Potential Geothermal Reservoirs Research Articles

Granitoids of the western Himalaya and Karakoram as potential geothermal reservoirs – A petrological, geochemical and petrophysical study

Hot springs in various granitoid and gneissic complexes in northern Pakistan indicate elevated geothermal gradients, which warrants their evaluation for geothermal applications. Evaluation of such prospects requires extensive knowledge of rock properties and their interaction with reservoir fluids. Our contribution provides first-order information on petrophysical, geochemical, and petrographic descriptions derived from outcrop analogs. About seventy samples of (mostly) granitoids and gneisses from the Nanga Parbat Massif (NPM), the Kohistan-Ladakh Batholith (KLB), and the Karakoram Batholith (KB) were collected. Biotite, K-feldspar, and plagioclase in the granitoid and gneiss show weak to moderate alteration, which increases in shear zones, suggesting fracture-assisted fluid interaction. Geochemical results indicate that the gneisses and granites of the NPM are mostly peraluminous S-type and show enrichment in most LREEs and depletion in HREEs. In addition, depleted Sr and Ba and enriched Rb and U in granites indicate partial melting and high fractionation. On the contrary, granitoids from the KLB are of calc-alkaline I-type and characterized by the depletion of REEs and the enrichment in Ba and Sr. The granitoids of the KB, due to their different magmatic histories, are more diverse, ranging from older I-type calc-alkaline granodiorite, and younger I-type alkaline syenite and S-type calc-alkaline granite. They show an overall enrichment in HREEs along with Ba, Th, Ta, Sr, and Lu. Allanites from syenite of the KB show significant concentrations of Th and U.Petrophysical measurements reveal low matrix porosities (0.6–3.5 %), with primarily average thermal conductivities (1.48–3.37 W m−1K −1) and thermal diffusivities (0.68–1.95 ∙10–6 m2 s−1), with minor variation in specific heat capacities (744–767 J kg−1 K−1). The NPM displays higher average thermal conductivity, thermal diffusivity, and lower porosity than the KB, while the petrophysical properties of the KLB range between these two domains.The geothermal systems in the area operate due to the thickening of a granitoid-dominant crust, which is enriched with radiogenic elements. The fault zones provide channels for meteoric water to access deeply these hot and thermally conductive rocks in the subsurface. After heat exchange, the water is discharged back to the surface as hot springs. The present data set provides a better understanding of regional geothermal regimes from petrological, geochemical, and petrophysical perspectives and assists in numerical modeling for potential geothermal assessment.

Potential geothermal reservoir systems in the Kenyan Great Rift Valley and volcanic region assessed by ambient noise analysis

To identify potential geothermal reservoirs in the Kenyan Great Rift, we analyzed ambient noise data to reveal seismic velocities and map azimuth-based Rayleigh-wave phase velocities. We extracted phase velocity dispersion curves from the ambient noise cross-correlation using the zero-crossing method and constructed S-wave velocity models using 3D surface-wave tomography. S-wave velocities were lower under Silali and Paka volcanoes (i.e., in magmatic regions) and in fractured areas (i.e., geothermal reservoirs) normal to the rift axis. We detected magmas at depths of >3 km beneath the volcanoes. Low-velocity anomalies may be related to fluid migration pathways. Most potential geothermal reservoirs are located above magma bodies and below the rift axis and are associated with intensive fracturing and porous rocks. Our approach used a limited number of seismometers, making it useful for application in areas with restricted access and challenging deployment; therefore, it could be used for the assessment of preliminary results in other magmatically active regions.

Potential Geothermal Reservoir Systems in the Kenyan Great Rift Valley and Volcanic Region Assessed by Ambient Noise Analysis

Potential Geothermal Reservoir Systems in the Kenyan Great Rift Valley and Volcanic Region Assessed by Ambient Noise Analysis

Petrophysical characterization, BIB-SEM imaging, and permeability models of tight carbonates from the Upper Jurassic (Malm ß), SE Germany

