characterization lead to high investment risk and a lack of economic viability. The increase in energy costs and the intensified search for climate-neutral energy has recently intensified the search for suitable reservoirs for geothermal use. In this context, the development times of geothermal projects must be shortened, and possible reservoirs must be systematically surveyed. Exploration plays one or even an essential role. Even if the state assumes costs and risks, concepts for exploration must be developed according to the current state of the art.

A closer look reveals that even in areas with a high information density, more than knowledge about the subsurface beyond the hydrocarbon reservoirs is needed for geothermal use. The exploration goal in geothermal energy so far has been mainly structural exploration. To better estimate the reservoir quality, understanding the depositional space and subdivision into different facies is necessary. These tasks are then the basis for estimating petrophysical parameters for reservoir characterization. Here, newer interpretation techniques help to perform such tasks quickly.

An example of an interpretation of a 3D seismic dataset from the Bavarian Molasse illustrates this. The Tertiary sediments of the Molasse basin are composed of marine sequences, redeposition of debris fans of the adjacent Alpine region and drainage systems of the basin. Due to regionally different uplift and subsidence of the basin, east-west directed trends exist in the respective depositional systems. In the region studied here, these changes are extreme. The goal was to find and map evidence of fluvial systems in the depositional settings. The seismic patterns are very heterogeneous, and there are only a few continuous reflectors to subdivide the study area. Indications of fluviatile deposits are not evident at first.

By calculating local inclinations and inversion techniques, a variety of so-called phantom horizons can be generated and then visualized with different seismic attributes. This way, fluviatile patterns could be recognized in a 300m thick part of the Aquitaine. The mapping of these patterns followed a further step by exploiting the corresponding signals' similarity and spatial connections. Machine learning can perform further facies discrimination but cannot recognize these fluviatile patterns.

The work showed that existing data, with appropriate processing and new interpretation methods, can be efficiently used to search new geothermal reservoirs systematically. The scientific challenge that industrial orders cannot solve is a more detailed analysis of the evolution of the sedimentation space. However, this is necessary for the transfer of results to neighbouring areas.      

How to cite: von Hartmann, H. and Ofori-Amanfo, N. Y.: Challenges for geothermal exploration- fluviatile systems and new interpretation techniques, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12919, https://doi.org/10.5194/egusphere-egu23-12919, 2023.

The Northern Apennines hinterland is normally referred to as a back-arc area affected by magmatism since the late Neogene, with intrusive and extrusive magmatic rocks being scattered through the southern Tuscany and the Tuscan Archipelago. This setting makes this geothermal area one of the most important worldwide.  The very high geothermal gradient of the Tuscan Magmatic Province (up to 200–1000 mW/m2 in the central part of the Larderello-Travale geothermal field) fuels vigorous fluid flow that promotes a widespread geothermal activity. The plumbing magmatic systems progressively migrated eastwards. In Tuscany, the Elba Island is proposed to be the ancient (and now exhumed) analogue of the Larderello-Travale geothermal system where Pliocene to Pleistocene granites have been found at about 3 km depth. The occurrence of such magmatic bodies links to one of the most debated structures of the region, namely the K-horizon seismic reflector. Early authors suggest that this level may represent the transition between ductile and brittle geological units. Other authors propose that the k-horizon may represent a level where the magmatic brines released by the cooling plutons are stored at super-critical conditions.

To understand the spatial and temporal relationship between tectonics and eastwards migration of magmatism, we deployed a seismic network composed of 30 broadband sensors that complemented the permanent INGV network from September 2020 to August 2021. We used QuakeMigrate to build the seismic catalogue and identified 915 events. The seismic activity is well spread across the region and occurs in swarm sequences in areas marked by higher geothermal gradients, particulary in the Larderello-Travale geothermal field. We investigate the seismic clusters and compare them against the locations obtained with the permanent monitoring network. Our results significantly reduce the magnitude of completeness for the Northern Appennines hinterland and provide new insights into the tectonics of the region. This study will be continued with a newly funded project that will use ambient noise methods acquired by nodal networks to provide high-resolution tomographic inversions of the geothermal systems of the Tuscan Magmatic Province.

How to cite: Nicollet, K., Muñoz, F., Savard, G., Montanari, D., d'Ambrosio, M., Saccorotti, G., Piccinini, D., Piana Agostinetti, N., Porras Loria, J. L., Michailos, K., Minetto, R., Bonini, M., Del Ventisette, C., and Lupi, M.: Seismic methods to investigate the interplay between magmatism and tectonics in the Northern Apennines hinterland, Tuscan Magmatic province, Italy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12965, https://doi.org/10.5194/egusphere-egu23-12965, 2023.

