ERE2.4

In geothermal exploration, regional 3D geological models, covering the full extent of a sedimentary basin, can be an efficient basis to assess spatial variations in the deep thermal field and related extractable energy. The term “geothermal potential” has been defined in different ways, considering characteristics of an intended geothermal plant and/or the geological reservoir. In this contribution, we concentrate on the latter geological aspects and combine 3D geological with 3D thermal models of the Berlin/Brandenburg region to assess “heat in place” as a quintessential part of the geothermal potential of differently composed geological units.

Our “heat in place” calculations correspond to a volumetric quantification of contained energy distributed across a series of litho-stratigraphic units showing variable thickness, mean temperature, porosity, density, and specific heat capacity. We set special focus on estimating how the calculated heat varies between different thermal models, derived from either purely conductive heat transport or coupled thermal-hydraulic simulations.

As part of the Northeast German Basin, the Berlin/Brandenburg region is known to be potentially suitable for deep geothermal energy exploitation. As this source of energy is not well-established yet, while energy demand from renewables is increasing in the metropolitan area of Berlin, this study aims at contributing to a more efficient decision making on promising sites for geothermal energy production by means of a series of new geothermal potential maps.

How to cite: Benoit, L., Bott, J., and Scheck-Wenderoth, M.: Insights into the deep geothermal potential from heat in place calculations of the Berlin/Brandenburg region (Germany), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5300, https://doi.org/10.5194/egusphere-egu22-5300, 2022.

Issues of sustainability and climate change are among the driving forces of current research aiming to remedy the grave consequences of greenhouse gas emissions. The consequent system change into the usage of regenerative energy sources through the replacing of fossil fuels represents the most promising solution. This structural change poses large challenges not only for metropolitan areas, but also for smaller towns, which intend to increasingly utilize renewable energies. One such example is the city of Minden in the northernmost part of the state of North Rhine-Westphalia, which serves as case study for this project. The city is geologically situated in the inverted part of the Lower Saxony Basin. This represents a sub-basin in the western part of the North German Basin, which is one of three economically viable regions for geothermal energy production in Germany. The investigation of the feasibility of generating electricity and heat through the exploitation of the deep geothermal reservoirs was driven based on the requirements of a company situated in Minden, which exclusively uses fossil fuels for their industrial processes. In order to generate steam, the production requires high temperatures from the subsurface (> 140°C).

The evaluation of geothermal projects conducted in the North German Basin, regional studies and interpretation of 2D seismic data confirmed the existence of the so-called Wiehengebirge Syncline with outcropping Jurassic rocks in the mountain range to the south of Minden. The compilation of all available data and models highlighted Mesozoic layers of the middle Bunter sandstone (Volpriehausen-Solling formations, 4.0-4.5 km), the Keuper (Stuttgart formation, 3.0-3.3 km) and the Dogger (Bathonium-Callovium formations, 1.8-2.4 km) as potential target horizons within the syncline. Moderate to good hydraulic properties are anticipated for the siliciclastic targets. Furthermore, the hydraulic productiveness of a fault mapped in the immediate vicinity of the investigation site was determined through dynamic reservoir simulations.

Moreover, drilling data provided by the hydrocarbon industry from the gas fields to the north of the city (e.g. Uchte) were used to set up a thermo-hydraulic model in order to determine temperatures and flow rates. These should ultimately specify the most promising formations within the afore mentioned Mesozoic units for further geothermal exploration and developments.

Summarizing, first results of this study have confirmed the deep geothermal energy potential of the western part of the North German Basin and the city area of Minden.

How to cite: Knaub, O., Jüstel, A., Bussmann, G., Wellmann, F., and Kukla, P.: Geothermal exploitation in the inverted part of the Lower Saxony Basin: A case study from the Minden area, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12492, https://doi.org/10.5194/egusphere-egu22-12492, 2022.