Tight carbonate rocks are important hydrocarbon and potential geothermal reservoirs, for example, in CO2-Enhanced Geothermal Systems. We report a study of outcrop samples of tectonically undeformed tight carbonates from the upper Jurassic “Malm ß” formation in Southern Germany near the town of Simmelsdorf (38 km NE of Nuremberg) to understand bulk petrophysical properties in relation to microstructure and to compare models for permeability prediction in these samples. We applied Archimedes isopropanol immersion, Helium pycnometry, mercury injection, gamma density core logging, and gas permeability measurements, combined with microstructural investigations and liquid metal injection (LMI-BIB-SEM). In addition, ultrasonic velocity was measured to allow geomechanical comparison of stratigraphically equivalent rocks in the South German Molasse Basin (SGMB). Results show only small variations, showing that the formation is rather homogeneous with bulk porosities below 5% and argon permeabilities around 1.4E−17 m2. The presence of stylolites in some of the samples has neither a significant effect on porosity nor permeability. Pores are of submicron size with pore throats around 10 nm and connected as shown by Mercury injection and Liquid Metal injection. Samples have high dynamic Young’s Modulus of 73 ± 5 GPa as expected for lithified and diagenetically overmature limestones. Moreover, no trends in properties were observable toward the faults at meter scale, suggesting that faulting was post-diagenetic and that the matrix permeabilities were too low for intensive post-diagenetic fluid–rock interaction. Petrophysical properties are very close to those measured in the SGMB, illustrating the widespread homogeneity of these rocks and justifying the quarry as a reasonable reservoir analog. Permeability prediction models, such as the percolation theory-based Katz-Thompson Model, Poiseuille-based models, like the Winland, the Dastidar, the capillary tube, and the Kozeny-Carman Models, as well as several empirical models, namely, the Bohnsack, the Saki, and the GPPT Models, were applied. It is shown that the capillary tube Model and the Saki Model are best suited for permeability predictions from BIB-SEM and mercury injection capillary pressure results, respectively, providing a method to estimate permeability in the subsurface from drill cuttings. Matrix permeability is primarily controlled by the pore (throat) diameters rather than by the effective porosity.

Geospatial exploration of geothermal reservoirs in the Souss Massa regions of the anti-atlas and Moroccan Sahara

Geothermal reservoirs refer to any heat stored, naturally, in the earth's crust; it is considered as resource coming from a natural complex depending on the geological, geophysical aspect and territories geochemistry. Although this natural resource remains distinguished by his characters to be a renewable, ecological, universal and sustainable mining energy, it stills not, yet, sufficiently exploitable worldwide (El Bouazouli et al., 2020). The same finding was brought up in Moroccan context, despite the presence of some hot resources and capitalizable scientific research to develop this potential. Prospecting for underground geothermal resources is an expensive project that deserves to mobilize scientific and technical methods in order to overcome this risky impasse. Thus, Geomatics has been used, drawing on Remote Sensing for geo-spatial exploration in order to guide prospecting through the development of geothermal deposits potential maps at very low and medium depth on Moroccan territories (El Bouazouli et al., 2019).Furthermore, we explore smart tools to support short and long-term decision-making in terms of creating of clean energy mix and complementary mining energy as well as investment in energy exploration operations. And, as an experiment, we chose for this study, areas distributed mainly in the Moroccan Sahara and the Souss Massa Region, known, probably, by a potentially high geothermal gradient, and therefore constituting geothermal energy reservoirs. As for the exploration methodology adopted, it consists in processing of the imageries: LANDSAT 8, MODIS AND ASTER, in order to extract exploitation indices such as the surface temperature, the layer of lineaments and cracks density, and evidence of hydrothermal alteration that may indicate geothermal activity. The correlation between these three factors, will lead us to conclude that the Moroccan Sahara area is considered as one of the most potential geothermal reservoir areas in Morocco.In short, the application of remote sensing and GIS will allow us to calculate surface temperature as well as hydrothermal alteration indices and lineaments to identify promising areas for low-energy geothermal (Zhang et al, 2007; El Bouazouli et al., 2020), (Ouzzaouit et al., 2017)[13]. Admittedly, the use of electromagnetic waves, in particular infrared, will have made it possible to identify potential geothermal sites on the three pilot sites Lagouera, Guergarat and Al Mahbes in the Moroccan Sahara.

