Geothermal Field Research Articles

Heat mining from super-hot horizons of the Larderello geothermal field, Italy

Super-hot rock geothermal is an emerging source of renewable and carbon-free energy. This paper is the first attempt to explore fluid and heat flow dynamics in the reservoir-wellbore coupled system, to assess the power generation performance of a super-hot (>450 °C) enhanced geothermal system (EGS). We developed a high-performance code and built a 3-D wellbore-reservoir coupled model based on data from a recently completed deep-drilling project at Larderello, Italy. The general pattern of the super-hot EGS is characterized by a significant temperature plummet (>60 °C), after which the production fluid evolves from steam to a two-phase mixture till the end of the operation period. Reservoir pressure emerges as a key parameter to determine the temperature of the two-phase mixture. By realistically capturing phase transitions driven by coupled thermo-hydraulic processes during operations, our numerical model predicts a lower power generation efficiency compared to previous attempts based on ultra-simplified models. Although finalized at assessing the thermodynamic viability of a specific system, this modeling approach provides general information on fundamental thermo-hydraulic processes in the Earth crust that might be applied for the design of similar EGS projects elsewhere.

CO2 and heat energy transport by enhanced fracture permeability in the Monterotondo Marittimo-Sasso Pisano transfer fault system (Larderello Geothermal Field, Italy)

Carbon dioxide is one of the most important gasses naturally released from geothermal systems. Establishing the processes and pathways that regulate the CO2 diffuse degassing can provide valuable information for exploration and exploitation purposes of geothermal reservoirs. In this work, a high-resolution CO2 flux (with records up to 2927 g m–2 d–1) and soil temperature (with records up to 98.8 °C) survey was carried out along with detailed fracture parameters measurements in a selected area of the Monterotondo Marittimo-Sasso Pisano transfer fault (Larderello geothermal system, Tuscany, Italy). The main aim was to define the behavior of diffuse CO2 through the fault system and investigate how the soil CO2 flux and steam change with respect to the architecture of the fault damage zone (i.e., volumetric fracture intensity, permeability, and persistence of the fractures). The presence of multiple populations of CO2 flux suggested that three different transport mechanisms control soil degassing: i) purely diffusive, ii) mixed diffusive-advective, and iii) purely advective, characterized by efflux values of <20, between 20 and 300 and >300 g m–2 d–1, respectively. The spatial distribution of these fluxes well agrees with the fracture distribution and features of the Jurassic radiolarite (Diaspri Fm) dissected by NNE-striking faults. Areas with high CO2 emissions are indeed able to reveal major upflow areas from deep reservoirs through deep-reaching permeable fault zones. The interaction between pre-existing fractures and fracture-related fault-zone locally enhances the secondary rock permeability as highlighted by the correlation between Discrete Fracture Network (DFN) modeling and advective flux. Eventually, by normalizing the CO2 output to the fault strip (1350 m2), a release of CO2 equal to ∼155 t d–1 km–2 was estimated.

Lithium Occurrence in Italy—An Overview

Italy has no record of Li production, even though it is well known for its outstanding Li mineral specimens from the Elba Island pegmatites. Because of the current geopolitical situation, the opportunity for a systematic appraisal of resources is evident. Most European Li production comes from deposits associated with Late Paleozoic magmatic rocks. In Italy, such rocks occur extensively in Sardinia and Calabria, but their potential for Li is unknown, and deserves a more systematic exploration. Also of potential interest are the Permo–Triassic spodumene pegmatites in the Austroalpine units of the Central Alps. The Tertiary pegmatites (Elba Island and Central Alps) contain Li minerals, but do not appear large enough to warrant bulk mining. However, we notice that Tertiary–Quaternary magmatic rocks of the Tuscan and Roman magmatic provinces have systematically higher Li contents than those recorded in normal arc igneous rocks worldwide. Specifically, Tuscan granites contain up to 350 μg/g Li, mostly hosted by biotite (up to 4000 μg/g Li); the Capo Bianco aplite (Elba Island) contains up to 1000 μg/g. There are other small Li occurrences associated with Mn deposits and metabauxites, and there is a hypothetical potential for sediment-hosted deposits in the post-orogenic Lower Permian Alpine basins. However, the most promising potential seems to be associated with subsurface fluids. High-enthalpy fluids in geothermal fields may contain up to 480 mg/L Li. Lower-temperature thermal waters may also contain significant Li (&gt;10 mg/L). Moreover, a visionary, but not impossible, perspective may consider a deep injection of water to interact with, and extract Li from, magmatic rocks.