High enthalpy geothermal fields are typically associated with magmatic intrusions providing the heat source that makes the system working. To identify and locate these features, geothermal exploration generally uses expensive, time-consuming approaches that could also require complex logistics. Here we present the result of a pilot study carried out in the well-known Larderello-Travale geothermal field (Tuscany, Italy), exploring the possibility of an advantageous use of low temperature thermochronology to obtain useful information implementing the geothermal exploration workflow. The majority of the collected samples (except one retaining the apatite fission-track age of undisturbed, or almost undisturbed country rocks) cluster in a close time span ranging between 3.1±0.8 and 1.9±1.1 Ma, which clearly matches the known ages of magmatic bodies in the region. We propose that this approach can contribute to the identification of sectors recently affected by thermal perturbations that could have led to the development of hydrothermal systems. This approach has allowed us to achieve clues for the presence of subsurface magmatic intrusions even in areas where currently there are no direct indications. Given the small number and distribution of analyzed samples, our contribution represents a first attempt that demonstrates the potentiality of the method in geothermal exploration, but that needs to be verified by further studies involving a larger sampling density. Overall, our results suggest how low temperature thermochronology can be a powerful, fast, and cost-effective tool for geothermal exploration, to be used jointly with the classical methods.

How to cite: Montanari, D., Ruggieri, G., Bonini, M., and Balestrieri, M. L.: Low temperature thermochronology as a tool for geothermal exploration: A promising test from the Larderello-Travale Geothermal Field (Italy)., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13667, https://doi.org/10.5194/egusphere-egu23-13667, 2023.

Seismic noise interferometry (SNI) is based on the computation of seismic response by extracting correlated wavefields from continuous seismic recordings. This seismic response is understood as the Green’s function of the propagation medium and it is typically used to image the subsurface. Furthermore, a regular retrieval of this magnitude in time enables the identification of changes in the structural and mechanical properties of the subsurface and therefore, it can be also used for monitoring purposes.

Here, we use SNI to monitor the steam content in geothermal reservoirs. The massive extraction of fluids commonly causes a pressure drop in the rock matrix and land subsidence in the surroundings of production areas. Furthermore, the boiling point decreases yielding decompression boiling. Estimating the steam fraction at depth is challenging especially in reservoirs with two-phase fluids. In this work, we focus on the Hengill geothermal area (Iceland), where the steam ratio has increased by around 30% in the last 10 years due to the harnessing of geothermal energy.

In this area, we measure the land deformation with synthetic aperture radar interferometry (InSAR) and estimate seismic velocity changes with SNI. We observe that both magnitudes linearly decrease in the long term accordingly to the energy production. Besides, we model the expected seismic velocity drop with in-situ borehole data (temperature, pressure, and steam ratio) and conclude that the seismic velocity drop might be related to the steam evolution within the reservoir. These results offer an innovative way of estimating the steam content in the crust with a surface and non-invasive technique.

How to cite: Sánchez-Pastor, P., Wu, S.-M., Hokstad, K., Kristjánsson, B., Drouin, V., Gunnarsson, G., Ducrocq, C., Rinaldi, A., Obermann, A., and Wiemer, S.: Monitoring steam content in the crust with seismic noise, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14708, https://doi.org/10.5194/egusphere-egu23-14708, 2023.

About half of all heating in Sweden comes from district heating, which makes it the most common form of heating in Sweden. The Otaniemi deep drilling project by St1 in Finland was a game changer, as it launched the idea to feed district heating systems with fossil-free, sustainable geothermal heat from great depths. Göteborg Energi AB has been conducting an exploration drilling program over the last three years targeting major Precambrian deformation zones in radiogenic heat-producing granites. The in-situ temperatures of these granites are boosted by radioactive gamma-ray decay from heat producing elements (K, U, Th). The objective is to develop an engineered geothermal system at 5-7 km depth where the groundwater temperatures reach 120°C to feed the local district heating system. Knowledge of the state of stress is central for this development because it provides insights for the understanding bedrock stability, induced seismicity and fluid flow patterns.

A first 1 km deep sub-vertical borehole in Högsbo was fully cored in 2021, and a second 1 km long inclined borehole in Sisjön was completed in 2022, less than 5 km south of the first borehole. Downhole logging was conducted in both boreholes to constrain downhole temperature, natural gamma radiation, fracture occurrence and orientation of in situ stress.

