High-temperature Geothermal Resources Research Articles

Thermo-Hydro-mechanical coupling analysis of geothermal reservoirs: optimizing extraction capacities by revealing influential factors

Abstract Extraction of high-temperature geothermal resources from reservoirs is a challenge due to the complex interactions between temperature, permeability, and stress fields. Variations in physical fields are critical to thermal reservoir engineering. In this study, the coupling mechanism between temperature, permeability, and stress fields is systematically explored using theoretical analyses and numerical simulations, by utilizing the three-field coupling model. This study models the changes in porosity and permeability evolution during geothermal extraction, emphasizing the importance of the coupling term. The effects of pressure differences between injection and production wells, reservoir temperature variations, and fluid property changes on the temperature distribution and output thermal power within the geothermal reservoir were modeled and analyzed. The results reveal the significant effect of pressure differences between wells, and show that the geothermal extraction capacity of supercritical carbon dioxide (SC-CO2) is 1.83 times higher than that of water. Based on the statistical characteristics of the distribution pattern of natural fractures within the geothermal reservoir, the study simulated the distribution of natural fractures and developed a coupled THM model containing natural fractures. The results show that increasing the porosity of hydraulic and natural fractures can effectively increase the geothermal production capacity, especially the increase in the porosity of natural fractures is significant. These findings provide a key theoretical basis for understanding the THM coupling mechanism during geothermal extraction, and provide substantial support for optimizing the development and utilization of geothermal resources.

Mapping and resource evaluation of deep high-temperature geothermal resources in the Jiyang Depression, China

In China, geothermal resource utilization has mainly focused on resources at shallow and medium depths. Yet, the exploration of deep, high-temperature geothermal resources holds significant importance for achieving the “dual carbon” goals and the transition of energy structure. The Jiyang Depression in the Bohai Bay Basin has vast potential for deep, high-temperature geothermal resources. By analyzing data from 2187 wells with temperature logs and 270 locations for temperature measurement in deep strata, we mapped the geothermal field of shallow to medium-deep layers in the Jiyang Depression using ArcGIS and predicted the temperatures of deep layers with a burial depth of 4000 m. Through stochastic modeling and numerical simulation, a reservoir attribute parameter database for favorable deep, high-temperature geothermal areas was developed, systematically characterizing the spatial distribution of geothermal resources within a play fairway of 139.5 km2 and estimating the exploitable deep geothermal resource potential by using the heat storage method and Monte Carlo data analysis. The study reveals that the Fan 54 well block in the Boxing-Jijia region is of prime significance to develop deep, high-temperature geothermal resources in the Jiyang Depression. Strata from the Cenozoic to the Upper Paleozoic are identified as effective cap layers for these deep geothermal resources. The Lower Paleozoic capable of effectively storing thermal energy and possessing an exploitable resource volume up to 127 million tons of standard coal, is identified as a target system for the development of deep high-temperature geothermal resources, providing significant insights for the efficient development of geothermal resources in the Jiyang Depression.

Representative High-Temperature Hydrothermal Activities in the Himalaya Geothermal Belt (HGB): A Review and Future Perspectives

Southern Tibet and western Yunnan are areas with an intensive distribution of high-temperature geothermal systems in China, as an important part of the Himalayan Geothermal Belt (HGB). In recent decades, China has conducted systematic research on high-temperature geothermal fields such as Yangbajing, Gudui, and Rehai. However, a comprehensive understanding has not yet been formed. The objective of this study was to enhance comprehension of the high-temperature geothermal system in the HGB and to elucidate the hydrogeochemical characteristics of geothermal fluids. This will facilitate the subsequent sustainable development and exploitation of domestic high-temperature hydrothermal geothermal resources. To this end, this study analysed geothermal spring and borehole data from the Yangbajing, Gudui, and Rehai geothermal fields. Based on previous research results, the source, evolution, and reservoir temperature characteristics of geothermal fluids are compared and summarised. The main high-temperature geothermal water in the geothermal field is derived from the deep Cl-Na geothermal fluid. Yangbajing’s and Gudui’s geothermal waters are primarily recharged by snow-melt water, while Rehai’s geothermal water is mainly recharged by local meteoric water. The average mixing ratios of magmatic water in the Yangbajing, Gudui, and Rehai geothermal fields are 17%, 21%, and 22%, respectively. The Yangbajing and Gudui geothermal fields have a relatively closed geological environment, resulting in a stronger water–rock interaction compared to the Rehai geothermal field. As geothermal water rises, it mixes with shallow cold water infiltration. The mixing ratios of cold water in the Yangbajing, Gudui, and Rehai geothermal fields are 60–70%, 40–50%, and 20–40%, respectively. Based on the solute geothermometer calculations, the maximum geothermal reservoir temperatures for Yangbajing, Gudui, and Rehai are 237 °C, 266 °C, and 282 °C, respectively. This study summarises and compares the hydrogeochemical characteristics of three typical high-temperature geothermal fields. The findings provide an important theoretical basis for the development of high-temperature geothermal resources in the Himalayan Geothermal Belt.