Ground motion prediction equations as a proxy for medium properties variation due to geothermal resources exploitation

Sub surface operations for energy production such as gas storage, fluid injection or hydraulic fracking modify the physical properties of the crust, in particular seismic velocity and anelastic attenuation. Continuously measuring these properties may be crucial to monitor the status of the reservoir. Here we propose a not usual use of the empirical ground-motion prediction equations (GMPEs) to monitor large-scale medium properties variations in a reservoir during fluid injection experiments. In practice, peak-ground velocities recorded during field operations are used to update the coefficients of a reference GMPE whose variation can be physically interpreted in terms of anelastic attenuation and seismic velocity. We apply the technique to earthquakes recorded at The Geysers geothermal field in Southern California and events occurred in the St. Gallen (Switzerland) geothermal field. Our results suggest that the GMPEs can be effectively used as a proxy for some reservoir properties variation by using induced earthquakes recorded at relatively dense networks.

APPLICATION OF GENERALIZED DERIVATIVE OPERATOR ON BOUGUER ANOMALY FOR DETECTING GEOLOGICAL STRUCTURES

Generalized Derivative Operator (GDO) is one of the first-order derivative filters that could control the derivative’s direction by modifying the value of azimuth () and dip () parameters. This study aims to examine those GDO parameters on synthetic Bouguer anomaly and apply them to field data of the Silver Peak geothermal field to identify the geological structures. We use Python programs to conduct the GDO and other derivative operators such as horizontal gradient (HG), analytic signal amplitude (AS), as well Second Vertical Derivative (SVD) for comparison. The derivative operators are performed in the Fourier domain and spatial domain. The results from synthetic data show that GDO can amplify the response both on local and regional anomalies. Nevertheless, enhanced local and regional anomaly might be shown as the same maximum value of GDO. It appears that GDO disregard the influence of density contrast and depth variation of the anomalous body. Subsequently, anomaly enhancement of Silver Peak area shows that GDO anomaly concurred with geological map. GDO and SVD could amplify the response of geological structures such as intrusive granite, fault lineaments, and lithological contact, as well as the horst-graben structure, as mentioned in previous studies, that might be acting as fluid pathways for the Silver Peak geothermal system.

An Inside Sun: Lickanantay Volcanology in the Salar de Atacama

The need of establishing more substantive dialogs between the mainstream and Indigenous knowledge on volcanoes has been increasingly recognized. To contribute to this endeavor, in this article we present the basic volcanological understandings of the Lickanantay people in the Salar de Atacama Basin. The Salar de Atacama Basin is an active volcanic territory within the Central Volcanic Zone of the Andes (CVZA). From the El Tatio geothermal field to Socompa volcano, more than 19 active volcanoes surround the territory that the Lickanantay (Atacameño) people have inhabited for more than 11,000 years. Living around and with the geological dynamism of the CVZA for millennia, the Lickanantay communities have accumulated rich observational and ceremonial data on volcanoes and volcanism. Paradoxically, however, while the Atacameño people have thoroughly characterized the CVZA, the volcanology community has not been properly introduced to the ancestral knowledge articulated in the territory. In order to make traditional Atacameño perspectives on volcanoes, volcanic risk, and geo-cosmic interdependence more amply available to the volcanology community, in this article, we present a basic description of what we call Atacameño volcanology. By Atacameño volcanology, we understand the ancestral principles by which volcanoes are known and understood as partaking in larger processes of a cosmo-ecological formation. Specifically, we describe the basic volcanological notions arising from the Lickanantay ancestral knowledge—volcanic formation, functions, and behavior. Second, we focus on the El Tatio geothermal field to offer a situated example. Finally, we delineate some relevant elements of human–volcano interactions and volcanic risk management from an Atacameño perspective. In our conclusions we suggest that volcanology, particularly in the context of the Andes, needs to engage more substantially with the Atacameño or other ancestral systems of knowledge production to expand volcanological insights and respond to the call for decolonizing science.