An acoustic borehole televiewer was used to map fracture occurrence and their geometry, as well as to investigate if stress-induced failure has occurred in the wellbore. For vertical boreholes, drilled parallel with a principal (vertical) stress, borehole breakouts and drilling-induced tensile fractures reveal the orientation of minimum- and maximum horizontal stress, respectively, if the tangential stress concentration generated by the borehole overcomes the compressional and tensile strength of the rock mass, respectively. Here, the objective is to investigate stress-induced failure in an inclined borehole, that is not aligned with a principal stress.

We analysed and processed acoustic borehole images from the Sisjön borehole and mapped fractures occurrence and stress induced features (borehole breakouts and drilling-induced tensile fractures). We have detected some stress-induced features at about 400-500 m depth, but most stress-induced features occur below 800 m. Preliminary results of drilling-induced tensile fractures suggest an orientation of maximum horizontal stress of 145±10°N along the inclined borehole. Corresponding mean orientation from borehole breakouts is slightly lower (132±21°N). More detailed studies are required to confirm the preliminary observations, and to account for the borehole inclination.

How to cite: Ask, M. and Pierdominici, S.: Constraining horizontal stress orientations for geothermal exploration in the inclined Sisjön borehole, Southwest Sweden, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15263, https://doi.org/10.5194/egusphere-egu23-15263, 2023.

Deep geothermal aquifers can be a good source for industry applications with a high heat demand in the course of the energy transition. Industries such as paper production require heat sources that are independent of global market fluctuations, with predictable, reliable and green long-term supply. A number of clastic aquifers have been identified as potentially feasible for geothermal use within the North German Basin ranging from the lower Cretaceous to Middle Bunter Sandstone. Due to the extensive hydrocarbon exploration in the North German Basin, it is often possible to provide a local subsurface analysis of the geothermal potential at the location of the required use.

In this work, we present a case study from the town of Hoya in the southern North German Basin, where an abandoned hydrocarbon exploration well is available, as well as numerous wells in the vicinity (20 km) and some 2D seismic lines. From well log analysis, well reports, seismic data and an available 3D model of Lower Saxony, we evaluate the quality of different clastic reservoirs in the study area. We found? that many of the reservoirs that are known for good quality sandstones in the North German Basin are presented as shales at the paper mill in Hoya. Potentially usable sandstone reservoirs include the lower Cretaceous Aptian (top ca. 1600 m) and Valendis Sandstone (top ca. 1700 m), the middle Jurassic Dogger Delta 2 (top ca. 1950 m) and the Lower Triassic Hardegsen Sandstone (top ca. 3850 m). Surrounding well data, however, show a large spatial heterogeneity for these lithologies over relatively short distances. We include these geological uncertainties, temperature measurements from wells, permeability measurements from well tests and the specific requirements of a paper production company to evaluate the geological and economic feasibility, based on geothermal energy estimations and levelized costs of heat for the individual reservoirs.

How to cite: Kettermann, M., Ritzmann, O., Battermann, J., and Wellmann, F.: Potential for deep geothermal heat production in clastic rocks of the southern North German Basin – A case study from Hoya, Germany, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14105, https://doi.org/10.5194/egusphere-egu23-14105, 2023.

Geothermal energy is a cleaner and more sustainable source of power, which plays a key role in the transition to a low-carbon economy. Sustainable and safe exploitation of geothermal resources, however, depends on our ability to understand and manage the associated seismic risks. In 2018, Nature's Heat geothermal project began operations in Kwintsheul, Netherlands, aiming to supply heat to 64 hectares of greenhouses. Between July and October 2019, a temporary seismic array was installed to monitor for possible seismic activity at the site. Microseismic moment-tensor inversion is a valuable tool for understanding the mechanics and structure of geothermal reservoirs, and for optimizing their exploitation. It can be challenging, though, to apply this technique when there are high levels of ambient seismic noise, as is often the case in geothermal operations in densely populated areas. In this study, we evaluate the feasibility of inverting the centroid moment tensor of microseismic events in Kwintsheul, using probabilistic moment-tensor inversions. We first test the probabilistic inversion using synthetic recordings of ambient seismic noise, after which we apply the technique to the low-magnitude (Md=0.16) event recorded on July 14, 2019. Our results give insight into the challenges and limitations of applying moment-tensor inversion to low-magnitude events in the context of geothermal operations in the Netherlands.

How to cite: Draganov, D., Naranjo, D., Polychronopoulou, K., and Weemstra, C.: The potential of probabilistic moment-tensor inversions for the characterization of geothermal reservoirs in urban environments, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14534, https://doi.org/10.5194/egusphere-egu23-14534, 2023.