Geo thermal energy - Clean, safe and renewable - A review study

The rising demand of energy and exhausting sources of fossil fuels has compelled people all over the world towards a renewable, clean energy source i.e. geothermal energy. Geothermal resources are reservoirs of hot water that exist or are human made at varying temperatures and depths below the Earth’s surface. In side the great depths of the Earth, the temperature (1,250 °C) and pressure is sufficient to melt rock into magma which is termed as lava (750°C) once it comes out of the crust. This heat content is harnessed as geothermal energy and utilized for heat applications as well as generation of power. The majority of the world’s geothermal resources are located in the tropical Pacific Rim (Ring of Fire). The types of geothermal resources are categorized as convective hydrothermal systems; EGSs; conductive sedimentary systems; coproduction, with water from oil and gas fields; geopressure systems; and magma energy. Mostly, geothermal fluid can be used directly or indirectly depending on the enthalpy. High-temperature geothermal resources are primarily used for energy production, whereas low to medium ones are particularly equipped for non-electric applications. The extraction of geothermal energy from the grounds leads to a release of greenhouse gases like hydrogen sulfide, carbon dioxide, methane and ammonia etc. However, the amount of gas released is significantly lower than in the case of fossil fuels. Moreover, geothermal energy has proven its capacity to be a reliable, clean, and uninterrupted sustainable renewable energy.

Design of a cooling system from underground thermal energy storage (UTES, Underground) Thermal Energy Storage) based on experimental results

Geothermal energy is a renewable and clean source that has been used for electricity generation in some countries since the 50s, the main characteristic to be used in this application is that the subsoil must have a high temperature geothermal resource (+150 °C). However, it can also be used in applications such as air conditioning in places where the temperature is around 30°C; In Europe alone, there are more than one million thermal installations operating by harnessing geothermal energy. The objective of the work was to design a cooling system from the storage of underground energy, for that, it is essential to know the variation of subsoil temperatures during a certain period of time. For this purpose, sensors were used that were installed at different depths and by means of an Arduino, information of a whole year was stored; so that these data are as representative as possible of the energy storage conditions and the changes depending on the seasons that pass. Additionally, the characteristics of the soil (conductivity, humidity and composition) were taken into account, where the equipment is intended to be installed in subsequent works. For the determination of the necessary cooling load, the design requirements of the ASHRAE standard were used and for the design of the underground heat exchanger, references of designs recommended through experimental tests in other research works are included, together with internal fluid methodology and one-dimensional heat transfer. It includes elements that can help improve the dissipation of energy into the subsurface and maintain transfer properties as stable as possible. This design is designed for the air conditioning of a classroom of normal dimensions that are used in the University and therefore avoid the energy consumption of conventional air conditioning equipment.

Geochemical Evidence Constraining Genesis and Mineral Scaling of the Yangbajing Geothermal Field, Southwestern China

The Yangbajing geothermal field, a renowned high-temperature geothermal resource in Tibet of southwestern China, has been utilized for power generation for several decades. To improve geothermal exploitation in the Yangbajing, genesis and mineral scaling have yet to be further revealed. In this study, hydrochemistry and D-O-Sr isotopy were employed for analyzing genesis and mineral scaling in the Yangbajing geothermal field. The geothermal waters were weakly alkaline and had a high TDS content (1400–2900 mg/L) with the Cl-Na, Cl·HCO3-Na, and HCO3·Cl-Na types. The dissolution of silicate minerals (sodium and potassium feldspars) and positive cation exchange controlled the hydrogeochemical process. The geothermal water was recharged from snow-melted water and meteoric water originating from the Nyainqentanglh Mountains and Tangshan Mountains. The geothermal waters possessed the highest reservoir temperature of 299 °C and the largest circulation depth of 2010 m according to various geothermometers. The geothermal waters can produce CaCO3 and SiO2 scaling during vertical and horizontal transport. These achievements can provide a scientific basis for the sustainable development and conservation of the high-temperature geothermal resources in Yangbajing and elsewhere.

