Editorial

9 pages, 1010 KiB

Open AccessEditorial

Editorial of Special Issue “Enhanced Geothermal Systems and Other Deep Geothermal Applications throughout Europe: The MEET Project”

by Béatrice A. Ledésert, Ronan L. Hébert, Ghislain Trullenque, Albert Genter, Eléonore Dalmais and Jean Herisson

Geosciences 2022, 12(9), 341; https://doi.org/10.3390/geosciences12090341 - 13 Sep 2022

Cited by 4 | Viewed by 1371

Abstract

The MEET project is a Multidisciplinary and multi-context demonstration of Enhanced Geothermal Systems exploration and Exploitation Techniques and potentials, which received funding from the European Commission in the framework of the Horizon 2020 program [...] Full article

(This article belongs to the Special Issue Enhanced Geothermal Systems and other Deep Geothermal Applications throughout Europe: The MEET Project)

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Research

Jump to: Editorial

16 pages, 3224 KiB

Open AccessArticle

Use of Analogue Exposures of Fractured Rock for Enhanced Geothermal Systems

by D. C. P. Peacock, David J. Sanderson and Bernd Leiss

Geosciences 2022, 12(9), 318; https://doi.org/10.3390/geosciences12090318 - 26 Aug 2022

Cited by 3 | Viewed by 1394

Abstract

Field exposures are often used to provide useful information about sub-surface reservoirs. This paper discusses general lessons learnt about the use of deformed Devonian and Carboniferous meta-sedimentary rocks in the Harz Mountains, Germany, as analogues for a proposed enhanced geothermal reservoir (EGS) at [...] Read more.

Field exposures are often used to provide useful information about sub-surface reservoirs. This paper discusses general lessons learnt about the use of deformed Devonian and Carboniferous meta-sedimentary rocks in the Harz Mountains, Germany, as analogues for a proposed enhanced geothermal reservoir (EGS) at Göttingen. The aims of any analogue study must be clarified, including agreeing with people from other disciplines (especially reservoir modellers) about the information that can and cannot be obtained from surface exposures. Choice of an analogue may not simply involve selection of the nearest exposures of rocks of a similar age and type, but should involve consideration of such factors as the quality and geological setting of the analogue and reservoir, and of any processes that need to be understood. Fieldwork should focus on solving particular problems relating to understanding the EGS, with care being needed to avoid becoming distracted by broader geological issues. It is suggested that appropriate questions should be asked and appropriate analyses used when planning a study of a geothermal reservoir, including studies of exposed analogues. Full article

(This article belongs to the Special Issue Enhanced Geothermal Systems and other Deep Geothermal Applications throughout Europe: The MEET Project)

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18 pages, 5987 KiB

Open AccessArticle

Hydrothermal Numerical Simulation of Injection Operations at United Downs, Cornwall, UK

by Saeed Mahmoodpour, Mrityunjay Singh, Christian Obaje, Sri Kalyan Tangirala, John Reinecker, Kristian Bär and Ingo Sass

Geosciences 2022, 12(8), 296; https://doi.org/10.3390/geosciences12080296 - 29 Jul 2022

Cited by 4 | Viewed by 1783

Abstract

The United Downs Deep Geothermal Project (UDDGP) is designed to utilize a presumably permeable steep dipping fault damage zone (constituting the hydrothermal reservoir in a very low permeability granitic host rock) for fluid circulation and heat extraction between an injection well at 2.2 [...] Read more.

The United Downs Deep Geothermal Project (UDDGP) is designed to utilize a presumably permeable steep dipping fault damage zone (constituting the hydrothermal reservoir in a very low permeability granitic host rock) for fluid circulation and heat extraction between an injection well at 2.2 km depth (UD−2) and a production well at 5 km depth (UD−1). Soft hydraulic stimulation was performed to increase the permeability of the reservoir. Numerical simulations are performed to analyze the hydraulic stimulation results and evaluate the increase in permeability of the reservoir. Experimental and field data are used to characterize the initial reservoir static model. The reservoir is highly fractured, and two distinct fracture networks constitute the equivalent porous matrix and fault zone, respectively. Based on experimental and field data, stochastic discrete fracture networks (DFN) are developed to mimic the reservoir permeability behavior. Due to the large number of fractures involved in the stochastic model, equivalent permeability fields are calculated to create a model which is computationally feasible. Hydraulic test and stimulation data from UD−1 are used to modify the equivalent permeability field based on the observed difference between the real fractured reservoir and the stochastic DFN model. Additional hydraulic test and stimulation data from UD−2 are used to validate this modified permeability. Results reveal that the equivalent permeability field model derived from observations made in UD−1 is a good representation of the actual overall reservoir permeability, and it is useful for future studies. The numerical simulation results show the amount of permeability changes due to the soft hydraulic stimulation operation. Based on the validated permeability field, different flow rate scenarios of the petrothermal doublet and their respective pressure evolution are examined. Higher flow rates have a strong impact on the pressure evolution. Simulations are performed in the acidized enhanced permeability region to make a connection between the ongoing laboratory works on the acid injection and field response to the possible acidizing stimulation. Full article

