Hydrothermal Reservoir Research Articles

Interdependencies between physical, design and operational parameters for direct use geothermal heat in faulted hydrothermal reservoirs

Interest in direct use geothermal systems is increasing due to their ability to supply renewable, environmentally friendly heat. Such systems are mostly developed in conduction dominated geological settings where faults are often encountered. Interdependencies between physical, design and operational parameters make it difficult to assess the performance of such systems. Interaction with faults could potentially have adverse effects on system lifetime, generated Net Present Value (NPV) and produced energy. In this work a single doublet system in the enthalpy range of 140 kJ/kg to 350 kJ/kg is analysed using COMSOL Multiphysics. A choice of design (well spacing and placement), physical (layered reservoir, fault flow properties, fault throw) and operational (injection and production flow rates) parameters are considered in a full factorial design that includes 2430 3D reservoir simulations. Results show that fault flow properties characterization is more significant than fault throw structural characterization. For the considered reservoir properties, increasing the flow rate four times results in an NPV increase of a factor seven, despite the shorter system lifetime. A sealing fault renders the system lifetime less sensitive to the doublet positioning. Synthetic model results shown can serve as guidelines to reducing full scale field models. Importance and relevance of these results remains very high for horizontally homogeneous, layered reservoirs. The analysis expands the understanding of interdependencies for direct use geothermal systems and informs on their further development.

Potential of $$\hbox {CO}_{2}$$ based geothermal energy extraction from hot sedimentary and dry rock reservoirs, and enabling carbon geo-sequestration

Carbon capture and sequestration (CCS) is necessary to mitigate global warming caused by anthropogenic $$\hbox {CO}_{2}$$ emissions in the atmosphere. However, due to very high storage cost, it is difficult to sustain the CCS industry. The hot sedimentary and dry rock reservoirs with very high temperature can support both geothermal energy production, and carbon geosequestration economically, provided the $$\hbox {CO}_{2}$$ is used as a heat-carrying fluid with proper optimization of injection parameters according to reservoir conditions. In this paper we have reviewed past studies discussing the working mechanisms, pressure management strategies and various advantages of energy extraction from hydrothermal reservoirs by $$\hbox {CO}_2$$ plume geothermal technology and hot dry rock— enhanced geothermal system (EGS) technology. Past studies highlighted that due to very high thermal expansivity and mobility, supercritical $$\hbox {CO}_2$$ can produce more heat than water-EGS. For low enthalpy (around 50 $$^\circ$$C) and shallow (0.5–1.5 km) reservoirs, $$\hbox {CO}_2$$ can fetch more heat than water because of higher heat capacity. Other advantages of CCS and EGS are (i) the production of brine or $$\hbox {CO}_2$$ assisting to manage the reservoir pressure and restrict the fluid interference with neighboring reservoirs, (ii) the fluid loss, which is a significant concern in a water-EGS but for $$\hbox {CO}_{2}$$-EGS it is environmentally friendly, and (iii) higher pressure and cold fluid injection induced geological deformation and microseismicity are relatively less for $$\hbox {CO}_2$$-EGS than water-EGS. In this paper, we have also discussed various challenges of $$\hbox {CO}_2$$-EGS to enable CCS in hydrothermal reservoir and hot dry rock system.

A Major Hydrothermal Reservoir Underneath the Tatun Volcano Group of Taiwan: Clues from a Dense Linear Geophone Array

A dense linear geophone array is deployed across the Tatun volcano group (TVG) at the northern tip of Taiwan, where more than 7 million residents live in the Taipei metropolis. The array is composed of 50 geophones with a station spacing of ~ 200 m in average, and it is designed for striking in the NW–SE direction to record the many earthquakes in eastern Taiwan, where the Philippine Sea plate subducted beneath the Eurasia plate. The detailed examination of felt earthquakes shows consistent P-wave delays are recorded at particular stations of the array. The further forward modeling indicates there is a low-velocity zone (LVZ) at depths between ~ 0.5 and ~ 2.5 km beneath the major fumarole sites. Combining this preliminary result with previous studies including clustering seismicity, volcanic earthquakes, low-resistivity zone, strong degassing processes and shallow velocity structures, we suggest that the LVZ might be associated with the major hydrothermal reservoir at the TVG. The identification of the hydrothermal reservoir by the LVZ not only implies a potential volcanic threat, such as phreatic eruptions, in the future, but also provides the possibility of sustainable geothermal resources for replacing traditional nuclear and fossil fuel power plants. Detailed images of the LVZ and other volcanic structures will be obtained soon when dense geophone arrays with more than 600 geophones are deployed from 2020 to 2022.