Heat, noble gases and CO2 sources in geothermal fields of Mexico

The Trans-Mexican Volcanic Belt (TMVB) and the Gulf Extensional Province (Baja California) are the regions in Mexico with the highest heat flow and where the main geothermal energy resources are currently exploited. Here, previously published and newly collected heat and volatile species (He, Ar and CO2) data from 132 fluid samples of four geothermal areas belonging to TMVB (Los Azufres, Los Humeros, Acoculco and Cerritos Colorados) and two in Baja California (Cerro Prieto and Las Tres Vírgenes) were analyzed. The goal was to identify heat sources, understand how heat is transported into the geothermal reservoirs and how the multiple fluid sources (meteoric, magmatic and crustal) in the reservoir might impact the heat in each field. Distinct R/Ra ratios (where R is the measured 3He/4He ratio and Ra is the atmospheric ratio) are observed in the TMVB and Baja California geothermal fields, with those in the TMVB displaying significantly higher R/Ra values (7.14–7.27) than those in Baja California (1.21–6.62), indicating a higher contribution of mantle helium from possibly younger, active magmatic heat sources. Lower R/Ra values in Baja California might reflect local tectonic features, where reservoirs are primarily affected by crustal contributions from an old, subducted fossil slab related to the Farallon Plate. The analysis of heat to 3He ratios (or Q/3He) together with information provided by 4He/36Ar ratios suggest that heat and volatiles are transferred into the hydrothermal system by advection. The magma bodies beneath the geothermal areas of interest are sufficiently voluminous to provide aqueous fluids by exsolution to sustain this advective flow. The release of He and Ar from magma, during its degassing, appears to be controlled by their diffusivities, fractionating the pristine mantle 4He/36Ar ratio toward higher values, while advection transfers volatiles more rapidly than heat, fractionating the pristine mantle Q/3He ratios toward lower values. The Q/3He vs. 4He/3He relationship shows that boiling and dilution by meteoric water led to further fractionation of heat and volatiles, particularly in Los Azufres and Los Humeros fields, while Cerro Prieto has Q/3He ratios close to those at mid-ocean ridges or fractionated by circulation of low-heat, 4He-rich fossil waters. Distinct sources of heat and volatiles are also reflected in the CO2 content, the major dry gas component in these geothermal fields. Main sources of CO2 are mantle carbon (1%-51%) and limestone (34%–100%) from local sources and the subducting slab, although expected fractionation of the C isotopic composition and CO2/3He during magma ascent and degassing introduce uncertainties on the precise estimation of each source contributing C to the studied geothermal reservoirs. Yet, contribution of carbon from sediments might be minor (0%–35%), and mainly found in Cerro Prieto and Las Tres Virgenes fields located in Baja California, which are affected by the release of volatiles from the subducted Farallon plate and presence of connate waters in the reservoir of Cerro Prieto.

Combined Large-NSeismic Arrays and DAS Fiber Optic Cables across the Hengill Geothermal Field, Iceland

AbstractFrom June to August 2021, we deployed a dense seismic nodal network across the Hengill geothermal area in southwest Iceland to image and characterize faults and high-temperature zones at high resolution. The nodal network comprised 498 geophone nodes spread across the northern Nesjavellir and southern Hverahlíð geothermal fields and was complemented by an existing permanent and temporary backbone seismic network of a total of 44 short-period and broadband stations. In addition, we recorded distributed acoustic sensing data along two fiber optic telecommunication cables near the Nesjavellir geothermal power plant with commercial interrogators. During the time of deployment, a vibroseis survey took place around the Nesjavellir power plant. Here, we describe the network and the recorded datasets. Furthermore, we show some initial results that indicate a high data quality and highlight the potential of the seismic records for various follow up studies, such as high-resolution event location to delineate faults and body- and surface-wave tomographies to image the subsurface velocity structure in great detail.