(This article belongs to the Special Issue Enhanced Geothermal Systems and other Deep Geothermal Applications throughout Europe: The MEET Project)

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33 pages, 7332 KiB

Open AccessArticle

Fracture Transmissivity in Prospective Host Rocks for Enhanced Geothermal Systems (EGS)

by Johannes Herrmann, Valerian Schuster, Chaojie Cheng, Harald Milsch and Erik Rybacki

Geosciences 2022, 12(5), 195; https://doi.org/10.3390/geosciences12050195 - 03 May 2022

Cited by 3 | Viewed by 2104

Abstract

We experimentally determined the hydraulic properties of fractures within various rock types, focusing on a variety of Variscan rocks. Flow-through experiments were performed on slate, graywacke, quartzite, granite, natural fault gouge, and claystone samples containing an artificial fracture with a given roughness. For [...] Read more.

We experimentally determined the hydraulic properties of fractures within various rock types, focusing on a variety of Variscan rocks. Flow-through experiments were performed on slate, graywacke, quartzite, granite, natural fault gouge, and claystone samples containing an artificial fracture with a given roughness. For slate samples, the hydraulic transmissivity of the fractures was measured at confining pressures, p_c, at up to 50 MPa, temperatures, T, between 25 and 100 °C, and differential stress, σ, acting perpendicular to the fracture surface of up to 45 MPa. Fracture transmissivity decreases non-linearly and irreversibly by about an order of magnitude with increasing confining pressure and differential stress, with a slightly stronger influence of p_c than of σ. Increasing temperature reduces fracture transmissivity only at high confining pressures when the fracture aperture is already low. An increase in the fracture surface roughness by about three times yields an initial fracture transmissivity of almost one order of magnitude higher. Fractures with similar surface roughness display the highest initial transmissivity within slate, graywacke, quartzite and granite samples, whereas the transmissivity in claystone and granitic gouge material is up to several orders of magnitude lower. The reduction in transmissivity with increasing stress at room temperature varies with composition and uniaxial strength, where the deduction is lowest for rocks with a high fraction of strong minerals and associated high brittleness and strength. Microstructural investigations suggest that the reduction is induced by the compaction of the matrix and crushing of strong asperities. Our results suggest that for a given surface roughness, the fracture transmissivity of slate as an example of a target reservoir for unconventional EGS, is comparable to that of other hard rocks, e.g., granite, whereas highly altered and/or clay-bearing rocks display poor potential for extracting geothermal energy from discrete fractures. Full article

(This article belongs to the Special Issue Enhanced Geothermal Systems and other Deep Geothermal Applications throughout Europe: The MEET Project)

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23 pages, 2854 KiB

Open AccessArticle

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

by Katja E. Schulz, Kristian Bär and Ingo Sass

Geosciences 2022, 12(1), 35; https://doi.org/10.3390/geosciences12010035 - 12 Jan 2022

Cited by 3 | Viewed by 2852

Abstract

A hydrothermal doublet system was drilled in a fault-related granitic reservoir in Cornwall. It targets the Porthtowan Fault Zone (PTF), which transects the Carnmenellis granite, one of the onshore plutons of the Cornubian Batholith in SW England. At 5058 m depth (TVD, 5275 [...] Read more.

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

(This article belongs to the Special Issue Enhanced Geothermal Systems and other Deep Geothermal Applications throughout Europe: The MEET Project)

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18 pages, 4674 KiB

Open AccessArticle

Impact of Well Placement in the Fractured Geothermal Reservoirs Based on Available Discrete Fractured System

by Saeed Mahmoodpour, Mrityunjay Singh, Kristian Bär and Ingo Sass

Geosciences 2022, 12(1), 19; https://doi.org/10.3390/geosciences12010019 - 04 Jan 2022

Cited by 15 | Viewed by 2600

Abstract

Well placement in a given geological setting for a fractured geothermal reservoir is necessary for enhanced geothermal operations. High computational cost associated with the framework of fully coupled thermo-hydraulic-mechanical (THM) processes in a fractured reservoir simulation makes the well positioning a missing point [...] Read more.