Progress in research on the exploration and evaluation of deep geothermal resources in the Fujian-Guangdong-Hainan region, China

Deep geothermal resources in the Fujian-Guangdong-Hainan region, China, offer significant potential for sustainable energy. The diverse igneous rock formations along the southeast coast present intricate geological challenges that impede exploration and evaluation efforts. In this study, we address critical concerns related to the Fujian-Guangdong-Hainan region's deep geothermal resources, encompassing heat source composition, formation conditions, strategic favorable areas, and exploration directions. Our methods involve the analysis of regional geothermal reservoirs and cap rocks. Major findings include: the primary heat sources in the Fujian-Guangdong-Hainan region consist of the radioactive heat generation from granites in the crust, heat conduction in the mantle, and, in specific areas like Yangjiang and Shantou, melts within the middle and lower crust; the deep, high-temperature geothermal resources in the region predominantly reside in basins' depressed areas. These areas are characterized by the confluence of triple heat sources: heat from the Earth's crust, mantle, and other supplementary sources; our analysis led to the identification of three strategic areas favorable for deep geothermal resources in the Fujian-Guangdong-Hainan region. These are the Beibu Gulf Basin's continental area, the Yuezhong Depression, and the Fuzhou-Zhangzhou area.

Three-Dimensional Geological Modeling and Resource Estimation of Hot Dry Rock in the Gonghe Basin, Qinghai Province

The Gonghe Basin, situated on the northeastern margin of the Qinghai–Tibet Plateau, is a strike-slip pull-apart basin that has garnered considerable attention for its abundant high-temperature geothermal resources. However, as it is located far from the Himalayan geothermal belt, research on the geothermal resources in the Gonghe Basin has mainly focused on the heat source mechanism, with less attention given to the distribution and resource potential of hot dry rock. In this project, a comprehensive approach combining geological surveys, geophysical exploration, geochemical investigations, and deep drilling was employed to analyze the stratigraphic structure and lithological composition of the Gonghe Basin, establish a basin-scale three-dimensional geological model, and identify the lithological composition and geological structures within the basin. The model revealed that the target reservoirs of hot dry rock in the Gonghe Basin exhibit a half-graben undulation pattern, with burial depths decreasing from west to east and reaching a maximum depth of around 7000 m. Furthermore, the distribution of the temperature field in the area was determined, and the influence of temperature on rock density and specific heat was investigated to infer the thermal properties of the deep reservoirs. The Qiabuqia region, situated in the central-eastern part of the basin, was identified as a highly favorable target area for hot dry rock exploration and development. The volume method was used to evaluate the potential of hot dry rock resources in the Gonghe Basin, which was estimated to be approximately 4.90 × 1022 J, equivalent to 1.67 × 1012 t of standard coal, at depths of up to 10 km.

Integrated application of gravity, aeromagnetic, and electromagnetic methods in exploring the Ganzi geothermal field, Sichuan Province, China

The Ganzi geothermal field is located in the Songpan-Ganzi orogenic belt in Sichuan Province. Many hot springs are exposed along the Yalahe valley in Ganzi geothermal field, which is a favorable area for high-temperature geothermal resource exploration. However, the geological model of heat exchange, the regional structure controlling hydrothermal convection and the development model of geothermal reservoirs are still unclear. Therefore, further studies are necessary to meet the geothermal exploration requirements in the middle and deep strata of this geothermal field. In this study, a geological model of the geothermal system of Ganzi geothermal field is proposed. We are convinced that there exists a hydrothermal convection system in the Ganzi geothermal field, the heat transfer of which is accomplished through deep-rooted major faults. Therefore, the identification of deep-rooted major faults and the description of geothermal reservoirs are the research objects of the integrated geophysical methods. The main factors controlling the geothermal reservoirs in the deep-rooted Xianshuihe major fault and Yalahe fault zones are analyzed by using gravity, aeromagnetic, and electromagnetic methods and techniques. The analysis results of regional gravity and aeromagnetic anomalies show that the Xianshuihe major fault has produced obvious gravity and aeromagnetic anomalies on the surface, and thus the position and strike of this fault can be accurately predicted by inversion of the aeromagnetic anomalies. Geothermal reservoirs show low-resistivity anomalies in the electromagnetic profile. The inversion results of the controlled source audio-frequency magnetotelluric (CSAMT) data show that geothermal reservoirs are mainly developed along the Yalahe valley, and the west side of the valley is more favorable for geothermal exploration. This study is of guiding significance to the efficient exploitation and utilization of the Ganzi geothermal field.