The contribution of hydrothermal mineral alteration analysis and gas geothermometry for understanding high-temperature geothermal fields – The case of Ribeira Grande geothermal field, Azores

The Ribeira Grande geothermal field is located on the northern flank of Fogo Volcano (S. Miguel Island, Azores) and it is characterised by the presence of several active hydrothermal manifestations. An analysis of the RG5 geothermal well has been carried out by the recognition of neoformation minerals with depth and the estimation of equilibrium temperatures. The formation of hydrothermal minerals depends not only on temperature but also on subsurface geology. The mineral assemblage consisting of chlorite + quartz + haematite ± calcite ± anatase ± titanite + albite ± adularia (± dolomite) shows temperature above 235 °C, and the main alteration is divided into a smectite zone followed by a chlorite zone at depth. The adularia appearance evidences a boiling zone at the top of the reservoir and illite and chlorite also suggest fluid level fluctuations in the past. The H2/Ar gas geothermometry applied to the gases emitted by fumaroles points to a temperature of 256 °C for the reservoir feeding a fumarolic field located close to the RG5, which sustains the mineralogical research.

Genetic Mechanism of Geothermal Resources in the Qutan and Reshuizhou Geothermal Fields, Jiangxi Province, China: Evidence from Hydrogeochemical Characteristics of Geothermal Water

Abstract The hydrogeochemical characteristics and isotopic analyses of geothermal water are effective in the genetic study of a geothermal system. This study systematically investigated the geochemical and isotopic compositions of the geothermal water in the Qutan and Reshuizhou geothermal fields. The geothermal water in these two fields is of HCO3 (·F)-Ca·Na type and has undergone protracted water runoff and deep thermal cycle. Based on the mineral-water solubility equilibrium, the reservoir in the Qutan geothermal field has a temperature of 60–74°C and a depth range of 770–1003 m. The geothermal water in the Reshuizhou geothermal field has been mixed with cold water, which accounts for 58% in weight. The reservoir in the Reshuizhou geothermal field has a temperature range of 131-150°C and a depth range of 1953–2287 m. The geothermal water in the Qutan and Reshuizhou geothermal fields have 14C ages of 8251–7961 BC and 3514–2254 BC, respectively, which are consistent with the result of the 3H isotopic analyses. When recharging and transiting to geothermal water, the meteoric water underwent changes in geochemical compositions, which were dominantly controlled by water-rock interactions, including the precipitation of chalcedony and muscovite and the dissolution of calcite, fluorite, K-feldspar, and albite. In the Reshuizhou geothermal field, the hydrogeochemical compositions of the geothermal water are affected by cold water mixing.

Advanced characterization of hydrothermal flows within recharge and discharge areas using rare earth elements, proved through a case study of two-phase reservoir geothermal field, in Southern Bandung, West Java, Indonesia