Well placement in a given geological setting for a fractured geothermal reservoir is necessary for enhanced geothermal operations. High computational cost associated with the framework of fully coupled thermo-hydraulic-mechanical (THM) processes in a fractured reservoir simulation makes the well positioning a missing point in developing a field-scale investigation. To enhance the knowledge of well placement for different working fluids, we present the importance of this topic by examining different injection-production well (doublet) positions in a given fracture network using coupled THM numerical simulations. Results of this study are examined through the thermal breakthrough time, mass flux, and the energy extraction potential to assess the impact of well position in a two-dimensional reservoir framework. Almost ten times the difference between the final amount of heat extraction is observed for different well positions but with the same well spacing and geological characteristics. Furthermore, the stress field is a strong function of well position that is important concerning the possibility of high-stress development. The objective of this work is to exemplify the importance of fracture connectivity and density near the wellbores, and from the simulated cases, it is sufficient to understand this for both the working fluids. Based on the result, the production well position search in the future will be reduced to the high-density fracture area, and it will make the optimization process according to the THM mechanism computationally efficient and economical. Full article

(This article belongs to the Special Issue Enhanced Geothermal Systems and other Deep Geothermal Applications throughout Europe: The MEET Project)

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33 pages, 12554 KiB

Open AccessArticle

Fracture Spacing Variability and the Distribution of Fracture Patterns in Granitic Geothermal Reservoir: A Case Study in the Noble Hills Range (Death Valley, CA, USA)

by Arezki Chabani, Ghislain Trullenque, Johanne Klee and Béatrice A. Ledésert

Geosciences 2021, 11(12), 520; https://doi.org/10.3390/geosciences11120520 - 17 Dec 2021

Cited by 8 | Viewed by 2778

Abstract

Scanlines constitute a robust method to better understand in 3D the fracture network variability in naturally fractured geothermal reservoirs. This study aims to characterize the spacing variability and the distribution of fracture patterns in a fracture granitic reservoir, and the impact of the [...] Read more.

Scanlines constitute a robust method to better understand in 3D the fracture network variability in naturally fractured geothermal reservoirs. This study aims to characterize the spacing variability and the distribution of fracture patterns in a fracture granitic reservoir, and the impact of the major faults on fracture distribution and fluid circulation. The analogue target named the Noble Hills (NH) range is located in Death Valley (DV, USA). It is considered as an analogue of the geothermal reservoir presently exploited in the Upper Rhine Graben (Soultz-sous-Forêts, eastern of France). The methodology undertaken is based on the analyze of 10 scanlines located in the central part of the NH from fieldwork and virtual (photogrammetric models) data. Our main results reveal: (1) NE/SW, E/W, and NW/SE fracture sets are the most recorded orientations along the virtual scanlines; (2) spacing distribution within NH shows that the clustering depends on fracture orientation; and (3) a strong clustering of the fracture system was highlighted in the highly deformed zones and close to the Southern Death Valley fault zone (SDVFZ) and thrust faults. Furthermore, the fracture patterns were controlled by the structural heritage. Two major components should be considered in reservoir modeling: the deformation gradient and the proximity to the regional major faults. Full article

(This article belongs to the Special Issue Enhanced Geothermal Systems and other Deep Geothermal Applications throughout Europe: The MEET Project)

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15 pages, 3501 KiB

Open AccessArticle

Soultz-sous-Forêts Geothermal Reservoir: Structural Model Update and Thermo-Hydraulic Numerical Simulations Based on Three Years of Operation Data

by Clément Baujard, Pauline Rolin, Éléonore Dalmais, Régis Hehn and Albert Genter

Geosciences 2021, 11(12), 502; https://doi.org/10.3390/geosciences11120502 - 09 Dec 2021

Cited by 9 | Viewed by 3185

Abstract

The geothermal powerplant of Soultz-sous-Forêts (France) is investigating the possibility of producing more energy with the same infrastructure by reinjecting the geothermal fluid at lower temperatures. Indeed, during the operation of the powerplant, the geothermal fluid is currently reinjected at 60–70 °C in [...] Read more.

The geothermal powerplant of Soultz-sous-Forêts (France) is investigating the possibility of producing more energy with the same infrastructure by reinjecting the geothermal fluid at lower temperatures. Indeed, during the operation of the powerplant, the geothermal fluid is currently reinjected at 60–70 °C in a deep fractured granite reservoir, and the MEET project aims to test its reinjection at 40 °C. A 3D hydrothermal study was performed in order to evaluate the spreading of the thermal front during colder reinjection and its impact on the production temperature. In the first step, a 3D structural model at fault scale was created, integrating pre-existing models from 2D vintage seismic profiles, vertical seismic profiles, seismic cloud structure and borehole image logs calibrated with well data. This geometrical model was then adapted to be able to run hydrothermal simulation. In the third step, a 3D hydrothermal model was built based on the structural model. After calibration, the effect of colder reinjection on the production temperature was calculated. The results show that a decrease of 10 °C in the injection temperature leads to a drop in the production temperature of 2 °C after 2 years, reaching 3 °C after 25 years of operation. Lastly, the accuracy of the structural model on which the simulations are based is discussed and an update of the structural model is proposed in order to better reproduce the observations. Full article

(This article belongs to the Special Issue Enhanced Geothermal Systems and other Deep Geothermal Applications throughout Europe: The MEET Project)