Strata temperatures and geothermal resource evaluation in the Dongpu Depression, Bohai Bay Basin, North China

The development of geothermal resources in the Dongpu Depression can improve not only the economic benefits of the oilfield but also the ecological environment. Therefore, it is necessary to evaluate the geothermal resources in the region. Based on the heat flow, geothermal gradient and thermal properties, the temperatures and their distribution in different strata are calculated using geothermal methods, and the geothermal resource types of the Dongpu Depression are identified. The results show that the geothermal resources include low-temperature, medium-temperature and high-temperature geothermal resources in the Dongpu Depression. The Minghuazhen and Guantao Formations mainly include low-temperature and medium-temperature geothermal resources; the Dongying and Shahejie Formations include low-temperature, medium-temperature and high-temperature geothermal resources; the Ordovician rocks mainly include medium-temperature and high-temperature geothermal resources. The Minghuazhen, Guantao and Dongying Formations can form good geothermal reservoirs and are favorable layers for exploring low-temperature and medium-temperature geothermal resources. The geothermal reservoir of the Shahejie Formation is relatively poor, and the thermal reservoirs may be developed in the western slope zone and the central uplift. The Ordovician carbonate strata can provide thermal reservoirs for geothermal resources, and the Cenozoic bottom temperature is more than 150 °C except for most of the western gentle slope zone. In addition, for the same stratum, the geothermal temperatures in the southern Dongpu Depression are higher than those in the northern depression.

When less is more: How increasing the complexity of machine learning strategies for geothermal energy assessments may not lead toward better estimates

Previous moderate- and high-temperature geothermal resource assessments of the western United States utilized data-driven methods and expert decisions to estimate resource favorability. Although expert decisions can add confidence to the modeling process by ensuring reasonable models are employed, expert decisions also introduce human and, thereby, model bias. This bias can present a source of error that reduces the predictive performance of the models and confidence in the resulting resource estimates.Our study aims to develop robust data-driven methods with the goals of reducing bias and improving predictive ability. We present and compare nine favorability maps for geothermal resources in the western United States using data from the U.S. Geological Survey's 2008 geothermal resource assessment. Two favorability maps are created using the expert decision-dependent methods from the 2008 assessment (i.e., weight-of-evidence and logistic regression). With the same data, we then create six different favorability maps using logistic regression (without underlying expert decisions), XGBoost, and support-vector machines paired with two training strategies. The training strategies are customized to address the inherent challenges of applying machine learning to the geothermal training data, which have no negative examples and severe class imbalance. We also create another favorability map using an artificial neural network.We demonstrate that modern machine learning approaches can improve upon systems built with expert decisions. We also find that XGBoost, a non-linear algorithm, produces greater agreement with the 2008 results than linear logistic regression without expert decisions, because the expert decisions in the 2008 assessment rendered the otherwise linear approaches non-linear despite the fact that the 2008 assessment used only linear methods. The F1 scores for all approaches appear low (F1 score < 0.10), do not improve with increasing model complexity, and, therefore, indicate the fundamental limitations of the input features (i.e., training data). Until improved feature data are incorporated into the assessment process, simple non-linear algorithms (e.g., XGBoost) perform equally well or better than more complex methods (e.g., artificial neural networks) and remain easier to interpret.