Rare earth element (REE) analysis is effective for tracing water–rock interactions and solute transport in geothermal and groundwater systems. This study aimed to clarify hydrothermal flows within recharge and discharge areas of a geothermal system. We selected a well-known high-temperature geothermal field with two-phase reservoir in Southern Bandung, West Java (Indonesia) as a case study target, and collected 31 river/spring samples and 8 well rock samples. The measured total REE series concentrations ranged widely in the water samples, i.e., rivers: 3.4–35 ppt, cold springs: 0.3–284 ppt, and hot springs: 1.4 ppt to 102 ppm. Enrichment of light REEs (LREEs) in the water samples was characterized more clearly by Post Archean Australian Shale normalization than by chondrite normalization. Speciation analysis derived the following interpretations: dissolved REEs in the river water originated from free ions or Ln3+ that are typically contained in near-surface groundwater, and the dominant components were carbonate (LnCO3+) and Ln3+ complexes in the cold springs and LnCO3+ complexation in the hot springs. Steam and gas condensation into less-oxygenated groundwater can be indicated by rich LnCO3+. Only one hot spring was interpreted as being directly connected with the reservoir by strong H2S condensation and LREE enrichment. The water of the other cold and hot springs probably underwent dilution, evaporation, and magmatic gas condensation through shallow, deep, and deep-perched aquifers, permeable zones, and a major fault within the recharge and discharge areas. REE results also suggest that the main constituents of the aquifers are carbonates and silicate minerals and hydrothermally altered lahars, andesitic lava, and pyroclastic flow deposits. REEs facilitate detailed interpretation of geochemistry facies in shallow–deep waters of a hydrothermal system. Finally, integration of the analysis results of REEs, major anions and cations, water isotopes, and strontium isotopes improved a conceptual model of groundwater flows and recharge–discharge interactions in aquifers feeding a geothermal reservoir.

Semi-Analytical Method for 3D Transient Heat Transfer in Thermally Interacting Fields of Ground Heat Exchangers

The study of heat transfer in ground heat exchangers (GHEs) considering the fluid advection inside the pipes; the heat transfer between the fluid and the ground through the grout material; and the thermal interaction between GHEs is a challenging task. The present paper presents a new semi-analytical method that takes into account the aforementioned effects to consider both the short- to long-term effects of GHEs. The heat transfer between the fluid and grout was studied by a transient multipole expansion considering time-dependent fluid temperatures and an advection model for the pipes obtained from an energy balance on the heat carrier fluid. Thermal interactions were analyzed using an equivalent borehole method while penalizing the transient multipole expansion to include thermal interaction effects. Validation of the short-term predictions was performed by comparing the proposed model to experimental data found in the literature and to an FEA model. The proposed model was then compared with a FEA model in long-term simulations of a geothermal field comprised of 24 GHEs for multi-annual simulation. The method resulted in substantial reduction in computational time while preserving good accuracy when compared with the FEA model.

Simulation of cooling in a magma chamber: Implications for geothermal fields of southern Peru

Numerical simulations of a geothermal reservoir often assume that the primary heat source is a magmatic system; however, heat from the host rock and the coupled complex transport phenomena between magma chamber and host rock also need to be considered. This research numerically simulated the cooling history of an enclosed magma chamber and the thermal effects on the host rock around it from which geothermal energy could be extracted. Modeling of the magma body included natural convection, the effect of latent heat of phase change when the crystals are being formed between the liquidus temperature and the solidus temperature, and heat conduction when the temperature is below the solidus temperature. This study takes as an example a geothermal reservoir in southern Peru (Western Cordillera) whose heat source is a rhyolitic magma chamber like those that gave rise to the intrusive rocks of the Peru coastal cordillera. This analysis varies the chamber shape and uses three solidification temperature ranges for convection and conduction models above the rock formation temperature (solidus) to study the implications for heating of the host rock. This is the first study of its kind in this area. The center of an average-sized magma chambers takes approximately 500 ka to cool from 800 °C to 300 °C. Simulated cooling times between intrusion and solidus temperatures decreased 3 ka when convection was modeled along with conduction cooling. Cooling times decreased by up to 6 ka when the solidification temperature range was increased. The host rock temperature pattern depends strongly on the stage at which the magma chamber is modeled to begin cooling. The temperatures results near the surface of the host rock obtained in this work match well with measurements at hot springs founded in several places in the Western Cordillera. Application of the methodology proposed in this study can reduce uncertainties in planning geothermal energy extraction wells. The accuracy of the numerical model described here could be improved by including more ground data from exploration wells, e.g., soil stratigraphy and temperatures variation with depth.