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27 pages, 6506 KiB

Open AccessArticle

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

by D.C.P. Peacock, David J. Sanderson and Bernd Leiss

Geosciences 2021, 11(12), 501; https://doi.org/10.3390/geosciences11120501 - 09 Dec 2021

Cited by 6 | Viewed by 4591

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Special Issue Enhanced Geothermal Systems and other Deep Geothermal Applications throughout Europe: The MEET Project)

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23 pages, 109705 KiB

Open AccessArticle

Study of Corrosion Resistance Properties of Heat Exchanger Metals in Two Different Geothermal Environments

by Svava Davíðsdóttir, Baldur Geir Gunnarsson, Kjartan Björgvin Kristjánsson, Béatrice A. Ledésert and Dagur Ingi Ólafsson

Geosciences 2021, 11(12), 498; https://doi.org/10.3390/geosciences11120498 - 07 Dec 2021

Cited by 9 | Viewed by 3051

Abstract

Geothermal fluids harnessed for electricity production are generally corrosive because of their interaction with the underground. To ensure the longevity and sustainability of geothermal Organic Rankine Cycle (ORC) powerplants, the choice of heat exchanger material is essential. The performance of heat exchangers is [...] Read more.

Geothermal fluids harnessed for electricity production are generally corrosive because of their interaction with the underground. To ensure the longevity and sustainability of geothermal Organic Rankine Cycle (ORC) powerplants, the choice of heat exchanger material is essential. The performance of heat exchangers is affected by corrosion and scaling due to the geothermal fluids, causing regular cleaning, part replacement, and in the worst cases, extensive repair work. The properties of geothermal fluids vary between geothermal settings and even within geothermal sites. Differences in exposure conditions require different material selection considerations, where factors such as cost, and material efficiency are important to consider. This work studies in-situ geothermal exposure testing of four metals at two geothermal locations, in different geological settings. Four corrosion-resistant materials were exposed for one month at Reykjanes powerplant in Iceland and four months at Chaunoy oil field in France as material candidates for heat exchangers. The tested alloys were analysed for corrosion with macro- and microscopic techniques using optical and electron microscopes, which give an indication of the different frequencies of repairs and replacement. Inconel 625 showed no effects at Reykjanes and cracks at Chaunoy. The others (316L, 254SMO, and titanium grade 2) showed either corrosion or erosion traces at both sites. Full article

(This article belongs to the Special Issue Enhanced Geothermal Systems and other Deep Geothermal Applications throughout Europe: The MEET Project)

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28 pages, 3252 KiB

Open AccessArticle

Thermodynamic and Kinetic Modelling of Scales Formation at the Soultz-sous-Forêts Geothermal Power Plant

by Pierce Kunan, Guillaume Ravier, Eléonore Dalmais, Marion Ducousso and Pierre Cezac

Geosciences 2021, 11(12), 483; https://doi.org/10.3390/geosciences11120483 - 23 Nov 2021

Cited by 5 | Viewed by 2485

Abstract

Geothermal energy has been a subject of great interest since the 1990s in the Upper Rhine Graben (URG), where the first European Enhanced Geothermal System (EGS) pilot site has been developed, in Soultz-sous-Forêts (SsF), France. Several studies have already been conducted on scales [...] Read more.

Geothermal energy has been a subject of great interest since the 1990s in the Upper Rhine Graben (URG), where the first European Enhanced Geothermal System (EGS) pilot site has been developed, in Soultz-sous-Forêts (SsF), France. Several studies have already been conducted on scales occurring at the reinjection side at the geothermal plants located in the URG. It has been observed that the composition of the scales changes as chemical treatment is applied to inhibit metal sulfate. The purpose of this study was to model the scaling phenomenon occurring in the surface pipes and the heat exchangers at the SsF geothermal plant. PhreeqC, a geochemical modelling software, was used to reproduce the scaling observations in the geothermal plant during exploitation. A suitable database was chosen based on the availability of chemical elements, minerals, and gas. A thermodynamic model and a kinetic model were proposed for modelling the scaling phenomenon. The thermodynamic model gave insight on possible minerals precipitated while the kinetic model, after modifying the initial rates equation, produced results that were close to the expected scale composition at the SsF geothermal plant. Additional laboratory studies on the kinetics of the scales are proposed to complement the current model. Full article

(This article belongs to the Special Issue Enhanced Geothermal Systems and other Deep Geothermal Applications throughout Europe: The MEET Project)

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28 pages, 1936 KiB

Open AccessArticle

Two-Stage Geothermal Well Clustering for Oil-to-Water Conversion on Mature Oil Fields

by Josipa Hranić, Sara Raos, Eric Leoutre and Ivan Rajšl

Geosciences 2021, 11(11), 470; https://doi.org/10.3390/geosciences11110470 - 16 Nov 2021

Cited by 6 | Viewed by 2804

Abstract

There are numerous oil fields that are approaching the end of their lifetime and that have great geothermal potential considering temperature and water cut. On the other hand, the oil industry is facing challenges due to increasingly stringent environmental regulations. An example of [...] Read more.