The western Himalaya-Karakoram region in northern Pakistan has such hydrothermal features as hot springs and alteration zones. The heat source for these features remains unclear, with suggested mechanisms including radiogenic heat production from minerals, frictional heating caused by shearing along faults, residual heat from Miocene plutonic intrusions, metamorphic heat caused by tectonic collision, and heat advection related to rapid exhumation. In this study, we provide a quantitative estimation of the radiogenic heat production of 158 locations from different crystalline lithologies exposed in the three distinct tectonic domains of the western Himalayan-Karakoram region, i.e., Nanga Parbat-Haramosh Massif, Kohistan-Ladakh Batholith, and Karakoram Batholith. The radiogenic heat production values are calculated from the concentrations of the uranium (ppm), thorium (ppm), and potassium (wt%), which are determined directly in the field using a portable gamma spectrometer on exposures of Proterozoic to Tertiary crystalline rocks. The radiogenic heat production in the Nanga Parbat-Haramosh Massif ranges between 0.72 and 18.46 µWm^-3, with mean and median values of 7.12 and 6.74 µWm^-3, respectively. Furthermore, Proterozoic gneisses, Tertiary granites, and pegmatites within the Nanga Parbat-Haramosh Massif have mean radiogenic heat production values of 7.86, 10.67, and 6.47 µWm^-3, respectively. The radiogenic heat production in the Kohistan-Ladakh Batholith ranges between 0.42 and 5.16 µWm-3, averaging at 2.49 µWm^-3 with the highest mean of 3.68 µWm^-3 in granites and lowest 0.74 µWm^-3 in tonalites. The radiogenic heat production of the Karakoram Batholith ranges between 1.04 and 23.54 µWm^-3 with a mean of 5.84 µWm^-3 and a median of 4.45 µWm^-3. Within the Karakoram Batholith, the Tertiary granites have the highest mean radiogenic heat production of 11.17 µWm^-3,  while the lowest mean radiogenic heat production of 2.86 µWm^-3 is found in the Cretaceous diorites. Our results suggest that the Nanga Parbat-Haramosh Massif, which is composed of Proterozoic Indian plate basement rocks, has high concentrations of uranium, thorium, and potassium, and consequently a higher radiogenic heat production. This also correlates with similar high radiogenic heat-producing basement rocks exposed in southern India. The presence of high radiogenic heat production in Tertiary granites and pegmatites indicates mobilization and enrichment of incompatible uranium and thorium due to crustal evolution processes related to the Himalayan Orogeny. We suggest that high radiogenic heat production in Proterozoic rocks may have contributed significantly to the enhanced heat flux in the active Himalayan Orogen.

How to cite: Anees, M., Kley, J., Leiss, B., Wagner, B., and Shah, M. M.: Spatial variation of radiogenic heat production related to the crystalline rock types in the western Himalaya-Karakoram region of Pakistan, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7690, https://doi.org/10.5194/egusphere-egu22-7690, 2022.

To reveal the seismic velocity structure and anisotropy in potential geothermal fields, Kenya, we analyzed ambient noise data from nine vertical components of broadband seismometer stations. Our analysis is based on the cross-correlation of ambient noise data by extracting phase velocity dispersion curves via the zero-crossing method and then applying surface-wave inversion to estimate S-wave velocity structures. The results for both phase velocity and S-wave velocity structures show a clear velocity contrast in volcanic systems (i.e., Korosi-Paka-Silali) in the Kenyan Great Rift (KGR). The phase velocity structure (i.e., 1400 - 2200 km/s) significantly drops in the Silali trachytic shield volcano and the Pleistocene Paka shield volcano. Such velocity contrasts are also observed where they are parallel and perpendicular to the geological structure of the KGR. Most decreasing seismic velocities are normal to abundant faults and fractures in the inner trough of the KGR. This direction is aligned with local extension direction, linked to divergent plate boundaries. The resulting S-wave velocity structures further disclose the anomaly features that can indicate permeable or non-permeable layers. Permeable layers are extensively existing that can provide potential geothermal fluid accumulations. Most potential areas are below the rift floor between the Paka and Silali volcanoes, involved in intense faulting/fractures and young porous rocks (i.e., intercalated pyroclastic, trachyte, and basalt lavas). The lithocap zones are mostly less than 1 km. The magma heat sources are most likely deeper than 3 km at Paka and 4 km at Silali. Therefore, geothermal reservoirs can be relatively interconnected due to shallow magma-volcanic systems, existing groundwater, porous volcanic rocks (i.e., pumice) and intensive fractures controlled by main graben faults in the KGR, regardless of impermeable volcanic rocks or (ore) mineralization in this study area. This work was supported by Leading Enhanced Notable Geothermal Optimization (LENGO) of SATREPS (JICA-JST), Japan.