Reservoir rock characterization in Edremit geothermal field: Geochemical implications for possible fault zones

Reservoir lithology of Edremit geothermal field from western Anatolia is investigated by means of mineralogic-petrographic and geochemical analyses. Drill cuttings that belong to reservoir levels are analyzed macroscopically and microscopically, followed by the techniques of X-Ray Diffraction (XRD), Confocal Raman Spectroscopy (CRS) and Electron Probe Micro Analysis (EPMA) for detailed mineral characterizations. In the samples, dominant rock fragments are identified as granitic rocks while quartz, feldspars (plagioclase, orthoclase), micas (biotite, muscovite), carbonates (calcite, dolomite, ankerite) and amphiboles (actinolite, richterite, pargasite, hornblende, edenite) constitute the main mineral phases. CRS and EPMA studies reveal that the composition of plagioclase feldspars ranges from Ca-rich (bytownite) to Na-rich (oligoclase). The effects of cataclasis and alteration which are the indications of faulting and hydrothermal fluid activity, respectively, are detected from textural characteristics of the samples. These textures include cataclastic, mylonitic and mortar textures. The observed alterations are metasomatism and secondary uralitization. The changes in major and trace element concentrations along the well bore are deduced from the results of X-Ray Fluorescence (XRF) analyses. It was observed that changes in textural features which are associated with various grain sizes from coarse to fine, closely coincide with chemical variations with depth, leading to the identification of three distinctive zones at depth intervals of 900–928 m, 930–974 m and 976–1038 m. The correlation of geochemical findings regarding hydrothermal fluid effect (such as pyrite abundance, alteration features) with the information of a probable water leakage around 930 m depth, suggests that the middle part of these three zones is the potential passageway for the fluid. The lithogeochemical results also suggest that the fluids are likely to be Si-rich and (in deeper levels) Ca-rich which are in accordance with the hydrogeochemistry of the system that is previously investigated. This study points out the importance and practicality of geochemical studies as significant indicators utilized in geothermal exploration.

Optimizing Image Scale and Resolution to Obtain Lineament Related Structures using ALOS PALSAR Dual Orbit Data

Geological structures identification using conventional optical remote sensing encountered some problems due to atmospheric and surface conditions. Therefore, we exploited the Synthetic Aperture Radar (SAR) in Ascending and Descending orbits to characterize lineaments related to geological structures. We have analyzed two images from the Phased Array type L-band Synthetic Aperture Radar (PALSAR) onboard the Advanced Land Observing Satellite (ALOS). The adjustment of their resolution in the multilooking process was applied to obtain the best understanding between detected lineament and reference fault. The cross-polarized data of ALOS PALSAR level SLC were multilooked with variation of resolution from 15 to 120 m. Obtaining the optimum detected lineament-related faults, which applied the Yamaguchi criterion to extract the lineament visually in the multilooked images. The Kamojang-Guntur Geothermal Fields in West Java were selected as a study area because there is a challenge to confirm geological structure in the area with high alteration and high weathering conditions. According to the proposed criterion, we have obtained those images with resolution of 75 to 90 from the analysis images can represent surface geological structure conditions, and the correlation with existing published geological map. The field verifications have been confirmed that the geological structure indication from images has been proved with the existence of slickenside and morphological conditions such as drainage of rivers, ridges, and valleys. Based on lineaments, stress regimes, and field verifications, it was concluded that Yamaguchi criterion analysis with image resolution 75 to 90 m can describe the presence of regional structures and interpreted structures.