There are numerous oil fields that are approaching the end of their lifetime and that have great geothermal potential considering temperature and water cut. On the other hand, the oil industry is facing challenges due to increasingly stringent environmental regulations. An example of this is the case of France where oil extraction will be forbidden starting from the year 2035. Therefore, some oil companies are considering switching from the oil business to investing in geothermal projects conducted on existing oil wells. The proposed methodology and developed conversions present the evaluation of existing geothermal potentials for each oil field in terms of water temperature and flow rate. An additional important aspect is also the spatial distribution of existing oil wells related to the specific oil field. This paper proposes a two-stage clustering approach for grouping similar wells in terms of their temperature properties. Once grouped on a temperature basis, these clusters should be clustered once more with respect to their spatial arrangement in order to optimize the location of production facilities. The outputs regarding production quantities and economic and environmental aspects will provide insight into the optimal scenario for oil-to-water conversion. The scenarios differ in terms of produced energy and technology used. A case study has been developed where the comparison of overall fields and clustered fields is shown, together with the formed scenarios that can further determine the possible conversion of petroleum assets to a geothermal assets. Full article

(This article belongs to the Special Issue Enhanced Geothermal Systems and other Deep Geothermal Applications throughout Europe: The MEET Project)

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19 pages, 5489 KiB

Open AccessArticle

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

by Saeed Mahmoodpour, Mrityunjay Singh, Aysegul Turan, Kristian Bär and Ingo Sass

Geosciences 2021, 11(11), 464; https://doi.org/10.3390/geosciences11110464 - 09 Nov 2021

Cited by 18 | Viewed by 2803

Abstract

The deep geothermal energy project at Soultz-sous-Forêts is located in the Upper Rhine Graben, France. As part of the Multidisciplinary and multi-contact demonstration of EGS exploration and Exploitation Techniques and potentials (MEET) project, this study aimed to evaluate the possibility of extracting higher [...] Read more.

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

(This article belongs to the Special Issue Enhanced Geothermal Systems and other Deep Geothermal Applications throughout Europe: The MEET Project)

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24 pages, 8209 KiB

Open AccessArticle

Long-Term Evolution of Fracture Permeability in Slate: An Experimental Study with Implications for Enhanced Geothermal Systems (EGS)

by Chaojie Cheng, Johannes Herrmann, Bianca Wagner, Bernd Leiss, Jessica A. Stammeier, Erik Rybacki and Harald Milsch

Geosciences 2021, 11(11), 443; https://doi.org/10.3390/geosciences11110443 - 28 Oct 2021

Cited by 6 | Viewed by 2172

Abstract

The long-term sustainability of fractures within rocks determines whether it is reasonable to utilize such formations as potential EGS reservoirs. Representative for reservoirs in Variscan metamorphic rocks, three long-term (one month each) fracture permeability experiments on saw-cut slate core samples from the Hahnenklee [...] Read more.

The long-term sustainability of fractures within rocks determines whether it is reasonable to utilize such formations as potential EGS reservoirs. Representative for reservoirs in Variscan metamorphic rocks, three long-term (one month each) fracture permeability experiments on saw-cut slate core samples from the Hahnenklee well (Harz Mountains, Germany) were performed. The purpose was to investigate fracture permeability evolution at temperatures up to 90 °C using both deionized water (DI) and a 0.5 M NaCl solution as the pore fluid. Flow with DI resulted in a fracture permeability decline that is more pronounced at 90 °C, but permeability slightly increased with the NaCl fluid. Microstructural observations and analyses of the effluent composition suggest that fracture permeability evolution is governed by an interplay of free-face dissolution and pressure solution. It is concluded that newly introduced fractures may be subject to a certain permeability reduction due to pressure solution that is unlikely to be mitigated. However, long-term fracture permeability may be sustainable or even increase by free-face dissolution when the injection fluid possesses a certain (NaCl) salinity. Full article

(This article belongs to the Special Issue Enhanced Geothermal Systems and other Deep Geothermal Applications throughout Europe: The MEET Project)

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34 pages, 15647 KiB

Open AccessArticle

Sensitivity Analysis of FWI Applied to OVSP Synthetic Data for Fault Detection and Characterization in Crystalline Rocks

by Yassine Abdelfettah and Christophe Barnes

Geosciences 2021, 11(11), 442; https://doi.org/10.3390/geosciences11110442 - 27 Oct 2021

Cited by 1 | Viewed by 1772

Abstract

We have performed several sensitivity studies to assess the ability of the Full Wave Inversion method to detect, delineate and characterize faults in a crystalline geothermal reservoir from OVSP data. The distant goal is to apply the method to the Soultz-sous-Forêts site (France). [...] Read more.

