High-enthalpy Geothermal Systems Research Articles

Modeling aqueous association constants and mineral solubilities at subcritical and supercritical temperatures

The need for sustainable power generation has increased interest in the use of hydrothermal fluids for industrial applications. New high-enthalpy geothermal systems and biowaste-to-fuel processes are two relevant examples that employ supercritical fluids which require an in-depth understanding of complex chemical reactions occurring near the supercritical temperature of water (374 °C). As these processes operate in thermodynamic regimes that are not currently covered by a standard molar Gibbs energy of formation model, only empirical fits for single reaction systems are available which limit the use of multi-component phase equilibria calculations that are standard practice for less extreme environments. Here, we advance a standard molar Gibbs energy of formation model able to operate in these otherwise inaccessible thermodynamic states to include species needed for key mineral solubility systems and ion association reactions. This work extends a model based on molecular statistical thermodynamics (MST) into four new systems (Na3PO4-H2O, LiOH-H2O, KOH-H2O, and BaSO4-H2O) by extending the model to cover 10 new species. For each of these systems, model predictions were consistently within the experimental uncertainties for the new systems covered. A breakdown of MST contributions to the model revealed that electrostatic and hard sphere contributions were key to reproducing density dependencies of standard molar Gibbs energy of formation values around the critical point of water.

Modelling reinjection of two-phase non-condensable gases and water in geothermal wells

Steam production from high enthalpy geothermal systems is frequently accompanied by the emission of non-condensable gases (NCGs), initially dissolved in the liquid phase or mixed in the vapour phase at depth in the reservoir. Capturing and reinjecting geothermal gases (CO2, CH4, NH3, H2S, H2, …) together with condensed steam leads to a significant improvement of the environmental profile of geothermal power systems and helps with reservoir recharge and pressure support. Nowadays, there are several ongoing projects targeting the minimalization of the environmental impact of geothermal exploitation through reinjection of the NCGs. For low NCG content, the gases can be fully dissolved into the condensed water and reinjected. However, for a high concentration of NCGs, the dissolution is only partial, and a two-phase mixture needs to be reinjected to achieve this goal. We present here the numerical results for the reinjection process in two different geothermal sites and for two different injection scenarios, focusing on the case of two-phase (liquid–gas) flows. Three commercial software (UniSim®, PIPESIM, and OLGA) and one in-house developed code (GWellFM) were used, and their results were compared. The injection should always take place inside a liquid column in the well to avoid degassing and accumulation of the NCGs. When targeting high pressure reservoirs or when injecting high ratios of water to NCGs flow rates, mixing of the two phases should take place on the surface. Whereas for low pressure reservoirs or for high gas contents mixing must occur deep in the well through one or several mixing points to ensure either complete dissolution of the gas or the downward flow of a two-phase mixture with minimum compression on the surface. The uncertainties of two-phase downward flows and water/NCGs mixtures characterisations introduce additional complexities in the modelling of the reinjection process.

High-Enthalpy Geothermal Simulation with Continuous Localization in Physics

Simulation of heat production in high-enthalpy geothermal systems is associated with a complex physical process in which cold water invades steam-saturated control volumes. The fully implicit, fully coupled numerical strategy is commonly adopted to solve the governing system composed of mass and energy conservation equations. A conventional nonlinear solver is generally challenged by the strong nonlinearity present during phase transition because of a huge contrast of thermodynamics between hot steam and cool water. In the process of solution, due to the steam condensation, the reduction in fluid volume reduces the pressure in the control volume. This generates multiple local minima in the physical parameter space, which indicates the Newton initial guess should be carefully selected to guarantee the nonlinear convergence. Otherwise, the Newton iteration will approach a local minimum and the solution based on Newton’s update cannot converge and needs to be repeated for a smaller timestep. This problem brings simulation to the stalling behavior where a nonlinear solver wastes a lot of computations and performs at a very small timestep. To tackle this problem, we formulated continuous localization of Newton’s method based on an Operator-Based Linearization (OBL) approach. In OBL, the physical space can be parameterized in terms of operators with supporting points at different levels of resolution. During the simulation, the operator values at supporting points are obtained through reference physics and the remaining part of the space is interpolated. In this way, the nonlinear physical parameter space can be flexibly characterized with different degrees of accuracy. In our proposed approach, the nonlinear Newton iterations are performed in parameter space with different resolutions from coarse to fine. Specifically, the Newton solution under coarser resolution is taken as the initial guess for that under finer resolution. A coarser parameter space represents more linear physics, under which the nonlinear solver quickly converges to a localized solution near the true solution. With refinement in physics, the Newton iteration will approach the true solution and the stalling behaviour in the simulation is avoided. Therefore, a larger timestep can be utilized in the simulation compared with the conventional nonlinear solvers.