The CORRELATION BETWEEN 3-D MAGNETOTELLURIC INVERSION MODEL WITH DRILLING DATA IN PATUHA GEOTHERMAL FIELD

The Patuha geothermal field is located in West Java Province, Indonesia and developed by state owned company PT Geo Dipa Energi (Persero). The Commercial Operation Date (COD) for Patuha was in September 2014 with plant capacity of 1x60 MW. Until now, Patuha Unit I geothermal field has been running for almost 7 years. The current production wells have experienced a natural decline, which is showed by a reduction in production capacity to the initial production. This causes the steam supply to the Power Plant Unit I to be not optimal, so a make-up well program is needed. Furthermore, to support the addition of electricity production capacity from geothermal energy in Indonesia, the development of the Patuha Geothermal Field is planned to be carried out for the next Power Plant Unit’s expansion (Unit 2 and Unit 3). Nevertheless, determining the location for both make-up and development drilling might still pose high risks. This is especially because the development area (where production and injection wells are located) is only concentrated in the eastern area of the contract area. Geophysical data especially Magnetotelluric (MT) has an indispensable role considering the limited data and the limited number of existing wells that cover the entire Patuha prospect area. To understanding the subsurface feature and see the correlation between MT model with well results in Patuha Geothermal Field, MT and TDEM survey were conducted in the eastern and western parts of Patuha area with total 100 stations. Considering the complexity of the subsurface condition in volcanic area, 3-D inversion of the MT data will be the most representative approach to investigate geothermal system in Patuha Geothermal Field. An obvious subsurface resistivity distribution revealed by the 3-D inversion showed a good agreement with well results especially in mapping the temperature distribution both vertically and horizontally. Generally, the resistivity distribution consists of a conductive zone (1–10 ohm-m) at the shallow part overlying a reservoir zone with a rather higher resistivity range (20–60 ohm-m). The conductive zone (&lt;10 ohm-m) is correlated with Base of Conductor (BOC) of the wells that indicated by the presence of the argillic mineral. Meanwhile, the resistivity value around 15-20 ohm-m is correlated with Top of Reservoir of the production or injection well which is characterized by the presence of a convective temperature. In addition, from the results of resistivity mapping there is a very good correlation also in determining of reservoir boundary which is characterized by the presence of reverse temperature from the well. These results can be used as a guidance for better development strategy and drilling prognosis for the next drilling campaign especially in the area which limited number of wells.

Laboratory evidence for slip evolution of granite fractures due to chemical stimulation in geothermal reservoirs

Understanding the mechanisms of injection-induced seismicity is critical for managing the anthropogenic geohazard, which recently hinders the development of geothermal energy. Here we reanalyzed the seismic data recorded during the acid fluid injection at the geothermal field of Soultz-sous-Forêts (France) in 2005 and conducted a series of water injection experiments on granite fractures treated with dilute hydrochloric acid. We aimed to provide laboratory evidence for slip transition of the chemically treated fractures in geothermal reservoirs. We found that the amount of induced seismic events is promoted by the acid treatment, and the distribution of these seismic events is expanded in the post-acid water injection. Our experimental study demonstrated non-linear distributions of water pressure and frictional strength over the fracture during the water injection. The ratio of the shear stress variation to the fluid pressure variation reduces with a larger aseismic slip front amplification factor, which reflects the transition from the aseismic to seismic slip. The study revealed that the acid treatment potentially causes an increase in seismic event amount but a reduction in moment magnitude in the chemically treated region. The aseismic slip in the treated region may promote shear stress gradient along the fracture and result in the seismic events expanding in untreated regions. Additionally, the moment magnitude may be amplified when the fracture dictates neighboring fractures beyond the treated region.

Uranium enrichment associated with geothermal alteration: An example from Rehai, Tengchong volcanic area of China

For the continental rift‐related geothermal system, the hydrothermal alteration zones are rarely studied for uranium exploration. High uranium concentrations (up to 36 ppm) are shown in the altered sandstones at the Rehai geothermal field (RHGF) in Tengchong, China. To demonstrate the uranium enrichment mechanism, this study comparatively examines the trace element (uranium in particular) and rare earth element (REE + Y) concentrations of hot spring water, spring deposits (sinters and sulphates), and altered host sandstones from the RHGF of the Tengchong volcanic area. Along the central north‐south trending fault at Rehai, sinters and sulphates are precipitated from hot spring water, and the Neogene sandstones have been altered by hydrothermal fluids into zones formed of various clay minerals. The δD and δ18O values for the hot spring water indicate that the geothermal water originates from meteoric water. The spring water and hot spring deposits have identical REE distribution patterns which are featured with heavy REE (HREE) enrichment and negative Eu anomalies. Among the hot spring deposits, the yellowish earthy aggregates (tamarugite) deposited at the acidic Zhenzhu Spring have the maximum U concentration (27 ppm). The altered sandstones at Rehai, however, display the highest U concentrations (14–36 ppm). The U enrichment in the hot spring deposits and altered host rocks at the Rehai geothermal field is probably related to the multiple cycles of uranium leaching from granites, transportation upward along fault structures by geothermal water, decompression, and fixation in the hot spring deposits and/or alteration products. The results indicate that alteration zones associated with geothermal systems are also potential targets for uranium exploration.