Characteristics of Radiogenic Heat Production of Widely Distributed Granitoids in Western Sichuan, Southeast Tibetan Plateau

Abstract Investigating the genesis of geothermal resources requires a thorough understanding of the heat source mechanism, which is also a vital basis for the efficient exploration and utilization of geothermal resources. Situated in the eastern Himalayan syntax, western Sichuan is considered to be one of the main concentration regions of high-temperature geothermal resources in China. To date, various studies have been carried out to reveal the heat source and genesis of the abundant high-temperature resources in this area; however, studies on the contribution of the radioactive heat generated by the widely distributed granitoids to the high-temperature geothermal resources remain scarce. In order to resolve this knowledge gap, we attempted to obtain evidence from the geochemical data published in the literature in the past few decades. A total of 548 radiogenic heat production rate data were determined. The statistical data indicate that the average concentrations of the heat-producing elements U, Th, and K are 6.09±5.22 ppm, 26.74±16.78 ppm, and 3.51±0.82%, respectively. The calculated heat production values of the granitoids vary from 0.52 to 10.86 μW/m3, yielding an arithmetic average value of 3.74±2.15 μW/m3, which is higher than that of global Mesozoic–Cenozoic granites (3.09±1.62 μW/m3). Based on the heat production values, the capacity of the granitic batholiths to store heat was assessed, and the Dongcuo pluton was found to be the largest heat reservoir (382.88×1013 J/a). The distribution of the crustal heat flow was examined using the calculated heat production data and the stratigraphic structure obtained via deep seismic sounding in the study area. The results indicate that the crustal heat flow is 48.3–56.2 mW/m2, which is mainly contributed by the radioactive decay in the granitoids in the upper crust. The fact that it accounts for nearly half of the regional background heat flow indicates that the radiogenic heat from the granitoids is an important heat source for the formation of the thermal anomaly and the high-temperature geothermal resources in the study area. Thus, the results obtained in this study highlight the importance of the widely distributed granitoids to high-temperature geothermal resources in western Sichuan.

Geothermal regime and implications for basin resource exploration in the Qaidam Basin, northern Tibetan Plateau

• Geothermal field characteristics of the Qaidam Basin, northeastern Tibetan Plateau, were analysed. • The crustal contribution to the surface heat flow was approximately 55%. • The existence of abnormally high-temperature zones significantly dominated the hydrocarbon distribution. • The Qaidam Basin is favourable for low- and medium-temperature geothermal resources development. • Certain deep high-temperature geothermal resource is prospective if using abandoned oil wells. The Qaidam Basin was formed under the background of the continuous uplift of the Tibetan Plateau, and its tectonic location imparts unique geothermal regime. The geothermal regime of the Qaidam Basin was studied based on oil-test static temperatures from 60 wells, 195 thermal conductivities measured by the optical scanning method, and 142 radiogenic heat production values. The geothermal gradient in the Qaidam Basin is 17.1–47.6 °C/km, with an average of 31.3 °C/km, and the thermal conductivity is 0.523–4.379 W/m‧K. The heat flow is 28.3–83.1 mW/m 2 , with an average of 59.6 mW/m 2 , and it is “high in the west and low in the east.” The heat flow can exceed 70 mW/m 2 in the northern marginal fold-and-thrust belt of western Kunlun and Nanyishan and generally exceeds 65 mW/m 2 in the Mangya Depression. The average radiogenic heat production rate (HPR) is 2.53 μW/m 3 , which is close to the HPR of granite in northern Tibetan Plateau. The crustal and mantle heat flows in the Qaidam Basin are 32.9 mW/m 2 and 26.7 mW/m 2 , respectively. The crustal contribution to the surface heat flow was approximately 55 %. The geothermal regime may be dominated by lithospheric thickness, HPR of sedimentary cover, and extra heat production related to late Cenozoic tectonic movement. The proven hydrocarbon reserves are primarily distributed around hydrocarbon-generating kitchens, and the existence of abnormally high-temperature zones significantly influences hydrocarbon distribution. The Qaidam Basin satisfies the fundamental temperature conditions for the development of low- and medium-temperature geothermal resources using abandoned oil and gas drilling wells.