The dynamic evolution of the Lahendong geothermal system in North-Sulawesi, Indonesia

This study uses an integrated approach to characterize the dynamic evolution of the power-producing high-enthalpy geothermal system of Lahendong, North-Sulawesi, Indonesia. Lahendong has two primary reservoirs, the southern and the northern, which have been utilised for electricity production for more than twenty years. The main focus of this study is the characterisation of heat and mass flows in the system with respect to geological structures and permeability distribution. Also, it delineates how the geothermal system has evolved and the spatial variation of the response resulting from prolonged utilization of the reservoirs. This research implemented geological structure analysis on recent surface fault mapping and pre-existing fault studies from literature. Further, the study analysed well data comprising well pressure, enthalpy, drilling program reviews and tracer tests. Hydrochemical investigation compiled new and old surface and subsurface hydrochemical evolution in both the temporal and spatial domain. The results confirm several fault trends in the study area: NE-SW and NW-SE are the major striking directions, while E-W and N-S are less dominant. The most apparent trends are NE-SW striking strike-slip faults, NW-SE thrust faults and N-S and E-W striking normal faults. The faults compartmentalize the reservoir. A comparison of the southern and the northern reservoir shows that the southern one is more controlled by faults; both reservoirs rely on fractures as permeability provider and are controlled by shallow hydrogeology, as derived from the integrated analysis of transient well data. Geochemical analysis shows that the reservoir fluids have generally a higher eelectrical conductivity and are closer to fluid-rock equilibrium, probably due to boiling, compared to spring waters. Spring waters have generally become more acidic, which is an expected result of reservoir boiling and increased steam input to near-surface waters. The spatial distribution of changes shows a permeability evolution over time and also the role of structural permeability in response to changing reservoir conditions. Observing and recording reservoir data is highly important to understand the reservoir response to production and ensure the long-term sustainability of the system. Additionally, the data is critical for making a major difference in the reservoir management strategy.

A laboratory study of hydraulic fracturing at the brittle-ductile transition

Developing high-enthalpy geothermal systems requires a sufficiently permeable formation to extract energy through fluid circulation. Injection experiments above water’s critical point have shown that fluid flow can generate a network of highly conductive tensile cracks. However, what remains unclear is the role played by fluid and solid rheology on the formation of a dense crack network. The decrease of fluid viscosity with temperature and the thermally activated visco-plasticity in rock are expected to change the deformation mechanisms and could prevent the formation of fractures. To isolate the solid rheological effects from the fluid ones and the associated poromechanics, we devise a hydro-fracture experimental program in a non-porous material, polymethyl methacrylate (PMMA). In the brittle regime, we observe rotating cracks and complex fracture patterns if a non-uniform stress distribution is introduced in the samples. We observe an increase of ductility with temperature, hampering the propagation of hydraulic fractures close to the glass transition temperature of PMMA, which acts as a limit for brittle fracture propagation. Above the glass transition temperature, acoustic emission energy drops of several orders of magnitude. Our findings provide a helpful guidance for future studies of hydro-fracturing of supercritical geothermal systems.

Analysis of fluid characteristics and estimation of geothermal reservoir temperature in Kaleosan Area, North Minahasa Regency

The appearance of manifestation characterizes the existence of geothermal potential. This research was conducted in the Kaleosan area of North Minahasa Regency, in North Sulawesi Province, Indonesian, which has a manifestation of hot water with a temperature of 90°C with pH of 6.89, and there are silica deposits around it, all of which are characteristics of a water-dominated geothermal reservoir. This research was carried out by analyzing the characteristics of the surface fluid samples of the Kaleosan hot water manifestations. Analysis of fluid characteristics using the Q-SO4-HCO3and Na-K-Mg. From the Q-SO4-HCO3diagram, it is found that the geothermal fluid in Kaleosan is a type of chloride. The high chloride content is thought to be due to the increase in geothermal fluids containing CO2and condensing in shallow aquifers. From the Na-K-Mg diagram, it is obtained that the Kaleosan hot springs are immature water where this manifestation fluid has undergone dilution and cooling by meteoric water during its trip to the surface. From the quartz silica geothermometer, the reservoir temperature estimate is 236.6°C, which indicates that the geothermal reservoir in the Kaleosan area is a high enthalpy geothermal system.