Geophysical Analysis of Thermo-physical Properties of Rocks in Ikogosi field for Geothermal Energy Prospect.

The thermal and physical properties of rock units from Ikogosi Warm Spring (IKGWS) in the Southwestern part of Nigeria were examined in order to characterize and explore its geothermal prospect and to provide an insight into the different thermal properties of rocks of the study area. A total of 40 rock samples made up of granite, quartzite and gneiss series were collected from the outcrops of the study area and analyzed for Thermal conductivity (TC), Radiogenic heat production (RHP), Heat flow (HF), Porosity, Density and surface spring temperature measurements. The RHP values for all samples varied from 1.8 to 3.5μWm-3 with an average value of 2.5μWm-3 and standard deviation (SD) of 0.4 while the heat flow values varied from 14 to 27mWm-2 with an average of 19mWm-2 and SD of 3.4. The TC values varied from 2.95 to 4.11 with a mean of 3.49mWK-1 with a SD of 0.4 while the porosity values varied from 0.21-1.15 % with a mean of 0.62% with SD of 0.39. The density values varied from 2.68 to 2.85gcm-3 with a mean of 2.76gcm-3 and SD of 0.067. The surface temperature of the spring varied from 32 to 45°C with a mean of 38.9°C. From these results the average RHP and HF values estimated from all samples was below the 4μWm-3 and 100mWm-2 recommended value of heat to be considered for economic importance. Thus, the IKGWS geothermal field cannot be explored for power generation but for other geothermal activities and may be classified as a low enthalpy geothermal system (

The impact of geothermal fluid discharge on drainage water and groundwater quality in terms of toxic contaminants in the agricultural Harran Plain, Turkey

The Karaali Geothermal field located in Harran Plain, which has a great agricultural potential for the development of our country, has a great importance because of being a thermal tourism and greenhouse heating source in Sanliurfa province. In this study, the aim was to determine heavy metal contamination, which has direct and indirect serious effects on human health, due to geothermal causes. In this context, a total of 11 sampling points in the Karaali Geothermal fluid (KG), the closest drainage channels where geothermal water is discharged (DW1, DW2, DW3, DW4 and DW5), and the closest groundwater to these points (GW1, GW2, GW3, GW4 and GW5), were sampled during 8 months between May 2020 and March 2021. Heavy metal concentrations were obtained in the order F>Sr>B>Ba>Al>Li>Fe>Cr>V>Mo>Zn>Cu>Zr>Be>Mn>Cd>Co>Pb>Ag>Ni for groundwater and Al>Fe>Sr>F>B>Mn>Ba>Li>Cu>Zn>Cr>Ni>Co>Mo>Be>Pb>Zr>Cd>Ag>V for drainage water. Al, Fe and Mn values were determined to exceed the acceptable values in terms of suitability of the water in the drainage channels for reuse and agricultural irrigation purposes. In groundwater, the limit values in terms of suitability for drinking water were exceeded for Al, B, F, Fe and Mo parameters. In the diagram of pH and metal load, all the samples were classified as “near neutral–high metal.” The study found that six factors in both drainage and groundwater factor analysis separately explained 86.1% and 81.2% of the total variation in the data, respectively. In the cluster, good results were obtained with three different similarity groups for drainage waters and five different similarity groups for groundwater. These results show that the factors responsible for toxic contamination in the drainage channels and especially in the groundwater are primarily related to the use of geothermal fluid in greenhouse cultivation and spa tourism, excessive use of pesticides and fertilizers in agricultural activities, and geogenic processes.