Keywords: ambient noise, S-wave velocity, seismic anisotropy, shallow magma-reservoir systems, rifting and volcanism

How to cite: Chhun, C., Tsuji, T., and Ikeda, T.: S-wave velocity structure in Kenyan potential geothermal fields inferred from ambient seismic noise data analysis, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1578, https://doi.org/10.5194/egusphere-egu22-1578, 2022.

Multiple operators use geothermal wells in Lower Bavaria and Upper Austria for balneological (medical and wellness) applications and for energy and/or heat mining purposes. These applications depend heavily on the well‘s hydrochemical and geophysical stability (mineralization, pressure, temperature).

At the moment, wells are submitted to inspection once a year, which includes the analysis of ions, pH, pressure, temperature etc. These „offline“ analyses, while covering a large set of parameters, obviously fail to show intra-annual variability within the measured parameters. On the other hand, some geothermal wells are being monitored quasi-continually. These „online“ sensors, however, only cover a small set of selected parameters, such as electric conductivity, temperature and pressure.

This study aims at forecasting hydrochemical and physical stability based on annual measurements by assessing the degree of intra-annual variability covered or neglected by the yearly measurements. The results are required for a sound assessment of possible adverse effects of other exploration activities and short term variations of the withdrawal rates reflecting the demand for heat, energy and/or spa water. To do this, we followed the concept of virtual sensors and their correlation to detailed yearly measurements.

We found that, while annual measurements, when taken approximately in the same season of the year, do match the data sampled online quite well, intra-annual variability at the examined wells was quite strong for some parameters and not represented by the offline data. Thus, annual data can be used to make predictions regarding long-term variability. In order to forecast intra-annual variability, higher temporal resolution is necessary. While not a replacement for the detailed analyses, the virtual sensors presented here provide a robust method to trigger further actions.

How to cite: Dietmaier, A. and Baumann, T.: Forecasting high resolution variations in deep geothermal wells based on low resolution measurements utilizing virtual sensors, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2634, https://doi.org/10.5194/egusphere-egu22-2634, 2022.

   With the development of geothermal energy in response to the 21st century environmental challenges, it appears that developing geothermal energy in off-grid regions can be a durable investment, providing electricity production and various direct thermal use, and bypassing the usual development of fossil energy in remote areas. In this context, the European Union funded project “Geothermal Village”, within the Horizon 2020 LEAP-RE program, aims to set up the geothermal exploration process for isolated areas in the East African Rift Valley. A field mission at Lake Abhé (Djibouti) was set up in late 2021, with geophysical, geochemical and geological researchers, in order to investigate the hydrothermal features of the area and to assess the feasibility of setting up a small-scale geothermal plant for local communities. 

    The area analysed with electrical resistivity tomographies (ERT) on the lake’s eastern shore is characterized by the presence of linear chains of active travertine chimneys built up on top of fluvio-lacustrine deposits. Chimney alignment orientations match those of the major structural lineaments observed through the basaltic units surrounding the lake. The ERT survey was set up with the aim of describing the main accessible fault structures, getting the resistivity information of the local lithologies to calibrate the magneto-tellurics (MT) survey, and getting the chargeability information to describe alteration processes over the geothermally active zones. Profiles of spontaneous polarization have also been done over the majority of ERT profiles.

    The method used was technically challenging, because of field conditions (especially because of the very low resistivity top layers) and incompatibilities between the data logger and the cable set. To process the data, pre-processing has been implemented with Python scripts, which allows poor quality data to improve, essentially thanks to geometrical reallocation of electrodes. The injection protocol is a randomized Wenner, in order to minimize noise due to near-electrode polarisation with steel electrodes.