We have performed several sensitivity studies to assess the ability of the Full Wave Inversion method to detect, delineate and characterize faults in a crystalline geothermal reservoir from OVSP data. The distant goal is to apply the method to the Soultz-sous-Forêts site (France). Our approach consists of performing synthetic Full Wave 2D Inversion experiments using offset vertical seismic and comparing the estimated fields provided by the inversion, i.e., the estimated underground images, to the initial reference model including the fault target. We first tuned the inversion algorithmic parameters in order to adapt the FWI software, originally dedicated to a sedimentary context, to a crystalline context. In a second step, we studied the sensitivity of the FWI fault imaging results as a function of the acquisition geometry parameters, namely, the number of shots, the intershot distance, the maximum offset and also the antenna length and well deviation. From this study, we suggest rules to design the acquisition geometry in order to improve the fault detection, delineation and characterization. In a third step, we studied the sensitivity of the FWI fault imaging results as a function of the fault or the fault zone characteristics, namely, the fault dip, thickness and the contrast of physical parameters between the fault materials and the surrounding fresh rocks. We have shown that a fault with high dip, between 60 and 90° as thin as 10 m (i.e. lower than a tenth of the seismic wavelength of 120 m for Vp and 70 m for Vs) can be imaged by FWI, even in the presence of additive gaussian noise. In summary, for a crystalline geological context, and dealing with acceptable S/N ratio data, the FWI show a high potential for accurately detecting, delineating and characterizing the fault zones. Full article

(This article belongs to the Special Issue Enhanced Geothermal Systems and other Deep Geothermal Applications throughout Europe: The MEET Project)

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28 pages, 8237 KiB

Open AccessArticle

Spring Water Geochemistry: A Geothermal Exploration Tool in the Rhenohercynian Fold-and-Thrust Belt in Belgium

by Marina Cabidoche, Yves Vanbrabant, Serge Brouyère, Vinciane Stenmans, Bruno Meyvis, Thomas Goovaerts, Estelle Petitclerc and Christian Burlet

Geosciences 2021, 11(11), 437; https://doi.org/10.3390/geosciences11110437 - 22 Oct 2021

Cited by 1 | Viewed by 1845

Abstract

Spring water geochemistry is applied here to evaluate the geothermal potential in Rhenohercynian fold and thrust belt around the deepest borehole in Belgium (Havelange borehole: 5648 m MD). Fifty springs and (few) wells around Havelange borehole were chosen according to a multicriteria approach [...] Read more.

Spring water geochemistry is applied here to evaluate the geothermal potential in Rhenohercynian fold and thrust belt around the deepest borehole in Belgium (Havelange borehole: 5648 m MD). Fifty springs and (few) wells around Havelange borehole were chosen according to a multicriteria approach including the hydrothermal source of “Chaudfontaine” (T ≈ 36 °C) taken as a reference for the area. The waters sampled, except Chaudfontaine present an in-situ T range of 3.66–14.04 °C (mean 9.83 °C) and a TDS (dry residue) salinity range of 46–498 mg/L. The processing methods applied to the results are: hierarchical clustering, Piper and Stiff diagrams, TIS, heat map, boxplots, and geothermometry. Seven clusters are found and allow us to define three main water types. The first type, locally called “pouhon”, is rich in Fe and Mn. The second type contains an interesting concentration of the geothermal indicators: Li, Sr, Rb. Chaudfontaine and Moressée (≈5 km East from the borehole) belong to this group. This last locality is identified as a geothermal target for further investigations. The third group represents superficial waters with frequently high NO₃ concentration. The application of conventional geothermometers in this context indicates very different reservoir temperatures. The field of applications of these geothermometers need to be review in these geological conditions. Full article

(This article belongs to the Special Issue Enhanced Geothermal Systems and other Deep Geothermal Applications throughout Europe: The MEET Project)

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40 pages, 16866 KiB

Open AccessArticle

Fluid-Rock Interactions in a Paleo-Geothermal Reservoir (Noble Hills Granite, California, USA). Part 2: The Influence of Fracturing on Granite Alteration Processes and Fluid Circulation at Low to Moderate Regional Strain

by Johanne Klee, Arezki Chabani, Béatrice A. Ledésert, Sébastien Potel, Ronan L. Hébert and Ghislain Trullenque

Geosciences 2021, 11(11), 433; https://doi.org/10.3390/geosciences11110433 - 20 Oct 2021

Cited by 6 | Viewed by 2822

Abstract

Fracture connectivity within fractured granitic basement geothermal reservoirs is an important factor controlling their permeability. This study aims to improve the understanding of fluid–rock interaction processes at low to moderate regional strain. The Noble Hills range (Death Valley, CA, USA) was chosen as [...] Read more.