Controls of dolomitization and bed thickness on fracture networks in Lower Carboniferous carbonates in southern Belgium

Fracture networks in limestone and dolostone quarries in southern Belgium were analyzed as analogues to fractured subsurface Lower Carboniferous carbonates in the Campine-Brabant Basin (northern Belgium, southern Netherlands and westernmost Germany), which are considered as potential geothermal reservoirs. This study focuses on the difference in fracture density, intensity and connectivity in dolostone and limestone quarry walls, as well as on the relationship between bed thickness and fracture density/intensity. Bedding planes and fractures were interpreted based on orthorectified images of quarry walls. Statistical tests show that fracture density and intensity are significantly larger in the dolostones than in the limestones, which is explained by rock mechanical properties. Fracture connectivity is also highest in the studied dolostones. Interpreted bed thickness is significantly larger in limestones than dolostones, but no correlation is present between bed thickness and fracture density or intensity. This may be explained by the low percentage of bed-bounded fractures in the quarry walls. Fracture density, intensity and connectivity are key parameters influencing permeability, especially in carbonates with low matrix porosity. The results suggest that, in light of fracture properties, Lower Carboniferous dolostones might possess better geothermal reservoir characteristics than limestones.

Multivariate and geochemical analyses of brines in Devonian strata of the Western Canada Sedimentary Basin for geothermal energy development

Geothermal favorability maps in the Western Canada Sedimentary Basin (WCSB) are focused on depth and temperature. The transfer of mass and heat by a geothermal system, however, depends on the brine's transport and thermodynamic properties, which are predominantly controlled by its solute geochemistry. As an ongoing attempt to identify, map, and model potential geothermal reservoirs in sedimentary formations in the WCSB, we collect brine samples from various databases and perform rigorous quality control to identify representative formation waters from the carbonate platforms and reefs of the Leduc and Swan Hills formations, and the sandstone of the Granite Wash and Gilwood formations. We carry out multivariate analysis on 1963 representative samples using major chemical components, i.e., sodium, potassium, calcium, magnesium, chloride, bicarbonate, sulfate, and brine's pH. Principal component and cluster analyses identify distinguishable hydrogeochemical groups and the corresponding influential constituents. We then examine the direct influence of fluid chemistry on the engineering aspects of geothermal energy development in terms of parasitic loads and geochemical risks. The high salinities and associated high densities in one of the groups generate significant parasitic loads, e.g., greater than 55% at 110 °C. The geochemical modeling in each group demonstrates the potential scaling risks of dolomite, calcite, and brucite. The geochemical approaches presented here can be applied to geothermal energy development in sedimentary basins around the world.

Characteristics of Precambrian basement intruded by Cretaceous geological intrusions in Monteregian Igneous Province and their impacts on regional thermal structure

With the progress of geothermal exploration in deep buried geological bodies, high radiogenic geological intrusions have become the hot spot in recent years. However, the assessment of the complex structure, lithology of geological intrusions by the geophysical methods has uncertainty, making it a challenging to accurately predict the thermal structure around the geological intrusions. In southern Québec, Canada, recent studies show that a relative high surface heat flux has been detected in the region enclosed by Montréal, Salaberry-de-Valleyfield and Saint-Jean-sur-Richelieu, around the southwest of the Monteregian Hills, which belong to the Early Cretaceous alkaline and carbonatite intrusions. It is not clear whether these Monteregian intrusions have impacts on the thermal anomaly of the Montréal, Salaberry-de-Valleyfield and Saint-Jean-sur-Richelieu region. The objective of this paper is to numerically investigate the thermal structure in the thermal anomaly region, considering the impact of different Monteregian intrusions. The simplified Monteregian intrusions are embedded into a three-dimensional geological model consisting of the sedimentary formations in the St. Lawrence Lowlands and the simulator Underworld2 is used for the thermal modelling. Simulation results show that the geological intrusions in this region have large impacts on the thermal structure at the local-scale, depending on the radiogenic heat production, thermal conductivity, emplacement depth and size. Temperature in the sedimentary formations may be lower or higher than that of the adjacent geological intrusions, highly depending on the thermal physical characteristics of these intrusions. Furthermore, the complex fault systems also strongly control the thermal distribution in different fault blocks, making the Potsdam Group sandstone located between the Grand-St-Esprit and Notre-Dame-du-Bon-Conseil faults as the potential geothermal reservoir. Cited as: Liu, H., Ban, S., Bédard, K., Giroux, B. Characteristics of Precambrian basement intruded by Cretaceous geological intrusions in Monteregian Igneous Province and their impacts on regional thermal structure. Advances in Geo-Energy Research, 2022, 6(3): 206-220. https://doi.org/10.46690/ager.2022.03.04

Thermal springs and active fault network of the central Colca River basin, Western Cordillera, Peru