Quantitative relationship between plume emission and multiple deflations after the 2014 phreatic eruption at Ontake volcano, Japan

The phreatic eruption of Mount Ontake in 2014 caused local-scale subsidence and a mass discharge of water–vapor plumes from vents. A previous study of InSAR data analysis modeled the local subsidence as a deflation of a shallow hydrothermal reservoir (~ 500 m beneath the vents), and speculated that it was associated with plume emission continuing just after the eruption. In addition, combination of the InSAR and GNSS data implies that another, deeper deflation source (~ 3–6 km beneath the vents) contributes to the baseline contraction of the GNSS data. In this study, we estimated daily mass flux of the emitting plumes using photographed images, and compared the temporal behavior of the discharged mass with that of deflation of the two sources in order to clarify their association. The temporal profiles of the shallow deflation volume and the discharge mass both show evidence of decay, but with different characteristics; the deflation volume progress was approximated by a single exponential decay with a long relaxation time (379–641 days), whereas the discharge mass displayed a sum of a linear trend and an exponential decay with shorter relaxation time (47 days). This discrepancy, along with GNSS data, suggests the contribution of a deep deflation source with a short relaxation time (20–40 days). Estimation of mass balance between the emitting plume and fluids discharged from both shallow and deep sources revealed that more than 70% of the discharged mass came from the deep source. Based on the estimated mass balance, phase state of the shallow reservoir was estimated as a single-phase, liquid-rich reservoir. The fast decay of the deep deflation may reflect rapid depressurization due to violent fluid discharge at the onset of the eruption. In contrast, the slow decay of the shallow deflation suggests that it had a minor role in the eruption. However, such a wet reservoir has the potential to induce volcanic hazard such as snow-melting lahar for future eruptions, requiring monitoring the volcano, which will probably shift to pre-eruptive re-pressurized phase, until the future eruption.

The whisper of the hydrothermal seismic noise at Ischia Island

This work is devoted to the study of the background seismic noise at Ischia Island. We highlight two persistent independent signals, with dominant frequency peaks in the range 1−2Hz and 3−4Hz, respectively.The first signal is the most energetic and persistent and its polarization properties are well defined, revealing a shallow propagation likely controlled by both tectonic/volcanic structures and morphological features. This signal is well correlated at all the stations in the Casamicciola area and its group velocity is estimated as 500−600m/s.The second signal shows similar polarization properties but the temporal pattern of its amplitude is strongly affected by anthropogenic noise, revealing a modulation on a diurnal scale. On the investigated spatial scales, any clearly correlated signal is not evidenced.We interpret the 1−2Hz correlated wave-packets as the persistent whisper of the shallow circulation of fluids, which are a mixing of sea and meteoric water and thermal fluids of the hydrothermal reservoir. In the conceptual scheme of the self-sustained musical instruments, we hypothesize that the solid structure of the shallow hydrothermal system of Ischia is formed by a network of channels, continuously excited by the circulating hydrothermal fluids, which produce a whispered sound (i.e. the seismic noise). This mechanism is able to produce persistent self-sustained oscillations that we observe as the 1−2Hz seismic signal and, eventually, higher modes.

The environmental impacts and the carbon intensity of geothermal energy: A case study on the Hellisheiði plant

Geothermal energy, alongside other low-carbon and renewable energies, is set to play a key role in decarbonising the power generation industry to meet the Paris Agreement goal. Thus far the majority of Life Cycle Assessment (LCA) studies focused on enhanced geothermal plants. However, conventional geothermal plants that harness hydrothermal reservoirs dominate the production of electricity from geothermal energy worldwide. This article focuses on Hellisheiði, a combined heat and power double flash geothermal plant located in Iceland, with an installed capacity of 303.3 MW of electricity and 133 MW of hot water. The study has a twofold goal: (i) identify hot spots in the life cycle and, where possible, suggest improvements, and (ii) understand the potential of geothermal energy to decarbonise the power generation industry. First, a detailed LCA study has been performed on Hellisheiði, with cradle-to-grave system boundaries and detailed site-specific data obtained from the literature. The analysis identifies consumption of diesel for drilling and use of steel for wells casing and construction of the power plant as the main hot spots. Second, carbon intensities of electricity production for various possible configurations of the Hellisheiði power plant (including single flash, and power-only production) have been compared with those of other geothermal plants and other energy sources. Different allocation procedures have been used to allocate impacts between electricity and hot water where necessary, and Monte Carlo simulations have been used to estimate uncertainties of Hellisheiði’s carbon intensities. The comparison shows that the carbon intensity of Hellisheiði is in the range of 15–24 g CO2-eq./kWh, which is similar to those of binary cycle geothermal plants, solar (photovoltaic) and hydropower, lower than other geothermal technologies and fossil-based technologies, and higher than nuclear and onshore wind.