The Characteristics and Laws of Fracture Damage in the Long-Term Production Process of High-Temperature Geothermal Resources

The Characteristics and Laws of Fracture Damage in the Long-Term Production Process of High-Temperature Geothermal Resources

An evaluation framework for production performance of high-temperature fractured and karstified geothermal reservoirs: Production mechanism, sensitivity study, and key parameters ranking

High-temperature (>150 °C) geothermal resources have great potential for sustainable power generation. These geothermal resources can be exploited efficiently through highly permeable geological structures, i.e. tectonic fractures and dissolution caves, which thus are categorized as fractured and karstified geothermal reservoirs. However, the study of long-term cold fluids injection into these high-temperature geothermal systems is faced with significant unsolved challenges, i.e. the simulation of coupled porous media flow and free flow in multiscale and heterogeneous fracture-cave networks with distinct thermo-hydro-mechanical (THM) coupling effects. To address these challenges, we develop a novel coupled THM model based on the discrete fracture-cave network approach. We then apply this model to perform the analysis of THM coupling mechanism, sensitivity study, and key parameters ranking to identify the development characteristics of high-temperature geothermal reservoirs. Results show that with respect to coupled THM processes, the fluid flow and heat transfer are dominated by connected fracture clusters and are strongly influenced by the inclusion of caves. However, the behavior of solid deformation is governed by the distribution of caves. As to the reservoir thermal performance under different working fluids, i.e. water and CO2, the water-based geothermal reservoir exhibits longer service life, greater global thermal power, and higher heat extraction rate than the CO2-based one. Regarding sensitivity analysis and key factors ranking, fracture attributes (fracture length and density) are the two most important parameters in regulating the geothermal extraction performance. Caves fall out as the third most significant parameter. However, the operating parameters (i.e. injection temperature, injection pressure, production pressure, well length, and well distance) play a limited role in influencing thermal performance compared with geometrical parameters of fractures and caves. The developed model and insights obtained from our research have crucial implications for understanding and optimizing the thermal production of high-temperature fractured and karstified geothermal reservoirs.

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

Hydrothermal alteration is a common process in active geothermal systems and can significantly change the physiochemical properties of rocks. To improve reservoir assessment and modeling of high-temperature geothermal resources linked to active volcanic settings, a detailed understanding of the reservoir is needed. The Los Humeros Volcanic Complex, hosting the third largest exploited geothermal field in Mexico, represents a natural laboratory to investigate the impact of hydrothermal processes on the rock properties through andesitic reservoir cores and outcropping analogs. Complementary petrographic and chemical analyses were used to characterize the intensities and facies of hydrothermal alteration. The alteration varies from argillic and propylitic facies characterized by no significant changes of the REE budget indicating an inert behavior to silicic facies and skarn instead showing highly variable REE contents. Unaltered outcrop samples predominantly feature low matrix permeabilities (< 10–17 m2) as well as low to intermediate matrix porosities (< 5–15%), thermal conductivities (0.89–1.49 W m−1 K−1), thermal diffusivities (~ 0.83 10–6 m2 s−1), and sonic wave velocities (VP: ~ 2800–4100 m s−1, VS: ~ 1600–2400 m s−1). Average magnetic susceptibility and specific heat capacity range between 2.4–7.0 10–3 SI and 752–772 J kg−1 K−1, respectively. In contrast, the hydrothermally altered reservoir samples show enhanced porosities (~ 7–23%), permeabilities (10–17–10–14 m2), and thermal properties (> 1.67 W m−1 K−1; > 0.91 10–6 m2 s−1), but a significant loss of magnetic susceptibility (10–3–10–6 SI). In particular, this latter characteristic appears to be a suitable indicator during geophysical survey for the identification of hydrothermalized domains and possible pathways for fluids. The lack of clear trends between alteration facies, alteration intensity, and chemical indices in the studied samples is interpreted as the response to multiple and/or repeated hydrothermal events. Finally, the proposed integrated field-based approach shows the capability to unravel the complexity of geothermal reservoir rocks in active volcanic settings.

Geothermal deep closed-loop heat exchangers: A novel technical potential evaluation to answer the power and heat demands

This paper investigates and optimises the thermal performance of deep closed-loop heat exchanger (DCHE) systems by applying a computational numerical approach. The investigated DCHE configuration accounts for two deep vertical boreholes, an injection and a production well, connected by a horizontal borehole at depth and an insulated pipeline at the surface, establishing an effective closed-loop system. First, a parametric sensitivity study explores the effects of the environmental, design and operating variables on the production temperature. The simulation uses realistic geological and geothermal conditions, depths, circulation rates and injection temperatures. Two complex numerical models are then solved for site-specific DCHEs in different geological scenarios: a foreland basin and a convergent margin hosting low-to-intermediate and high-temperature geothermal resources, respectively. Production temperatures beyond 40–60 °C and 100 °C, sustainable for both heat and electric power generation, are obtained, depending on the geothermal conditions and closed-loop dimensions. Furthermore, circulation rates of 0.02–0.04 m3 s−1 are cost-effective, and the system's efficiency and sustainability increase when a fluctuating and periodic heat extraction strategy is employed. When efficiently operated, DCHEs are a viable solution for renewable energy production and should be integrated into the local heat market and distribution network infrastructure. Field of researchEarth and Planetary Sciences; Energy; Environmental Sciences