Modeling of multiphase mass and heat transfer in fractured high-enthalpy geothermal systems with advanced discrete fracture methodology

Multiphase mass and heat transfer are ubiquitous in the subsurface within manifold applications. The presence of fractures over several scales and complex geometry magnifies the uncertainty of the heat transfer phenomena, which will significantly impact, or even dominate, the dynamic transport process. Capturing the details of fluid and heat transport within the fractured system is beneficial to the subsurface operations. However, accurate modeling methodologies for thermal high-enthalpy multiphase flow within fractured reservoirs are quite limited. In this work, multiphase flow in fractured geothermal reservoirs is numerically investigated. A discrete-fracture model is utilized to describe the fractured system. To characterize the thermal transport process accurately and efficiently, the resolution of discretization is necessarily optimized. A synthetic fracture model is firstly selected to run on different levels of discretization with different initial thermodynamic conditions. A comprehensive analysis is conducted to compare the convergence and computational efficiency of simulations. The numerical scheme is implemented within the Delft Advanced Research Terra Simulator (DARTS), which can provide fast and robust simulation to energy applications in the subsurface. Based on the converged numerical solutions, a thermal Péclet number is defined to characterize the interplay between thermal convection and conduction, which are the two governing mechanisms in geothermal development. Different heat transfer stages are recognized on the Péclet curve in conjunction with production regimes of the synthetic fractured reservoir. A fracture network, sketched and scaled up from a digital map of a realistic outcrop, is then utilized to perform a sensitivity analysis of the key parameters influencing the heat and mass transfer. Thermal propagation and Péclet number are found to be sensitive to flow rate and thermal parameters (e.g., rock heat conductivity and heat capacity). This paper presents a numerical simulation framework for fractured geothermal reservoirs, which provides the necessary procedures for practical investigations regarding geothermal developments with uncertainties.

Geochemical Exploration of Geothermal Manifestations: Study Case of Bajawa Volcanic Complex, Flores Island, East Nusa Tenggara Province

Currently Indonesia occupy third place in the World demand of geothermal energy with using of 1375 Mwe (Mega Watt electrical). Energy utilization is only used around 4 – 5 % of geothermal energy capacity owned by Indonesia. Surely, this fact encourages many researcher to develop geothermal field with multidisciplinary sciences. One of potential geothermal areas is Nage geothermal field located in Bajawa City, Ngada District, East Nusa Tenggara Province. Four hot springs and fumarole were founded in the area. The Exploration using geochemical analysis in the form of descriptive method and analysis of geothermal fluid data. Based on regional geology, Bajawa volcanic complex composed of three periods of volcanic, first is old volcanic that formed Nage Caldera and Old Inierie volcanoes, then formed Bajawa Cinder Cone unit with North – South and North East – Southwest alignment, this alignment surely controlled as a linear eruptions. And the last is Inierie unit, this unit is active stratovolcano. The results of data obtained from one fumarole and four hot springs sample were taken from Wolo puti fumarole, Bena hotspring, Keli hotspring, Nage I hotspring and Nage II hotspring. Based on the relationship of chemical element ratio and plotting of triangular Giggenbach diagram shows that Keli and Bena hot springs are sulphate type, this hotspring controlled by Wolo puti fumarole as upflow of the geothermal system then Keli and Bena hot springs are in the outflow zone, which has mixed by meteoric water. Nage I and Nage II hot springs are chloride type, which has different geothermal system from Keli and Bena hot springs. Based on the geothermometer calculation of the Na-K-Mg diagram, the temperature of reservoir is 220° - 340° C, which is included of high enthalpy geothermal system.

Decompression boiling and natural steam cap formation in high-enthalpy geothermal systems

Many high-enthalpy geothermal systems exhibit vapor-rich boiling zones at shallow depths (<1 km), commonly known as steam caps. While the spatial extent and vapor content of steam caps often increases in response to fluid extraction and subsequent pressure decline, the geologic factors controlling steam cap formation and development are poorly understood. Numerical simulations of groundwater convection driven by a transiently cooling intrusion elucidate a natural mechanism by which steam caps can develop from initially liquid-dominated boiling zones. In the simulations, intensive decompression boiling and steam cap development occurs after the heat of a subsurface magmatic intrusion is exhausted by prolonged hydrothermal convection. A reduction in the strength of the upflow leads to a fluid pressure decrease of 2–4 MPa in boiling zones beneath an impermeable cap rock. As the vertical pressure gradient decreases, approaching vapor-static, zones of liquid-vapor counterflow (i.e., heat pipes) develop at the base of steam caps, efficiently isolating overlying vapor-rich zones from underlying liquid. The reservoir rock permeability and the cap rock thickness are main controls on the enthalpy and spatial extent of steam caps, with thicker and higher enthalpy (potentially superheated) steam caps developing in intermediate permeability reservoir rocks (∼ 10−15 m2). Due to the importance of transient changes in geothermal system structure to steam cap development, the dynamics of natural steam cap formation may not be captured in standard approaches to natural state reservoir modeling, which express the role of the heat source in terms of fixed boundary conditions.