Thermal waters and vapor discharges (hot springs, geysers, solfataras, and fumaroles) are common phenomena in volcanic regions at active plate boundaries, and the Central Andes are no exception. The Colca River basin in S Peru is a highly diversified and complex thermal region with unresolved questions on the origin of thermal fluids, reservoir temperature, and connections with tectonic and/or volcanic activity. To answer these, we used hydrogeochemical analysis of 35 water samples from springs and geysers, together with isotopic (δ18O and δD) analysis, chemical and mineral studies of precipitates collected in the field around these outflows, and field observations. We aimed (1) to recognize the geochemistry of thermal waters and precipitates in the central part of the Colca River basin, (2) to identify fluid sources and their origin, (3) to estimate the temperature of a potential geothermal reservoir, and (4) to discuss the regional active tectonic and volcanic framework of this geothermal region and mutual relationships.Our results corroborate a heterogeneous and complex geothermal system in the central part of the Colca River basin, with contrasting hydrogeochemical and physical properties, variable isotope composition, different reservoir temperatures, and associated precipitates around thermal springs. Processes controlling water chemistry are closely related to the Ampato-Sabancaya magmatic chamber's activity and tectonic structures that allow complex interactions of meteoric waters with magmatic fluids and gases. With a considerable gradient of pressure owing to local relief and deep incision in the Colca Canyon, these processes led to the differentiation of the thermal waters into three main groups. (1) Chloride-rich, mainly sodium chloride, thermal waters are of meteoric origin but mature within the geothermal reservoir possibly fed by magma degassing. These waters' chemical and isotopic composition results from water-rock interaction and mixing with magmatic waters within the reservoir. These waters discharge at the bottom of the Colca Canyon and Valley, presenting a broad hydrogeochemical spectrum and highly variable mineral phases precipitating at the outflows. The reservoir temperature estimated for these waters ranges from 180 to 200 °C. The group of hottest springs and geysers at the bottom of the Colca Canyon waters are fully equilibrated, with the reservoir temperature ~ 240 °C. (2) Sulfate-rich waters are shallow meteoric waters heated by ascending gases that form an independent group referring to the local water circulation, often controlled by tectonic barriers. (3) Bicarbonate-rich waters are the intermediate meteoric waters, divided into two hydrochemical groups: waters partially equilibrated with reservoir rocks and more similar to chloride-rich waters or additionally enriched with SO4 and more similar to sulfate-rich waters.Studied thermal springs show a clear spatial correlation with active and seismogenic crustal W- to NW-tracing normal and strike-slip faults. These act as barriers to infiltrating meteoric waters, provide pathways to hydrothermal solutions and gases assisting in meteoric water heating, and yield passages for pressured by lithostatic load and heated waters to ascend to the surface.

Identification of Geothermal Reservoirs in South Cameroon from a Combined Landsat 8 and AMT Data

This geothermal source prospecting work is carried out in order to respond to the energy deficit affecting the populations of South Cameroon in particular. The study combines audio-magnetotelluric (AMT) and Landsat 8 data for the geolocation of reservoirs, as well as the geological and tectonic information. The processing of these data defined 5 profiles and led to two-dimensional models of resistivity sections for each profile. The interpretation of the data identified a highly conductive zone with a resistivity ranged 1–10 Ω·m at an estimated depth of 1200 m in the locality of Menve’ele. The Landsat 8 data collected in this area during the same period enabled the elaboration of temperature and lineament anomaly maps of the study area. High-temperature areas with high lineament predominance correlate better with areas of low resistivity. For example, Menve’ele (A1) has a surface temperature range of 20°C–30°C and a high density of lineaments. By correlating these results with those provided by previous authors, the Menve’ele locality may be the target of a potential geothermal reservoir. A 3D resistivity model is built to better understand the variations of resistivity and their depths. The geological and tectonic information of the region confirm the tectonic origin of the faults and lineaments as well as the predominance of metamorphic and sedimentary formations.