Geothermal potential of Sedimentary Basins, especially of the Swiss Molasse Basin

Sedimentary basins usually have significant geothermal potential. Deep aquifers are key components. The factors, conditions, and processes that define and control the potential are: processes during basin formation like sedimentation, karstification, fracturing; rock porosity, permeability/fluid content; depth/temperature; hydrogeology; production sustainability. They are demonstrated on selected examples: USA basins, Paris Basin, Molasse Basin. Of the latter, the French, German and Austrian parts are treated first and then the Swiss Molasse Basin (SMB) in more detail.The various efforts undertaken to assess and quantify the SMB potential are described, example maps presented. The realizations of the SMB potential so far are really modest: of 10 deep drilling projects performed in various locations to date only one is successful, two are a partial success. The official Swiss energy strategy EN2050 includes electricity supply in the future; this assigns 4.4 TWh to geothermal sources in 2050. This would be delivered from geothermal power plants, foreseen are 250 MWe installed capacity from hydrothermal reservoirs and another 250 MWe from petrothermal (EGS) sources. Only the SMB could host hydrothermal resources; the current data don’t make much hope. In principle, EGS plants could take heat (and convert it to electricity) from below the SMB. The EGS technology itself has great potential but it is still in the proof of concept stage. Intensive R&D is ongoing in several countries, however, very substantial funding will be needed to answer the many questions still open.

Anomalous concentrations of arsenic, fluoride and radon in volcanic-sedimentary aquifers from central Italy: Quality indexes for management of the water resource

659 water samples from springs and wells in the Sabatini and Vicano-Cimino Volcanic Districts (central Italy) were analyzed for arsenic (As), fluoride (F−) and radon (222Rn) concentrations. Waters mostly sourced from a shallow and cold aquifer hosted within volcanic rocks, which represents the main public drinking water supply. Cold waters from perched aquifers within sedimentary formations and thermal waters related to a deep hydrothermal reservoir were also analyzed. The highest concentrations of As and F− were measured in the thermal waters and attributed to their enhanced mobility during water-rock interaction processes at hydrothermal temperatures. Relatively high concentrations of As and F− were also recorded in those springs and wells discharging from the volcanic aquifer, whereas waters hosted in the sedimentary units showed significantly lower contents. About 60% (As) and 25% (F−) of cold waters from the volcanic aquifer exceeded the maximum allowable concentrations for human consumption. Such anomalously high levels of geogenic pollutants were caused by mixing with fluids upwelling through faulted zones from the hydrothermal reservoir. Chemical weathering of volcanic rocks and groundwater flow path were also considered to contribute to the observed concentrations. Cold waters from the volcanic aquifer showed the highest 222Rn concentrations, resulting from the high contents of Rn-generating radionuclides in the volcanic units. Approximately 22% of these waters exceeded the recommended value for human consumption. A specific Quality Index (QI), comprised between 1 (very low) and 4 (very high), was computed for each water on the basis of As, F− and 222Rn concentrations and visualized through a spatial distribution map processed by means of geostatistical techniques. This map and the specific As, F− and 222Rn maps can be regarded as useful tools for water management by local authorities to both improve intervention plans in contaminated sectors and identify new water resources suitable for human consumption.