    The main results show fault structures, and allow to determine fault offsets for some basalt blocs. It also provides resistivity values for the sedimentary top layer, allowing with the basalt resistivity for a low depth calibration of the MT survey, and a preliminary assessment of the distribution of pore water salinity across the area. The chargeability values are less accurate, showing in the best cases good signatures of the chimney’s structure, and demonstrating in the worst cases the artefact effects of the very conductive values of this salty area. 

    In conclusion, this survey has faced a lot of technical issues, but the results are defining more precisely the previous surveys done on this area. The results can be used for geothermal modelling (describing the fluid path within the reservoir and along the chimney’s structures), water exploration (with the salinity assessment), and geotechnical safety (depth of hardened rock for drilling).

How to cite: Piolat, L., Géraud, Y., Diraison, M., Walter, B., Chibati, N., Favier, A., Ibrahim-Ahmed, N.-A., and Magareh, H.-M.: Electrical geophysical exploration of Lac Abhé’s geothermal system, Djibouti, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5775, https://doi.org/10.5194/egusphere-egu22-5775, 2022.

Vintage seismic lines from the area of the city of Aachen from the early 1980s were reprocessed and interpreted in order to characterize the deeper subsurface for its geothermal energy potential. In focus are Lower Carboniferous and Middle to Upper Devonian carbonates as possible karstification is assumed due to the occurrence of thermal springs in the region. Further, extensional NW-SE striking faults of the Tertiary Lower Rhine Graben Rift may provide opportunities for geothermal field development.

The Paleozoic units northwest of the Variscan Rhenish Massif range from Lower Devonian to Upper Carboniferous. Upper Devonian to Upper Carboniferous strata was thrusted onto folded Upper Carboniferous rocks of the foreland molasse basin (e.g. Wurm Syncline) during the Variscan orogeny. As a result, a complex tectonized belt with (at least) three major NE-SW striking thrusts developed, which are cropping out in the region of the city of Aachen. Later, during Upper Cretaceous and Cenozoic times, the region experienced SW-directed rifting and block faulting orthogonal to the Variscan strike as well as inversion movements. Along two of these Paleozoic thrusts, the Aachen Thermal Springs are arising from the subsurface with a temperature of up to 72°C assuming pathways for thermal water from karstified rocks in the deeper subsurface of the Inde Synclinorium. In addition, the strata is exposed along the margins of the Inde Synclinorium or was encountered in wells such as the RWTH-1 well located in the Wurm Syncline or Frenzer Staffel 1 in the Inde Syncline. While the stratigraphy of the geological units is well known from outcrops, their facies distribution and related lithology in the subsurface and within the different fault blocks of the thrust system remains fairly unknown. This was shown from the drilling of the well RWTH-1 in 2004 where the Kohlenkalk (platform carbonates) and the Massenkalk (reef carbonates) were expected but not encountered despite cropping out 10 kilometers to the southwest.

For this investigation, two roughly NW-SE trending seismic lines (~70 km long) were interpreted. The lines were acquired perpendicular to the strike of the Variscan structural elements. The seismic data imaging is challenging due to highly consolidated rocks and very high impedances, partly steep inclination of bedding planes due to thrusting and back-thrusting, low angle faulting (thrust planes), and possible 3D effects where migration is difficult. However, based on the new processing, surface geology correlation, integration of well data, and back-stripping check-ups, we were able to map the key thrust outlines at depths and lithology targets based on their characteristic reflectivity pattern. The most prominent feature on the seismic data is a sharp reflection, which dips approx. south from a very shallow level (~1.000 m) below 6.000 m. The feature was also evident on vintage DEKORP-data and is interpreted as the seismic expression of the Aachen Thrust. Current models can thus be validated or adapted based on our sweet spot maps including the deeper tectonic fabric of the region and regional distribution of carbonate sequences with possible insights on karstification to de-risk future geothermal developments in the area.

How to cite: Jüstel, A., Ritzmann, O., Strozyk, F., and Kukla, P.: Interpretation of seismic reflection vintage lines from the Variscan Fold and Thrust Belt in the Aachen region, Germany: Implications for geothermal exploration, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9276, https://doi.org/10.5194/egusphere-egu22-9276, 2022.