Fracture connectivity within fractured granitic basement geothermal reservoirs is an important factor controlling their permeability. This study aims to improve the understanding of fluid–rock interaction processes at low to moderate regional strain. The Noble Hills range (Death Valley, CA, USA) was chosen as a naturally exhumed paleo geothermal reservoir. A series of petrographic, petrophysical, and geochemical investigations, combined with a fracture distribution analysis, were carried out on samples collected across fracture zones. Our results indicate that several generations of fluids have percolated through the reservoir. An increase of (1) the alteration degree; (2) the porosity values; and (3) the calcite content was observed when approaching fracture zones. No correlation was identified among the alteration degree, the porosity, or the calcite content. At a local scale, samples showed that the degree of alteration does not necessarily depend on the fracture density or on the amount of the strain. It is concluded that the combined influence of strain and coeval fluid–rock interaction processes drastically influence the petrophysical properties of fracture zones, which in turn impact geothermal production potential. Full article

(This article belongs to the Special Issue Enhanced Geothermal Systems and other Deep Geothermal Applications throughout Europe: The MEET Project)

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20 pages, 5664 KiB

Open AccessArticle

Analysis of Enhanced Geothermal System Development Scenarios for District Heating and Cooling of the Göttingen University Campus

by Dmitry Romanov and Bernd Leiss

Geosciences 2021, 11(8), 349; https://doi.org/10.3390/geosciences11080349 - 19 Aug 2021

Cited by 9 | Viewed by 3031

Abstract

The huge energy potential of Enhanced Geothermal Systems (EGS) makes them perspective sources of non-intermittent renewable energy for the future. This paper focuses on potential scenarios of EGS development in a locally and in regard to geothermal exploration, poorly known geological setting—the Variscan [...] Read more.

The huge energy potential of Enhanced Geothermal Systems (EGS) makes them perspective sources of non-intermittent renewable energy for the future. This paper focuses on potential scenarios of EGS development in a locally and in regard to geothermal exploration, poorly known geological setting—the Variscan fold-and-thrust belt —for district heating and cooling of the Göttingen University campus. On average, the considered single EGS doublet might cover about 20% of the heat demand and 6% of the cooling demand of the campus. The levelized cost of heat (LCOH), net present value (NPV) and CO₂ abatement cost were evaluated with the help of a spreadsheet-based model. As a result, the majority of scenarios of the reference case are currently not profitable. Based on the analysis, EGS heat output should be at least 11 MW_th (with the brine flow rate being 40 l/s and wellhead temperature being 140 °C) for a potentially profitable project. These parameters can be a target for subsurface investigation, reservoir modeling and hydraulic stimulation at a later stage. However, sensitivity analysis presented some conditions that yield better results. Among the most influential parameters on the outcome are subsidies for research wells, proximity to the campus, temperature drawdown and drilling costs. If realized, the EGS project in Göttingen might save up to 18,100 t CO₂ (34%) annually. Full article

(This article belongs to the Special Issue Enhanced Geothermal Systems and other Deep Geothermal Applications throughout Europe: The MEET Project)

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33 pages, 9676 KiB

Open AccessArticle

Fluid-Rock Interactions in a Paleo-Geothermal Reservoir (Noble Hills Granite, California, USA). Part 1: Granite Pervasive Alteration Processes away from Fracture Zones

by Johanne Klee, Sébastien Potel, Béatrice A. Ledésert, Ronan L. Hébert, Arezki Chabani, Pascal Barrier and Ghislain Trullenque

Geosciences 2021, 11(8), 325; https://doi.org/10.3390/geosciences11080325 - 31 Jul 2021

Cited by 10 | Viewed by 3073

Abstract

Only few data from geothermal exploited reservoirs are available due to the restricted accessibility by drilling, which limits the understanding of the entire reservoir. Thus, analogue investigations are needed and were performed in the framework of the H2020 MEET project. The Noble Hills [...] Read more.

Only few data from geothermal exploited reservoirs are available due to the restricted accessibility by drilling, which limits the understanding of the entire reservoir. Thus, analogue investigations are needed and were performed in the framework of the H2020 MEET project. The Noble Hills range, located along the southern branch of the Death Valley pull-apart (CA, USA), has been selected as a possible granitic paleo-reservoir. The aim is to characterize the pervasive alteration processes affecting this granite, away from the influence of the faults, in terms of mineralogical, petrophysical and chemical changes. Various methods were used as petrographic, geochemical and petrophysical analyses. Mineral changes, clay mineralogy, bulk rock chemical composition, calcite content and porosity were determined on different granite samples, collected in the Noble Hills granite, far from the faults and in the Owlshead Mountains, north of the Noble Hills, considered as its protolith. In order to complete the granite characterization, the metamorphic grade has been studied through the Noble Hills granite body. This complete characterization has allowed distinguishing the occurrence of three stages of alteration: (1) a pervasive propylitic alteration characterized by calcite-corrensite-epidote-K-white mica assemblage, (2) a more local one, only present in the Noble Hills granite, producing illite, kaolinite, illite/smectite, calcite and oxides, characteristic of the argillic alteration, which overprints the propylitic alteration and (3) weathering evidenced by the presence of montmorillonite in the Owlshead Mountains, which is considered as negligible in both granites. Alteration was also outlined by the correlation of the loss on ignition, representing the hydration rate, to porosity, calcite content and chemical composition. Moreover, the Kübler Index calculated from illite crystals allowed to identify a NW-SE temperature gradient in the Noble Hills. Full article