The Effects of Regional Fluid Flow on Deep Temperatures (Hesse, Germany)

A successful utilization of deep geothermal resources requires accurate predictions about the distribution of reservoir temperature as well as of the hydraulic processes exerting a direct influence on the productivity of geothermal reservoirs. The aim of this study was to investigate and quantify the influence that regional thermo-hydraulic processes have on the geothermal configuration of potential reservoirs in the German Federal State of Hesse. Specifically, we have addressed the question of how the regional thermal and hydraulic configuration influence the local hydro-thermal reservoir conditions. Therefore, a 3D structural model of Hesse was used as a basis for purely hydraulic, purely thermal and coupled 3D thermo-hydraulic simulations of the deep fluid flow and heat transport. As a result of our numerical simulations, Hesse can be differentiated into sub-areas differing in terms of the dominating heat transport process. In a final attempt to quantify the robustness and reliability of the modelling results, the modelling outcomes were analyzed by comparing them to available subsurface temperature, hydraulic and hydrochemical data.

Preliminary conceptual model of the Cerro Blanco caldera-hosted geothermal system (Southern Puna, Argentina): Inferences from geochemical investigations

The Cerro Blanco Caldera (CBC) is the youngest collapse caldera system in the Southern Central Andes (Southern Puna, Argentina). The CBC is subsiding with at an average velocity of 0.87 cm/year and hosts an active geothermal system. A geochemical characterization of emitted fluids was carried out based on the chemical and isotopic compositions of fumaroles, and thermal and cold springs discharged in this volcanic area with the aim of constructing the first hydrogeochemical conceptual model and preliminary estimate the geothermal potential. The main hydrothermal reservoir, likely hosted within the pre-caldera basement rocks, has a Na+-Clˉ(HCO3)ˉ composition with estimated temperatures ≥135 °C. The unconsolidated, fine-grained Cerro Blanco ignimbrite likely acts as the cap-rock of the hydrothermal system. The presence of phreatic eruption breccias in the surrounding area of the geothermal fumaroles supports the effectiveness of the pyroclastic deposit as sealing rocks. The isotopic data of water (δ18O and δD) indicate a meteoric recharge of the hydrothermal reservoir, suggesting as recharge areas the sectors surrounding the CBC, mainly towards the W and NW where large outcrops of the pre-caldera basement exist. A fault-controlled hydraulic connection between the hot springs and the hydrothermal reservoir is proposed for the Los Hornitos area. The fumaroles show the typical compositional features of hydrothermal fluids, being dominated by water vapor with significant concentrations of H2S, CH4 and H2. Considering the high geothermal gradient of this area (∼104 °C/km) and the relatively high fraction of mantle He (∼39%) calculated on the basis of the measured R/Ra values, the hydrothermal aquifer likely receives inputs of magmatic fluids from the degassing magma chamber. The preliminary geothermal potential at CBC was evaluated with the Volume Method, calculating up to E = 11.4*1018 J. Both the scarce presence of superficial thermal manifestations and the occurrence of an efficient cap-rock likely contribute to minimize the loss of thermal energy from the reservoir. The results here presented constitute the necessary base of knowledge for further accurate assessment of the geothermal potential and ultimately the implementation of the geothermal resource as a viable energy alternative for small localities or mining facilities isolated from the National Interconnected System due to their remote localization.

An Analytical Algorithm of Porosity–Permeability for Porous and Fractured Media: Extension to Reactive Transport Conditions and Fitting via Flow-Through Experiments Within Limestone and Dolomite

Accurate prediction of permeability evolution is essential for forecasting the long-term performance and lifetime of hydrothermal reservoirs, an important goal in the geothermal, ore, and petroleum industries. Erol et al. (Transp Porous Media 120(2):327–358, 2017. https://doi.org/10.1007/s11242-017-0923-z ) introduced a general (non-empirical) analytical Kozeny–Carman type equation for predicting matrix and fracture permeability during single-phase, non-reactive flow. Here we incorporate the equation into an algorithm for addressing the influence on porous and fractured media permeability of the transient reactive processes of mineral dissolution and precipitation. Analytical algorithm predictions are identical to permeability values measured during fluid circulation through limestone and dolomite core samples from the Campine Basin deep geothermal system in Belgium. Benchmarking used identical values for initial hydraulic aperture dimension and porosity, measured during fluid circulation based on nondestructive micro-CT imaging. Analytical algorithm predictions of reactive surface area and fracture porosity are similar to results based on the TOUGHREACT reactive transport code. TOUGHREACT implements several well-established power-law models for predicting permeability, notably Civan (AIChE J 47(2):1167–1197, 2001. https://doi.org/10.1002/aic.690470206 ) and Verma and Pruess (J Geophys Res Solid Earth 93:1159–1173, 1988. https://doi.org/10.1029/jb093ib02p01159 ). However, these models rely on specification of empirical exponents, which are not straightforward to measure. Our results suggest that a more general, computationally inexpensive analytical method can lead to accurate permeability calculation.