Potential Geothermal Reservoir Systems in the Kenyan Great Rift Valley and Volcanic Region Assessed by Ambient Noise Analysis

Potential Geothermal Reservoir Systems in the Kenyan Great Rift Valley and Volcanic Region Assessed by Ambient Noise Analysis

Potential Geothermal Reservoir System in the Kenyan Great Rift Valley and Volcanic Region Assessed by Ambient Noise Analysis

Potential Geothermal Reservoir System in the Kenyan Great Rift Valley and Volcanic Region Assessed by Ambient Noise Analysis

Use of Mohr Diagrams to Predict Fracturing in a Potential Geothermal Reservoir

Inferences have to be made about likely structures and their effects on fluid flow in a geothermal reservoir at the pre-drilling stage. Simple mechanical modelling, using reasonable ranges of values for rock properties, stresses and fluid pressures, is used here to predict the range of possible structures that are likely to exist in the sub-surface and that may be generated during stimulation of a potential geothermal reservoir. In particular, Mohr diagrams are used to show under what fluid pressures and stresses different types and orientations of fractures are likely to be reactivated or generated. The approach enables the effects of parameters to be modelled individually, and for the types and orientations of fractures to be considered. This modelling is useful for helping geoscientists consider, model, and predict the ranges of mechanical properties of rock, stresses, fluid pressures, and the resultant fractures that are likely to occur in the sub-surface. Here, the modelling is applied to folded and thrusted greywackes and slates, which are planned to be developed as an Enhanced Geothermal System beneath Göttingen.

The optimization of a potential geothermal reservoir using abandoned wells: a case study for the forest reserve field in Trinidad

The worldwide dependence on non-renewable energy sources continues as existing energy systems have been built on these supplies. There is an established link between these conventional energy resources, greenhouse gas (GHG) emissions and climate change and its associated negative effects. As a signatory to the Paris Agreement, Trinidad and Tobago (TT) has been exploring strategies to reduce GHG emissions and the use of geothermal energy is one potential option. Through enhanced geothermal systems, TT, even without extensive volcanic heat reservoirs, can still develop this renewable source by utilizing oil reservoirs. This study evaluates the possibility of designing and evaluating geothermal systems using wells from the Forest Reserve fields in South-western Trinidad as a case study. The Forest Reserve fields have a high number of abandoned oil and gas wells which can be re-used for tapping the required heat and reducing the requirement for drilling of new injection and production geothermal wells. Key information and data from these wells and reservoirs were used as input for CMG software to model, design and evaluate the feasibility of the geothermal reservoir and wells as being possible and viable for geothermal production. A base model was constructed in the CMG software which was subjected to three essential manual sensitivity analyses (well distance, stream temperature, and injection pressure) to obtain an optimized model which was then subjected to hydraulic fracturing. The optimal model of the retrofitted geothermal systems demonstrated to be the best case scenario due to the shallow nature of the reservoir in the area of interest. Six retrofitted geothermal systems (3 injector wells and 3 producer wells) showed that capable capacity of 3.3721 MWe for a 25-year period with an Internal Rate of Return of 190% and an Net Present Value of US$1,431,263,840.00 utilizing a Minimum Acceptable Rate of Return of 10%, Capital Expenditure of US$12MM, Operating Expenditure of US$2MM and a cost of geothermal electricity at US$0.05 per kWh. These results were then used in Crystal Ball to apply Monte Carlo simulations where it confirmed that the project is 100% economically feasible. The cumulative carbon dioxide reduction after the 25-year period was 50,062,500 tons of CO2.

Oilfield geothermal resources of the Upper Assam Petroliferous Basin, NE India

Extracting geothermal energy from the oil-producing fields is an experimental venture globally. The exploitation and utilization of geothermal energy can partly reduce the larger dependence on conventional non-renewable energy sources like oil, gas, coal, and other fossil fuels, and has a bright prospect. The Upper Assam Basin is a mature petroliferous basin of NE India, where there are several hundred low production, high water cut, or abandoned oil and gas wells that can be retrofitted as geothermal wells instead of drilling new ones. This will help bridge the gap of growing energy demand and limited supply in energy-deficient state like Assam. Situated away from the active plate boundaries and in lack of active volcanism, the Upper Assam Basin remains a low-to-medium enthalpy geothermal fluid regime. The deeper reservoir in this regard can, therefore, be the best candidate for the introspection of the potential geothermal energy reservoir reconnaissance. The selection of a deeper horizon considered in the present case has been the stratified reservoirs of the Lakadong-Therria (Lk-Th) Formation, Sylhet Group of the Lower Eocene age occurring at a variable depth of 3400 m–4600 m. The Lk-Th Formation possesses a fair-quality reservoir with lateral continuity and favourable petrophysical properties. In this study, representative gamma-ray (GR) and resistivity (R) logs were examined to work out lithology, and bed boundary demarcation, etc. The total Formation thickness varies from 97 to 157 m; the individual sand body thickness is up to 6 m. Other reservoir parameters, e.g., porosity (Φ = 8–33 %), water saturation (Sw = 4.57–95.15 %), geothermal gradient (2.71 °C/100m to 3.92 °C/100 m at 4300 m and 3608 m) respectively, and theoretical estimate of high heat flux in the range 70–100 mW/m2/s, are the necessary yard-stick to measure the subsurface geothermal reserves. Efficient energy extraction will have the potential in facilitating energy utilization for industrial purposes, especially in tea processing units present nearby oilfields and also for power generation by the binary mechanism.