As part of the energy transition effort in Switzerland, the ‘GEothermies’: exploration program carried out in the Geneva Basin and its neighbouring areas (Fig. 1) for direct geothermal heat production and storage is ongoing in the Molasse sedimentary basin. Results from previous hydrocarbon exploration and subsequent geothermal drilling campaigns confirmed the presence of five possible stratigraphic targets for geothermal projects.

This study focuses on the shallowest: the Cretaceous-Cenozoic Transition (CCT), a poorly known stratigraphic interval marked by a sedimentary hiatus related to the tectonic phase and the climate that dominated the area at this period. It displays good geothermal potential for its flow rate due to the occurrence of karstification and open fracture systems in the Lower Cretaceous characterized by low-porosity carbonates. The Alpine orogenesis has subaerially exposed and faulted the Lower Cretaceous generating karsts that were infilled with Eocene and Oligocene coarse sediments: the ‘Sidérolithique’, a quartz-rich sandstone facies, and the ‘Gompholite’, a polygenic conglomerate facies.

A better understanding of this stratigraphic interval is provided by tackling the following aspects:

• Sedimentological framework and diagenetic history of the Cenozoic sediments based on outcrop data and description of cores, further refined by geochemical and mineralogical analyses aimed at reconstructing their depositional processes and environment,
• Petrophysics, reservoir properties and evaluation of their geothermal reservoirs potential,
• Regional 2D geological model based on outcrops, wells record, 2D and 3D seismic data in order to predict their lateral and vertical distribution.

The CCT is identified in a few outcrops in the Salève, the Vuache and the Jura, in 35 wells (Fig. 1) and in seismic data, with a depth ranging between 31.20 m (well SPM-3) and 1376 m (well Thônex; Fig. 1). The most significant ‘Sidérolithique’ deposit of 130 m thickness was recorded at 630 m depth in the geothermal exploration well GEo-02 drilled in 2019. According to cores and outcrops observations, the ‘Sidérolithique’ facies have a variable clay content of kaolinite and chlorite as well as a variable abundance of siderite. In the Geo-02 well, a kaolinite-rich content at the bottom of the 'Sidérolithique' shows a mixed signature of the doline palaeosol with the sandstone fill. Despite the presence of clay and siderite, in the Gex wells, porosity values in the CCT deposits ranges from 0 to 20 % while permeability values can reach up several hundreds of mD (Fig. 2) indicating overall good reservoir potential.

Ultimately, this study aims at improving the predictive capability of the geological model of the Canton of Geneva and neighbouring France and provides new insights on the potential of the CCT deposits as geothermal reservoirs.

Figure 1. Location of the study area with the 35 wells that recorded the CCT. Two NW-SE major faults are showed in red: the Vuache Fault and the Arve Fault.

Figure 2. Summary of porosity and permeability values from 86 plugs sorted by facies from the Gex wells including different units in the CCT interval from younger Molasse (left) to older Lower Cretaceous units (right).

How to cite: Crinière, A. and Moscariello, A.: Reservoir geology of the Cretaceous-Cenozoic Transition in the context of geothermal exploration in the Geneva Basin and neighbouring France (Switzerland & France), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11499, https://doi.org/10.5194/egusphere-egu22-11499, 2022.

There is an increased interest in Scandinavia for development of deep geothermal energy as fossil-free heat-source of district heating system. As part of exploration for Enhanced Geothermal System (EGS) to 5-7 km depth where the groundwater temperature is 120°C can potentially be extracted at 5-7 km depth, Göteborg Energi AB has cored a 1 km deep borehole in Högsbo, southwest Sweden. The objectives of testing was to (1) measure in situ temperatures and derive the geothermal gradient, (2) measure thermal properties of the bedrock, and (3) constrain hydrogeological and mechanical properties of the fracture zone.

The target for drilling and a subsequent EGS facility, is a heat-producing granite that is overlain by a regional fracture zone with inferred elevated permeability. The in-situ temperature of this type granite is boosted by radioactive gamma-ray decay from heat producing elements (K, U, Th).