Seismic and Geodetic Evidences of a Hydrothermal Source in the Md 4.0, 2017, Ischia Earthquake (Italy)

AbstractSeismic events characterize active hydrothermal and volcanic areas and may be due to magma/fluid migration, hydrothermal pressurization, gravitational instability, and local tectonics. On 21 August 2017, an Md 4.0 earthquake occurred at Ischia volcanic island (Italy), within an active hydrothermal system. We analyze seismic, Global Positioning System, and interferometric synthetic aperture radar data to shed light on the source mechanism of such an event. The low‐frequency content (2 Hz), the low stress drop (0.01 MPa), and a low S/P spectral ratio suggest the involvement of fluids in the source mechanism. The focal mechanism suggests a mixed shear‐tensile (opening) rupture with the P first arrivals showing up movements in the nearest stations. Geodetic data describe an E‐W elongated area of coseismic subsidence overlapping a WSW‐ENE fault bounding the hydrothermal reservoir at depth. The modeled deformation field is consistent with a two‐source model consisting of a WSW‐ESE striking, north dipping normal fault, and a closing subhorizontal crack. This closure immediately followed an initial opening related to a fluid pressurization event responsible for the earthquake. We show that moderate magnitude earthquakes in active hydrothermal areas may be associated with the pressurization/depressurization cycles of a hydrothermal reservoir due to self‐sealing processes and not to the arrival of new fluids from depth. Other events like that recorded at Ischia, which have affected the island in historical times, are not necessarily associated with ‘volcanic unrest’ episodes and imply the occurrence of fault‐valve mechanisms. Therefore, the dynamics of hydrothermal systems must be taken into account in the seismic hazard evaluation.

Surface displacements of Aso volcano after the 2016 Kumamoto earthquake based on SAR interferometry: implications for dynamic triggering of earthquake–volcano interactions

SUMMARYThe 2016 Kumamoto earthquake involved a series of events culminating in an Mw 7.0 main shock on 2016 April 16; the main-shock fault terminated in the caldera of Aso volcano. In this study, we estimated surface displacements after the 2016 Kumamoto earthquake using synthetic aperture radar interferometry analysis of 16 Phased Array Type L-band Synthetic Aperture Radar-2 images acquired from 2016 April 18 to 2017 June 12 and compared them with four images acquired before the earthquake. Ground subsidence of about 8 cm was observed within about a 3 km radius in the northwestern part of Aso caldera. Because this displacement was not seen in data acquired before the 2016 Kumamoto earthquake, we attribute this displacement to the 2016 Kumamoto earthquake. Furthermore, to estimate the source depth of the surface displacement, we applied the Markov chain Monte Carlo method to a spherical source model and obtained a source depth of about 4.8 km. This depth and position are nearly in agreement with the top of a low-resistivity area previously inferred from magnetotelluric data; this area is thought to represent a deep hydrothermal reservoir. We concluded that this displacement is due to the migration of magma or aqueous fluids.

Hydrothermal alteration environments and recent dynamics of the Ischia volcanic island (southern Italy): Insights from repeated field, mineralogical and geochemical surveys before and after the 2017 Casamicciola earthquake

This study presents the results of repeated surveys conducted in hydrothermally altered areas on the active volcanic island of Ischia (in the Gulf of Naples, southern Italy). Data were obtained from field work, in situ temperature measurements, X-ray diffraction, optical and scanning electron microscopy, EDS-SEM micro-analysis, infrared spectroscopy and whole-rock geochemistry analyses on samples collected in October 2016, September 2017, March 2018 and November 2018, both prior to and following the damaging seismic event that occurred in August 2017 at Casamicciola. Mineralogical results point to the evolution of acid sulfate zones along the northern and western flanks of a resurgent block located in the central part of the island, in strict relation with endogenous fluid outgassing indicated in the literature. Mineralogy and whole-rock geochemistry of these acid sulfate zones reveal magmatic-hydrothermal environments in association with major structures that enable fluid circulation. It is interpreted that rare coarser alunites within dominant steam-heated and supergene alteration mineral assemblages are associated with environments that are no longer active, but which are supplied by magmatic vapours. In addition, collected data indicate slow endogenous degassing from hydrothermal reservoirs. Data from September 2017 show variations in the sulfate assemblage within the acidic sulfate zone located near the earthquake's epicentre, and acidification and the transitory appearance of alum-(K) is apparent under a nearly unchanged endogenous supply. Meteorological conditions of late summer could, in principle, justify the appearance of alum-(K), but its coexistence with alunogen and soil temperatures refute this possibility. Alum-(K) is interpreted to be the product of alunite decomposition at a local temperature of ca. 100 °C. Assuming a correct space-time relationship between the appearance of alum-(K) and the earthquake, it is thus inferred that mechanical activation by tectonic stress is the possible cause of alunite decomposition.