(This article belongs to the Special Issue Enhanced Geothermal Systems and other Deep Geothermal Applications throughout Europe: The MEET Project)

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28 pages, 13622 KiB

Open AccessEditor’s ChoiceArticle

Multiscale Characterization of Fracture Patterns: A Case Study of the Noble Hills Range (Death Valley, CA, USA), Application to Geothermal Reservoirs

by Arezki Chabani, Ghislain Trullenque, Béatrice A. Ledésert and Johanne Klee

Geosciences 2021, 11(7), 280; https://doi.org/10.3390/geosciences11070280 - 03 Jul 2021

Cited by 23 | Viewed by 3216

Abstract

In the basement fractured reservoirs, geometric parameters of fractures constitute the main properties for modeling and prediction of reservoir behavior and then fluid flow. This study aims to propose geometric description and quantify the multiscale network organization and its effect on connectivity using [...] Read more.

In the basement fractured reservoirs, geometric parameters of fractures constitute the main properties for modeling and prediction of reservoir behavior and then fluid flow. This study aims to propose geometric description and quantify the multiscale network organization and its effect on connectivity using a wide-ranging scale analysis and orders scale classification. This work takes place in the Noble Hills (NH) range, located in the Death Valley (DV, USA). The statistical analyses were performed from regional maps to thin sections. The combination of the length datasets has led to compute a power law exponent around −2, meaning that the connectivity is ruled by the small and the large fractures. Three domains have been highlighted in the NH: (1) domain A is characterized by a dominance of the NW/SE direction at the fourth order scale; (2) domain B is characterized by a dominance of the E/W and the NW/SE directions at respectively the fourth and third order scales; (3) domain C is also marked by the E/W direction dominance followed by the NW/SE direction respectively at the fourth and third order scale. The numerical simulations should consider that the orientation depends on scale observation, while the length is independent of scale observation. Full article

(This article belongs to the Special Issue Enhanced Geothermal Systems and other Deep Geothermal Applications throughout Europe: The MEET Project)

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23 pages, 11536 KiB

Open AccessArticle

Scaling in a Geothermal Heat Exchanger at Soultz-Sous-Forêts (Upper Rhine Graben, France): A XRD and SEM-EDS Characterization of Sulfide Precipitates

by Béatrice A. Ledésert, Ronan L. Hébert, Justine Mouchot, Clio Bosia, Guillaume Ravier, Olivier Seibel, Éléonore Dalmais, Mariannick Ledésert, Ghislain Trullenque, Xavier Sengelen and Albert Genter

Geosciences 2021, 11(7), 271; https://doi.org/10.3390/geosciences11070271 - 28 Jun 2021

Cited by 9 | Viewed by 3385

Abstract

The Soultz-Sous-Forêts geothermal site (France) operates three deep wells for electricity production. During operation, scales precipitate within the surface installation as (Ba, Sr) sulfate and (Pb, As, Sb) sulfide types. Scales have an impact on lowering energy production and inducing specific waste management [...] Read more.

The Soultz-Sous-Forêts geothermal site (France) operates three deep wells for electricity production. During operation, scales precipitate within the surface installation as (Ba, Sr) sulfate and (Pb, As, Sb) sulfide types. Scales have an impact on lowering energy production and inducing specific waste management issues. Thus scaling needs to be reduced for which a thorough characterization of the scales has to be performed. The geothermal brine is produced at 160 °C and reinjected at 70 °C during normal operation. In the frame of the H2020 MEET project, a small heat exchanger was tested in order to allow higher energy production, by reinjecting the geothermal fluid at 40 °C. Samples of scales were analyzed by XRD and SEM-EDS, highlighting that mostly galena precipitates and shows various crystal shapes. These shapes can be related to the turbulence of the flow and the speed of crystal growth. Where the flow is turbulent (entrance, water box, exit), crystals grow quickly and mainly show dendritic shape. In the tubes, where the flow is laminar, crystals grow more slowly and some of them are characterized by well-developed faces leading to cubes and derived shapes. The major consequence of the temperature decrease is the increased scaling phenomenon. Full article

(This article belongs to the Special Issue Enhanced Geothermal Systems and other Deep Geothermal Applications throughout Europe: The MEET Project)

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