Knowledge of the state of stress is central to understand bedrock stability, induced seismicity and fluid flow patterns. Acoustic borehole televiewer logging was conducted to map fracture occurrence and their geometry, as well as to investigate of 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.

An unexpected large number of stress indicators has been observed, both borehole breakouts and drilling induced tensile fractures. We observed two types of borehole breakouts: (1) Well-developed borehole breakouts that are clearly visible on both travel-time and amplitude logs; and (2) Poorly-developed borehole breakout that are best visible on the Amplitude log. The shallowest stress indicators are observed from 172-174 m depth, where both types of indicators show NNW-SSE orientation of maximum horizontal stress. This orientation is confirmed from deeper observation, regardless of type of stress indicator. It appears that the regional fracture zone is not influencing the orientation of the stress field in the borehole.

The observations of overlapping drilling induced tensile fractures and borehole breakouts, from such a shallow depth is interesting. It raises questions regarding the magnitudes of stress and the strength of the rock mass. We note that elevated temperature during drilling may induce thermal stresses that favors the formation of drilling induced tensile fractures. On the other hand, the heat producing granites also are known to be probe to weathering. Further studies are required to understand the observed stress indicators, but it appears that the stress orientation is uniform.

How to cite: Ask, M. and Pierdominici, S.: Constraining horizontal stress orientations from acoustic borehole televiewer logs in Högsbo, Southwest Sweden: geothermal exploration, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12768, https://doi.org/10.5194/egusphere-egu22-12768, 2022.

Geothermal energy can be a limitless and CO2-free energy resource. However, moderate geothermal temperature gradients of ∼30 ^oC/km in most regions typically require employing so-called "Advanced" or "Enhanced" geothermal systems, called AGS and EGS, respectively, which require reservoirs with temperatures >150 ^oC. To access such high temperatures, we need to drill deeper than 5 km, i.e., in hard rock. The costs of drilling to such depths, using traditional rotary drilling, increase exponentially with depth and can be up to 80% of the total geothermal project investment. These high drilling costs can be reduced significantly with contactless drilling technologies (e.g., thermal spallation drilling, laser drilling, microwave drilling, and Plasma Pulse Geo-Drilling), as they avoid the lengthy tripping times associated with drill-bit damage.

PPGD uses high-voltage pulses of a few microseconds duration to fracture the rock, thereby drilling without any mechanical abrasion. Future PPGD costs may be as low as 10% of mechanical rotary drilling costs (Schiegg et al., 2015). Our PPGD research addresses two outstanding questions: (1) Understand the fundamental physics of the electric breakdown inside the rock and associated rock fracturing processes, which we investigate numerically (Ezzat et al., 2022, 2021; Vogler et al., 2020; Walsh and Vogler, 2020). (2) Evaluate the PPGD performance under deep-wellbore conditions of  ~5 km (i.e., high pore and lithostatic pressures, and high temperatures). Our ongoing numerical and experimental studies are expected to provide further insights into the applicability of PPGD for geothermal energy utilization.

First, we numerically model the formation of a plasma in rock pores, which constitutes the onset of rock failure during the PPGD process. These numerical models show the significant effect of the pore characteristics on the PPGD process and give insight into how future PPGD operations should be designed. Second, we conduct PPGD physical experiments, where we employ lithostatic pressures of up to 1500 bar, pore pressures of up to 500 bar, temperatures of up to 80 ^oC, and voltages of up to 300 kV. Concluding these experiments with the associated challenges shall demonstrate whether PPGD is efficient at great depths of up to 5 km. Combining our numerical and experimental results allows optimizing future PPGD operations.

References

Ezzat, M., Adams, B. M., Saar, M. O., and Vogler, D. (2022). Numerical modeling of the effects of pore characteristics on the electric breakdown of rock for plasma pulse geo drilling. Energies, 15(1).

Ezzat, M., Vogler, D., Saar, M. O., and Adams, B. M. (2021). Simulating plasma formation in pores under short electric pulses for plasma pulse geo drilling (ppgd). Energies, 14(16).