Lower carboniferous fractured carbonates of the Campine Basin (NE-Belgium) as potential geothermal reservoir: Age and origin of open carbonate veins

Within the Lower Carboniferous limestones in the boreholes of Poederlee, Heibaart and Turnhout (Campine Basin, NE-Belgium), open veins partially cemented by calcite and dolomite are considered important with regard to the geothermal reservoir potential of these limestones. Based on a detailed petrographic analysis, stable carbon and oxygen as well as clumped Δ47 isotope analysis and U/Pb age dating, the formation conditions of these veins have been reconstructed and were placed in the geodynamic evolution of the sedimentary basin. All host rocks are recrystallized, and the open carbonate vein cements often possess a similar depleted δ18OPDB signature as their recrystallized host rock. The latter thus indicates vein cement precipitation was likely rock-buffered. The δ13C signature of the host rock often displays a marine signature, while in the veins two isotopic clusters can be differentiated. The first cluster groups veins with cleavage twins, and are characterized by slightly to moderately depleted δ13C, reflecting a mixture of marine carbon with carbon derived from organic matter. This group typically occurs in the open veins from the microbially dominated Poederlee and Heibaart buildups characterised by on-lapping organic-rich Namurian black shales. One of these veins formed at 216 ± 6 °C, indicating that it crystallised during the Sudetic – Asturian deformation stage of late Carboniferous age, while others formed between 110 and 130 °C, indicating that they could have been formed during the late Carboniferous burial stage or after the Sudetic – Asturian deformation and uplift. The second cluster groups veins that may, but often do not, contain cleavage twins. They show host-rock buffered δ13C signatures. These veins have a Cretaceous (late Aptian/early Albian) and/or Paleocene U/Pb age and formed at temperatures dominantly between 50 and 80 °C based on clumped isotopes.

Geothermal Resources Recognition and Characterization on the Basis of Well Logging and Petrophysical Laboratory Data, Polish Case Studies

Examples from the Polish clastic and carbonate reservoirs from the Central Polish Anticlinorium, Carpathians and Carpathian Foredeep are presented to illustrate possibilities of using well logging to geothermal resources recognition and characterization. Firstly, there was presented a short description of selected well logs and methodology of determination of petrophysical parameters useful in geothermal investigations: porosity, permeability, fracturing, mineral composition, elasticity of orogeny and mineralization of formation water from well logs. Special attention was allotted to spectral gamma-ray and temperature logs to show their usefulness to radiogenic heat calculation and heat flux modelling. Electric imaging and advanced acoustic logs provided with continuous information on natural and induced fracturing of formation and improved lithology recognition. Wireline and production logging were discussed to present the wealth of methods that could be used. A separate matter was thermal conductivity provided from the laboratory experiments or calculated from the results of the comprehensive interpretation of well logs, i.e., volume or mass of minerals composing the rocks. It was proven that, in geothermal investigations and hydrocarbon prospection, the same petrophysical parameters are considered, and well-logging acquisition equipment and advanced methods of processing and interpretation, developed and improved for almost one hundred years, can be successfully used in the detection and characterization of the potential geothermal reservoirs. It was shown that the newest (current investment)—as well as the old type (archive)—logs provide useful information.

Characterization of discontinuities in potential reservoir rocks for geothermal applications in the Rhine-Ruhr metropolitan area (Germany)