Diffuse emission of CO2 and convective heat release at Nisyros caldera (Greece)

The diffuse emission of CO2 from the south east sector of Nisyros caldera (Lakki plain) has been measured during a detailed survey (~1400 soil CO2 flux measurements) performed in October 2018. The gas emissions are fed by hydrothermal sources and, in minor part, by the soil biogenic activity whose mean CO2 flux (4 g m−2 d−1) is here estimated for the first time. The total amount of hydrothermal CO2 reaches 92 ± 8 t/d, a value that is slightly higher than that estimated with the same method between 1999 and 2001 (74 ± 7 t/d). The gas is emitted by different diffuse degassing structures (DDSs), including volcanic-hydrothermal structures (craters and domes) and NE-SW and NW-SE-trending tectonic lineaments.Even if the total CO2 emission is not particularly high at Nisyros (close to the median of CO2 emissions measured in volcanoes worldwide), the process is very energetic. The thermal energy associated with the shallow condensation of the steam in the DDSs reaches ~60 MW, while we estimate at 134–270 MW the total amount of thermal energy involved in the convective rising of the deep geothermal liquids that transport the gas from the depth toward the surface. This large flux of energy could dramatically increase during future earthquakes by addition of heat and mass from a deep hydrothermal reservoir, potentially triggering hydrothermal explosions, as it happened several times in the past few centuries.

Insights for the Angra do Heroísmo hydrothermal reservoir conceptual model

Onshore and offshore hydrothermal activity over the Azores mostly links to young active large volcanic polygenic edifices. The surface features of upflowing fluids from the hydrothermal reservoirs include fumaroles, water springs, and gas emissions, as in the case of Guilherme Moniz and Pico Alto stratovolcanoes on Terceira Island, Azores. Geology, tectonics, and pressure and temperature of fluid dynamics affect possible lateral and fluid downflow, giving rise to hidden hydrothermal reservoirs. The several hydrothermal features identified on water springs, well fluid production temperature, radon gas emissions, and alteration rock minerals on the vicinities of Angra do Heroismo clearly outline a low-seated hot reservoir, the Angra do Heroismo hydrothermal reservoir. The top reservoir feature, hot water, might result from the mixing of low mineralized meteoric freshwater with a deeper upward hydrothermal fluid component. Several sites of the upflowing hot fluid are unknown, but most probably are linked to the closer young active stratovolcanoes of Guilherme Moniz, Pico Alto, and Santa Barbara. Taking into consideration the available data and interpretation, we estimate the range and probable distribution of the stored heat reserves using Monte Carlo stochastic analysis in the order of 12.99–46.90 MWe. The proximity to potential end users, namely the tourism industry, turns the probable resource into a plausible geothermal reserve. The exploration studies implementation here outlined would highly contribute to the conceptual model improvement.

Geochemical studies of thermal waters from Kanlaon Volcano, Negros Island, Philippines