Schiegg, H. O., Rødland, A., Zhu, G., and Yuen, D. A. (2015). Electro-pulse-boring (epb): Novel super-deep drilling technology for low cost electricity. Journal of Earth Science, 26(1):37–46.

Vogler, D., Walsh, S. D., and Saar, M. O. (2020). A numerical investigation into key factors controlling hard rock excavation via electropulse stimulation. Journal of Rock Mechanics and Geotechnical Engineering, 12(4):793–801.

Walsh, S. D. and Vogler, D. (2020). Simulating electropulse fracture of granitic rock. International Journal of Rock Mechanics and Mining Sciences, 128:104238.

How to cite: Ezzat, M., Börner, J., Vogler, D., Wittig, V., and Saar, M. O.: Plasma Pulse Geo-Drilling as a Low-cost Drilling Technology for Deep-geothermal Energy Utilization: Status and Challenges, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6705, https://doi.org/10.5194/egusphere-egu22-6705, 2022.

Numerical modelling of fluid flow and heat transport in geothermal reservoirs can be very challenging. One reason is the broad range of length-scales that control the flow behaviour, spanning several orders of magnitude from fractures (millimetre-scale) and wells (metre-scale) to facies architecture and faults (kilometre-scale). The usual approach for modelling geothermal reservoirs is to discretise the equations using the finite-volume method and variants thereof to ensure mass conservation, subdividing space onto a fixed, structured mesh. However, using a fixed mesh resolution across the entire model domain can be very computationally expensive, and prohibitive if the model must resolve many length-scales.

Here we report an efficient method to apply dynamic mesh optimisation (DMO) to model geothermal reservoirs. DMO is widely used in other areas of computational fluid dynamics because it offers significant advantages in providing higher resolution, multi-scale solutions at much lower computational cost. However, application of DMO to geothermal reservoir modelling has so far been very limited.

The method reported here uses a surface-based representation of all geological heterogeneity that should be captured in the model. The numerous surfaces in the model represent geologic features such as faults, fractures, and boundaries between rock types with different material properties. The surfaces bound rock volumes, termed geologic domains, within which material properties are constant. When simulating flow and heat transport, the mesh dynamically adapts to optimize the representation of key solutions metrics of interest such as temperature, pressure, flow velocity or fluid saturation, but the surface architecture is preserved. The advantage of this approach is that up-, cross- or down-scaling of material properties during DMO is not required, as the properties are uniform within each geologic domain.

We demonstrate the method using a number of example problems, including complex wells. Another advantage of our approach is that well trajectories are accurately represented as the mesh conforms to the wells. The well trajectory is also preserved during DMO. We show that more accurate results are obtained at lower computational cost as compared to conventional, fixed mesh approaches.

How to cite: Salinas, P., Regnier, G., Jacquemyn, C., Pain, C. C., and Jackson, M. D.: Geothermal reservoir modelling by using dynamic unstructured meshes for improved heat recovery in highly heterogeneous reservoirs, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12358, https://doi.org/10.5194/egusphere-egu22-12358, 2022.

Successful development of the geothermal energy sector requires an understanding of the geological systems and processes that occur within. In the North Alpine Foreland Basin, a hydrogeothermal play type, most sites are affected by inorganic precipitates which reduce the efficiency and safety of plants. Although the gas concentrations in the geothermal waters are generally low, any formation of an independent gas phase induces carbonate precipitation. A quantitative prediction of the precipitates, however, is not yet possible because kinetic data for gas phase induced precipitates is not available.

To address this issue laboratory experiments were run where a gas (CO₂, air) was induced into a column containing an aqueous solution (tap water, water with high salinity). The scalings produced were analyzed by Raman spectroscopy, a mass balance of the process including the dissolving of scaling by inducing CO₂ into the scaled column was based on ion chromatography data. The experiments show that by injecting air into tap water a full stripping of CO₂ occurs which is the experimental proof of the disruption of the lime carbonic acid equilibrium by gas bubbles. The salinity of the initial solution influences - in agreement to previous investigations - the polymorph: only aragonite crystals were detected in tap water (ionic strength: 8.5e-03 mol/L), whereas only calcite crystals showed up in tap water with additional 0.2 g/L NaCl (ionic strength: 1.2e-02 mol/L).  Precipitation was inhibited when 120 g/L NaCl were added to the tap water before stripping (ionic strength: 2.1 mol/L).