Abstract. The importance of research into clean and renewable energy solutions has increased over the last decade. Geothermal energy provision is proven to meet both conditions. Therefore, conceptual models for deep geothermal applications were developed for different field sites regarding different local conditions. In Bavaria, Germany, geothermal applications were successfully carried out in carbonate horizons at depths of 4000 to 6000 m. Matrix permeability and thermal conductivity was mainly studied in karstified carbonates from the Late Jurassic reef facies. Similar to Bavaria, carbonates are located in the east of the Rhenohercynian Massif, in North Rhine-Westphalia (NRW), for which quantification of the geothermal potential is still lacking. Compared to Bavaria, a supraregional carbonate mountain belt is exposed at the Remscheid-Altena anticline (in NRW) from the Upper Devonian and Lower Carboniferous times. The aim of our study was to examine the potential geothermal reservoir by field and laboratory investigations. Therefore, three representative outcrops in Wuppertal, Hagen-Hohenlimburg, and Hönnetal were studied. During field surveys, 1068 discontinuities (139 open fractures without any filling, 213 joints, 413 veins filled with calcite, and 303 fractures filled with debris deposits) at various spatial scales were observed by scanline surveys. These discontinuities were characterized by trace length, true spacing, roughness, aperture, and filling materials. Discontinuity orientation analysis indicated three dominant strike orientations in NNW–SSE, NW–SE, and NE–SW directions within the target horizon of interest. This compacted limestone layer (Massenkalk) is approximately 150 m thick and located at 4000 to 6000 m depth, dipping northwards at a dip angle of about 30 to 40∘. An extrapolation of the measured layer orientation and dip suggests that the carbonate reservoir could hypothetically extend below Essen, Bochum, and Dortmund. Our combined analysis of the field and laboratory results has shown that it could be a naturally fractured carbonate reservoir. We evaluated the potential discontinuity network in the reservoir and its orientation with respect to the prevailing maximum horizontal stress before concluding with implications for fluid flow: we proposed focusing on prominent discontinuities striking NNW–SSE for upcoming geothermal applications, as these (1) are the most common, (2) strike in the direction of the main horizontal stress, (3) have a discontinuity permeability that significantly exceeds that of the reservoir rock matrix, and (4) only about 38 % of these discontinuities were observed with a calcite filling. The remaining discontinuities either showed no filling material or showed debris deposits, which we interpret as open at reservoir depth. Our results indicate that even higher permeability can be expected for karstified formations related to the reef facies and hydrothermal processes. Our compiled data set, consisting of laboratory and field measurements, may provide a good basis for 3D subsurface modelling and numerical prediction of fluid flow in the naturally fractured carbonate reservoir.

Lithofacies classification of a geothermal reservoir in Denmark and its facies-dependent porosity estimation from seismic inversion

Characterization of geothermal reservoirs is an important step for exploration and development of geothermal energy, which is reliable and sustainable for the future. Based on the inversion results of seismic reflection data, lithofacies and porosity are predicted beyond well locations on a potential geothermal reservoir in the north of Copenhagen, onshore Denmark. To classify the lithofacies, a new system of Artificial Neural Networks-Hidden Markov Models is proposed to consider the complex spatial distribution of rock properties and the intrinsic depositional rules. Artificial Neural Networks can overcome the common Gaussian assumption for the distribution of rock properties. At the same time, the transition matrix in Hidden Markov Models provides the conditional probability for the lithofacies transitions along the vertical direction. After classification, the resulting lithofacies are used to constrain the porosity prediction, in which the Artificial Neural Networks is trained and applied within each type of lithofacies, as a regression process. The novelty of this approach is in the integration of statistics and computer science algorithms that allows capturing hidden and complex relations in the data that cannot be explained by traditionally deterministic geophysical equations. This workflow could also improve the prediction accuracy and the uncertainty quantification of the porosity distribution given rock properties.

Results of a walk‐above vertical seismic profiling survey acquired at the Thônex‐01 geothermal well (Switzerland) to delineate fractured carbonate formations for geothermal development

ABSTRACTThis paper deals with the design, acquisition and processing of vertical seismic profiling data collected in 2016 at the directional Thônex‐01 geothermal well, drilled in 1993 in the Geneva Canton (Switzerland). The aim of the study was to obtain additional information from existing‐abandoned well for downhole geophysical characterization of potential geothermal reservoirs in the Geneva area. The main results obtained by this study allowed an improved subsurface image by multi‐offset acquisitions near and around the deviated well. Specifically we were able to (1) improve the velocity model of the Geneva Basin, which, at present, is only constrained by one unique deep exploration well, located at the south‐west of the basin, far away from the study area; (2) image the transition between the Tertiary Molasse siliciclastic sediments and the Mesozoic carbonates; (3) define an acquisition strategy for future vertical seismic profiling surveys in support of the geothermal exploration campaigns in the area. This study has demonstrated the value of vertical seismic profiling method for the refinement of the subsurface geology at local scale, and its usefulness to assist the planning of second well associated with geothermal doublets.