Kanlaon is a stratovolcano situated in central Negros Island, Philippines. It has a record of 30 eruptions since 1866 ranging from phreatic to phreatomagmatic (VEI = 1–2) events. Geothermal manifestations in Kanlaon Volcano are evidenced by the presence of hot springs, boiling mud pools, and warm rivers with temperatures up to 95 °C. It has developed a significant hydrothermal system within the volcanic edifice. Through time, it evolved into two distinct hydrothermal systems independent of each other. A mature hydrothermal system, represented by Pataan thermal area is characterized by neutral Na + K chloride (bicarbonate) fluids and an immature system, represented by Hagdan spring is characterized by the presence of acid-sulfate waters.Representative water samples were collected between 2014 and 2017 to further characterize the two systems. Chemical and sulfur isotopic analyses were performed on thermal waters to classify the samples that are linked to the co-existence of these two hydrothermal systems. Results show that thermal spring's pH ranges between 2.5 and 8.1 and conductivity ranges between 88 μS/cm to 3930 μS/cm. Based on their Cl-SO4-HCO3 relative abundances, the thermal waters are classified as acid sulfate, acid-sulfate chloride, neutral chloride and neutral bicarbonate waters.Temperatures of deep-water hydrothermal reservoirs were calculated using fluid-mineral geothermometers and correlated with available borehole temperatures. The Na-K-Mg geothermometer was used to evaluate equilibrium between water and reservoir rocks. Results show that the samples are not in equilibrium with the minerals in the reservoir. Temperature of equilibrium for the mature hydrothermal system was extrapolated to 270 °C based on available borehole temperatures. The linear trend towards the equilibrium temperature in the Na-K-Mg geothermometer can be explained by either groundwater/meteoric dilution. This is consistent with the light sulfur isotopic signatures between δ34S = −3.4‰ and + 1.2‰ of the mature hydrothermal system (Mambucal), which imply that the origin of sulfates is linked to surficial oxidation of H2S or dilution and mixing of meteoric water. On the other hand, immature hydrothermal system (Hagdan) shows a significantly higher sulfur isotopic composition (δ34S = +8.2‰), which could either result from the disproportionation of magmatic SO2 or from the hydrolysis of elemental sulfur at high temperature between 100 and 350 °C.

Geothermal Linkage between a Hydrothermal Pond and a Deep Lake: Kuttara Volcano, Japan

Kuttara Volcano, Hokkaido, Japan, consists of temperate Lake Kuttara and the western Noboribetsu geothermal area. In order to explore geothermal relations between Lake Kuttara and the geothermal area, the heat budget of a hydrothermal pond, Okunoyu, was evaluated, and the heat storage change in the lower layer of Lake Kuttara was calculated by monitoring the water temperature at the deepest point. The lake water temperature consistently increased during the thermal stratification in June–November of 2013–2016. The heat flux QB at lake bottom was then calculated at a range of 4.1–10.9 W/m2, which is probably due to the leakage from a hydrothermal reservoir below the lake bottom. Meanwhile, the heat flux HGin by geothermal groundwater input in Okunoyu was evaluated at 3.5–8.5 kW/m2, which is rapidly supplied through faults from underlying hydrothermal reservoirs. With a time lag of 5 months to monthly mean QB values in Lake Kuttara, the correlation with monthly mean HGin in Okunoyu was significant (R2 = 0.586; p < 0.01). Applying Darcy’s law to the leakage from the hydrothermal reservoir at 260–310 m below the lake bottom, the time needed for groundwater’s passage through the media 260–310 m thick was evaluated at 148–149 days (ca. 5 months). These findings suggest that the hydrothermal reservoir below lake bottom and the underlying hydrothermal reservoirs in the western geothermal area are both connected to a unique geothermal source in the deeper zone as a geothermal flow system of Kuttara Volcano.

Isotopes & Geochemistry: Tools For Geothermal Reservoir Characterization (Kamchatka Examples)

The thermal, hydrogeological, and chemical processes affecting Kamchatka geothermal reservoirs were studied by using isotope and geochemistry data: (1) The Geysers Valley hydrothermal reservoirs; (2) The Paratunsky low temperature reservoirs; (3) The North-Koryaksky hydrothermal system; (4) The Mutnovsky high temperature geothermal reservoir; (5) The Pauzhetsky geothermal reservoir. In most cases water isotope in combination with Cl- transient data are found to be useful tool to estimate reservoirs natural and disturbed by exploitation recharge conditions, isotopes of carbon-13 (in CO2) data are pointed either active magmatic recharge took place, while SiO2 and Na-K geothermometers shows opposite time transient trends (Paratunsky, Geysers Valley) suggest that it is necessary to use more complicated geochemical systems of water/mineral equilibria.

Secondary minerals in the geyserites of the Geysers Valley (Kamchatka)

Secondary minerals assemblages that are deposited from thermal solutions at the top of geysers (Velikan, Bolshoy) were investigated. It is established that assemblages are represented mainly by opal and high-silica zeolites (mordenite and heulandite). As conditions of feeding hydrothermal reservoir change, minerals of the kaolinite group and smectites may appear.