The data were evaluated using a combination of hydrogeochemical calculation and precipitation kinetics (PhreeqC) with gas bubble kinetics (Python) and PEST++ for the final parameter adjustment. We successfully modelled the process of stripping, CO₂-injection and the experiments with higher salinity with one model.  This experimental proof of the gas phase induced carbonate precipitation and the new adequate description of the scaling process is a further step to predictive maintenance for geothermal sites and a more reliable holistic site assessment during the planning stage. Together, this improves the sustainability and the attractiveness of the geothermal energy sector to investors.

How to cite: Zacherl, L. and Baumann, T.: Gas phase induced carbonate precipitation - experimental proof and model verification, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2751, https://doi.org/10.5194/egusphere-egu22-2751, 2022.

Geothermal energy is considered as one of the renewable energy resources to meet the world's growing low-carbon energy demand. There are increasing number of oil wells to be abandoned and these wells could be potentially retrofitted to geothermal wells for sustainable purpose. There have been some studies and practices on retrofitting oil wells, but most of the existing retrofitting methods suffer from either low efficiency or high cost. In this study, we designed and modelled a novel retrofitting pattern to a single well featured with enhanced reservoir system (ERS). We established numerical models of ERS in abandoned oil reservoirs configured with a vertical well. We simulated the effects of reservoir initial oil saturation, thickness, permeability and different ERS designs on the production temperature and output power after 50 years. We found these effects have positive feedback on the production temperature and output power. In addition, we also modeled the temperature effect on oil-gas-water relative permeability as it would highly affect the oil viscosity and mobility in the oil reservoir. Moreover, through sensitive analyses of this retrofitted single well and the traditional doublet geothermal well, we found the retrofitted single well in our study could be as high as that from doublet geothermal well, implying that one single abandoned oil well could work either for direct use or power generation with economic yield but little retrofitting investment. Lessons learned in this study might also be applied to other geothermal scenarios, such as enhanced geothermal system.

How to cite: Jia, L., Li, K., and Zhang, C.: Modelling of a retrofitting methodology to revive abandoned oil reservoirs for geothermal exploitation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4982, https://doi.org/10.5194/egusphere-egu22-4982, 2022.

The monitoring of a geothermal reservoir during its development and production stage is crucial for sustainable and long-lasting utilization. Production wells can cut through several feeding aquifers resulting in discharging of a mixture of fluids originating from these different zones. As such, a sample obtained from a well head during conventional sampling represents an average chemical composition of the subsurface fluids. In contrast, a downhole sampler can collect fluids at precise depths and therefore providing an information about fluid properties at individual feed zones. As it has been observed in high temperature geothermal fields, lack of such a knowledge can lead to a decreased production efficiency and high cost of utilization. For example, extreme corrosion rates of perforated liners have been observed at specific depths due to localized mixing of fluids characterized by distinct compositions. Such damages could be avoided by identifying of such mixing depths before well flow test and by casing off these mis-matching feed zones through cement plugs or tiebacks. Similarly, scaling induced by fluid mixing, could be reduced by assessing appropriate casing depth, and therefore preventing the inflow of problematic fluids into the well.

The aim of this study is to design the downhole sampler that is capable of collecting fluids from high temperature wells at up to 300-400 °C during every stage of the geothermal utilization. The chemical data obtained from fluids at different depths will not only help to select the most energy efficient discharge fluids for improved productivity of the well. It will also contribute to the conceptual models of the reservoirs, hence, to better understanding of hydrothermal reservoirs through their production lifetime.

How to cite: Clark, D., Kaldal, G. S., Gunnarsson, B. S., Galeczka, I., Þorbjörnsson, I. Ö., Nielsen, S., and Sigurðardóttir, Á. K.: Geochemical monitoring of the geothermal reservoirs using a high-temperature downhole sampler, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12849, https://doi.org/10.5194/egusphere-egu22-12849, 2022.