Production of geothermal energy relies heavily on subsurface properties. In low-enthalpy geothermal regimes, thermal energy extracted with a borehole heat exchanger (BHE) is the subsurface heat conducted from the rock and grout to the BHE fluid. No subsurface fluids are pumped in or out of the BHE system in the process. However, groundwater flow can bring warm or cold fluid from the surroundings which changes the temperature profile in the well and the vicinity and thus it can affect well heat production.

The geology in Estonia provides a good opportunity to study the groundwater influence on the BHE systems. In Estonia, several hundred meters thick Lower Palaeozoic sedimentary rock sequence is mostly covered by relatively thin Quaternary sediments, while the underlying Lower Proterozoic crystalline basement rocks occur at depths from 150 m in North Estonia to over 600 m in South Estonia. The main groundwater aquifers are confined to the Palaeozoic sedimentary rocks. In North Estonia, the main groundwater aquifers overlying the crystalline basement rocks are Gdov, Voronka, Cambrian-Vendian, Ordovician-Cambrian, and Silurian-Ordovician aquifers. Those aquifers have different thermogeological and hydrogeological parameters shown by several groundwater hydrogeological and hydrodynamic studies and modelling. Only a few geothermal energy studies have been conducted in the Estonian geological setting and the groundwater effects on the geothermal energy budget on the shallow to medium depth geothermal energy systems have not yet been studied in detail.

In this study, we parametrized different hydraulic conditions to compare how the groundwater flow velocity impacts the thermal energy yield of the geothermal system. We modelled single BHEs of lengths of 400 m, 500 m and 1000 m and a BHE field of ten 400 m wells at three sites across Estonia. 400 m wells have a U-tube type heat collector grouted with bentonite clay and the 500 m and 1000 m wells have a coaxial heat exchanger of plastic pipe or a vacuum-insulated tube (VIT), respectively. Our results demonstrate that 1) the thermogeological parameters of the area, such as subsurface temperature and thermal conductivity, are the most significant factor in the thermal energy yield of the wells and 2) at all sites, shallow BHEs are sensitive to the added groundwater flow, whereas 1-kilometer-deep coaxial wells with VIT are the least sensitive to the addition. An interesting highlight is that the increase in the thermal energy yield is not consistent at different locations with the groundwater flow variation. At sites where the aquifers are located deep with respect to the borehole length, an increase in groundwater flow velocity brought more advantages than at sites where permeable layers are distributed more evenly or near the top of the geothermal well.

How to cite: Piipponen, K., Soesoo, A., Bauert, H., and Arola, T.: Modelling Impacts of Groundwater Flow on Borehole Heat Exchangers: Lessons Learned from Estonia, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12480, https://doi.org/10.5194/egusphere-egu23-12480, 2023.

Reliable delivery of economic well performance of geothermal projects is affected to a high degree by uncertain reservoir quality. Construction of multilateral wells is well known as an effective concept to overcome the challenges of reservoir heterogeneity or low permeability by increasing the reservoir contact. However, the drilling costs for these structures are currently very high and multilateral well construction with standard rotary steerable systems is complex. Canopus’ directional steel shot drilling system (DSSD) has the potential to enable the construction of short-radius multilaterals at rates attractive for geothermal applications.

As part of the European GEOTHERMICA project ‘DEPLOI the HEAT’, the operational performance and economic impact of Canopus’ DSSD system will be investigated. Full-scale tests at TNO’s Rijswijk Centre for Sustainable Geo-Energy (RCSG) and a field trial at the Hagerbach underground test facility (VSH) are planned for the first project year. The RCSG drilling rig enables full factory acceptance testing (FAT) of the drilling assembly before the trial at the VSH site is executed. The geology at the VSH site reflects conditions of typical mid-depth fractured limestone reservoirs in Switzerland and elsewhere. At VSH, a complete set of operational parameters and system longevity will be tested with a full-scale trial to prepare for live well deployment.

Further, a techno-economic assessment of multilateral structures drilled with Canopus’ DSSD system will highlight the potential for increasing or safe-guarding well productivity and economically de-risking geothermal projects. Several operators are involved in this project and the techno-economic assessment of the drilling technology will be based on several site-specific data sets provided by them. Stochastic modeling approaches will be implemented to generate an ensemble of equally probable realizations of permeable structures in the subsurface. Then, the performance and costs of different well geometries and multilateral configurations in different subsurface model realizations will be evaluated and compared.

The current status of this project will be presented with a focus on the factory acceptance testing at TNO, the VSH trial preparations and the workflow developed for the techno-economic assessment of this innovative multilateral drilling technology.