Potential areas of interest for the development of geothermal energy in La Réunion Island based on GIS analysis

La Réunion Island has been considered an attractive target for geothermal exploitation since the 1970s. However, to date, the island's geothermal resources are not being used. Here we use for the first time, a Geographic Information System (GIS) as a decision-making tool to target the most favorable zones for exploitation of moderate and high temperature geothermal resources. All data acquired since the 1970s (geology, geophysics, geochemistry) in addition to environmental constraints such as protection of the National Park that was recently classified as a UNESCO World Heritage Site, were analyzed using the weighted overlay method to delineate favorable areas for local geothermal exploration. Three thematic maps were constructed: geological and structural indicators, thermal indicators, and geophysical indicators. Cross-analysis of these maps highlights two zones that have high geothermal potential; six zones with unconfirmed geothermal potential; and two zones located within the National Park protected area and thus subject to stringent exploitation regulations. It appears that, while thermal anomalies exist in several areas, permeability of the geological formations is low or insufficient for traditional geothermal utilization. Future exploration phases should assess the feasibility of using enhanced geothermal system (EGS) technologies to obtain a satisfactory economic return.

Geochemical evidence for the nonexistence of supercritical geothermal fluids at the Yangbajing geothermal field, southern Tibet

Exploring and exploiting high-temperature (even supercritical) geothermal resources are significant to meet energy demands and reduce carbon emissions. The Yangbajing geothermal field is the most exploited in China, with the currently highest temperature (329.8 °C) measured in a geothermal well. However, whether there are supercritical geothermal fluids beneath the deep parts of this geothermal field is under controversy. In this paper, the water isotope, chemical compositions, and C–He isotopes of gas samples were collected and analyzed. The geothermal water originated from the mixing of meteoric water and magmatic water (25%). The sources of CO2 in the geothermal field were dominated by the thermogenic degassing of carbonates and metasediments in the crust while the radioactive decay of U and Th in granite is the dominated source of He. The temperatures of three different reservoirs are 150 ± 15 °C, 250 ± 10 °C, and ∼320 °C (with a depth of ∼8 km), respectively. These were obtained using dissolved gas, soil CO2 flux, and noble gas geothermometers. Unlike other supercritical geothermal fields worldwide with larger, shallower, basaltic magma chambers, the Yangbajing geothermal field has a deep-seated, small-scale, granitic magma chamber. Thus, its geological conditions are not conducive for gestating supercritical fluids. These results are of great significance for exploring and developing high-temperature (even ultra-high-temperature) geothermal resources in China.

Existence of High Temperature Geothermal Resources in the Igneous Rock Regions of South China

Large areas of Yanshan period granites with high heat production values (3–10 μW/m3) and mantle plume around Hainan province co-exist in Igneous Rocks Regions of South China (IRRSC). Surface manifestations are mainly warm/hot springs with temperatures below 90 °C and no typical phenomenon of high temperature resources have been observed. The main objective of this paper is to discuss the existence of high temperature geothermal resources and their possible locations under this kind of geothermal and tectonic background by analysis of high temperature heat sources, borehole temperature measurement, and reservoir temperature estimation. Two possible partial melts of the magma chamber were detected as high temperature heat sources in the Southern Leizhou Peninsular and North Hainan Island at a depth of 8–15 km. Other low resistivity zones in the upper crust are more likely caused by fluid in the formations or faults but not high temperature heat sources. This was also verified by borehole temperature measurement in these two areas, with maximum formation temperatures of 211°C and 185°C found, respectively. Reservoir temperatures from fluid geothermometers show lower temperatures of between 110–160°C for typical geothermal fields over the IRRSC but not in the Southern Leizhou Peninsular and Northern Hainan Island. In all, high temperature geothermal resources may be found in the Southern Leizhou Peninsular and on Northern Hainan Island.