How to cite: Reinicke, A., Deb, P., Saar, M. O., Zikovic, V., V., Lassnig, E., Knebel, M., and Blangé, J. J.: Novel Directional Steel Shot Drilling Technology for Short-Radius Multilaterals – Field Application and Commercial Impact, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14928, https://doi.org/10.5194/egusphere-egu23-14928, 2023.

A hydrochemical and geothermometric study of thermal waters in Topusko (Croatia) was conducted in order to improve the existing conceptual model of the Topusko hydrothermal system. The town of Topusko is located 80 km south of Zagreb at the SW edge of the Pannonian Basin System, which has favourable geothermal characteristics. The natural thermal springs of Topusko with temperatures up to 53°C and thermal waters from shallow wells with temperatures up to 65°C represent the second warmest natural thermal water source in Croatia. Water samples have been collected monthly since March 2021 from two natural thermal springs, i.e. Livadski izvor and Blatne kupelji springs, and the TEB-4 thermal well in the discharge area of the hydrothermal system. Furthermore, rainwater was sampled in the supposed recharge area. Chemical composition of groundwater is determined by the original composition of the infiltrated water as well as chemical reactions with various rocks along its circulation path. In-situ parameters, major anions and cations, stable water isotopes, radioactive isotope analysis of tritium (³H), and silica geothermometers were used to assess the origin of thermal waters in Topusko and their interaction with the thermal aquifer. Furthermore, a local meteoric water line was constructed and compared with the isotope ratio of the thermal waters. The results pointed to: i) the meteoric origin of thermal waters according to stable water isotopes; ii) Ca-HCO₃ hydrochemical facies suggesting that carbonate dissolution occurs in the aquifer, being consistent with the presence of carbonates in the stratigraphic logs of wells; iii) an equilibrium temperature in the reservoir of 88°C according to silica geothermometers; and iv) low tritium activity being consistent with sub-modern waters and a recharge before 1955. The monitoring of these thermal springs will continue for another year to collect a larger dataset and reinforce our conclusions. Additional analyses including radiocarbon dating and stable isotope composition from SO₄^2- will be conducted to provide a comprehensive hydrochemical characterisation of these thermal waters, which have been utilised since ancient Roman times, and intensively since 1980s, but have not been thoroughly investigated.

Acknowledgments: Presented research has been conducted in the scope of the project “Multidisciplinary approach to hydrothermal system modelling” (HyTheC) funded by the Croatian Science Foundation under grant number UIP-2019-04-1218.

How to cite: Pavić, M., Briški, M., Pola, M., and Borović, S.: Hydrogeochemical research of thermal waters from Topusko, Croatia, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14202, https://doi.org/10.5194/egusphere-egu23-14202, 2023.

Forecasting of downhole temperatures is of great interest for the development of deep geothermal energy, since the maximum production temperatures are important for the efficiency of the system. The production temperatures are mainly determined by the prevailing reservoir temperature, which is insufficiently known even in well-developed reservoirs. 

Most available temperature data from hydrothermal reservoirs are “bottom hole temperatures” (BHTs) that are usually measured during geophysical measurement programs, after each section of a deep geothermal well has been drilled. These measured temperatures are thermally disturbed by the preceding drilling fluid circulation and therefore show a high deviation from the undisturbed formation temperature, requiring correction of the BHT measurements. This is made possible by a variety of analytical and numerical BHT correction methods, all of which require different input parameters for each method. Those parameters are often documented with poor quality, incompletely, or not at all. It can be assumed that the inaccuracy of a corrected BHT value depends to a high degree on the errors of the input parameters and the choice of the correction method is secondary in this respect.

In order to perform a complete evaluation of corrected BHT values and to determine the range of errors, we corrected BHT values from over 300 current geothermal and old hydrocarbon wells in the Bavarian Molasse Basin in Southern Germany using a Monte Carlo approach. Thus, the corrected temperature was given as a density distribution rather than a discrete value after individual estimation of the error in the input parameters. This allows a prediction of the formation temperature based on risk scenarios, for example specifying a p10 or p90 case.

From the corrected temperatures and taking into account the individual variances studied (p10 and p90 values); we created a set of temperature gradients that take into consideration, if known, the discovered inflow zones and the slope changes in the stratigraphic layers. This approach provides a spatial representation of temperatures while accounting for error propagation by estimates in the correction process, as well as the extrapolation of point temperature data to gradients and the application of geo-statistical methods.

How to cite: Schölderle, F. and Zosseder, K.: Mapping regional variability of reservoir temperatures for hydrothermal use: a statistical model based on parameter uncertainty, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8110, https://doi.org/10.5194/egusphere-egu23-8110, 2023.