Role of CO2 in low to medium enthalpy geothermal systems in the Central Betic Cordillera (Spain)

There is growing interest in geothermal energy, which is considered as an efficient energy solution to mitigate rising atmospheric CO2. Besides known high enthalpy geothermal systems, increasing attention is paid to low temperature geothermal systems, as they are suitable for local use. Although geothermal production seems to be an environmentally advantageous renewable energy, it might result in significant CO2 emissions. In this study, we investigate the relationship between temperature, fugacity of CO2 (fCO2), and mineral buffers in the reservoir conditions, taking the low- to medium- enthalpy thermal waters in the Central Betic Cordillera as case study. Using geochemical modeling, three main groups of waters have been identified depending on temperature, buffering mineral assemblage, and fCO2 in their reservoir. A group of waters with a reservoir temperature ranging from 70 to 90 °C and located in the intramountain sedimentary basins shows a fCO2 in depth ranging from ~6 × 10−2 and 6 × 10−1. The reservoir chemistry of this water group seems to be mainly controlled by carbonates and evaporites displaying a fCO2 variation between depth and surface (ΔfCO2) of 10−1. Another intermediate group of waters, located in an active extension zone, displays lower temperature (50–60 °C) and fCO2 in the reservoir (from 10−3 to 10−2). Finally, the third group of waters, located on the metamorphic complexes contacts, show the highest estimated temperatures (130–140 °C) and fCO2 in the reservoir (1 to 102). The two latter groups suggest increasing buffering effect of alumino-silicates, in addition to carbonates and quartz. Therefore, we evidenced a strong relationship between temperature and fCO2 in the reservoir as well as the potential mineral buffers. We discussed the potential of geothermal systems as clean energy source based on the estimation of the CO2 emissions generated by the investigated thermal systems for a practical case of household heating.

Degassing and Cycling of Mercury at Nisyros Volcano (Greece)

Nisyros Island (Greece) is an active volcano hosting a high-enthalpy geothermal system. During June 2013, an extensive survey on Hg concentrations in different matrices (fumarolic fluids, atmosphere, soils, and plants) was carried out at the Lakki Plain, an intracaldera area affected by widespread soil and fumarolic degassing. Concentrations of gaseous elemental mercury (GEM), together with H2S and CO2, were simultaneously measured in both the fumarolic emissions and the atmosphere around them. At the same time, 130 samples of top soils and 31 samples of plants (Cistus creticus and salvifolius and Erica arborea and manipuliflora) were collected for Hg analysis. Mercury concentrations in fumarolic gases ranged from 10,500 to 46,300 ng/m3, while Hg concentrations in the air ranged from high background values in the Lakki Plain caldera (10-36 ng/m3) up to 7100 ng/m3 in the fumarolic areas. Outside the caldera, the concentrations were relatively low (2-5 ng/m3). The positive correlation with both CO2 and H2S in air highlighted the importance of hydrothermal gases as carrier for GEM. On the other hand, soil Hg concentrations (0.023-13.7 μg/g) showed no significant correlations with CO2 and H2S in the soil gases, whereas it showed a positive correlation with total S content and an inverse one with the soil pH, evidencing the complexity of the processes involving Hg carried by hydrothermal gases while passing through the soil. Total Hg concentrations in plant leaves (0.010-0.112 μg/g) had no direct correlation with soil Hg, with Cistus leaves containing higher values of Hg with respect to Erica. Even though GEM concentrations in the air within the caldera are sometimes orders of magnitude above the global background, they should not be considered dangerous to human health. Values exceeding the WHO guideline value of 1000 ng/m3 are very rare (&lt;0.1%) and only found very close to the main fumarolic vents, where the access to tourists is prohibited.

Sealing capacity of clay-cap units above the Cerro Pabellón hidden geothermal system (northern Chile) derived by soil CO2 flux and temperature measurements

Cerro Pabellón is a hidden high-enthalpy geothermal system (reservoir temperature up to 250°C) located in the Pampa Apacheta (northern Chile), where the first geothermal power plant in South America is currently operating. A fumarolic field, located on the summit of Apacheta Volcano, about 3.5kmW of Pampa Apacheta, is the only evidence of the presence of a degassing magma connected to the surface. CO2 soil flux and temperatures, on a grid of ~430 points, were measured to understand the absence of any other thermal manifestations. More than 90% of the measured CO2 flux values were below background, estimated at 1.35g·m−2·day−1 whereas the associated soil temperatures (mean value=19.4°C) carried out at ~10cm depth were related, at least partially, to an endogenous, though weak, thermal anomaly. No spatial correlation was observed between soil CO2 flux and temperature measurements, suggesting a minimal role played by the volcano-tectonic structures in the circulation of geothermal fluids. Soil temperature gradients measured on a grid of 85 points in the active fumarolic field (where acid waters and magmatic gases are discharged) revealed the only strong thermal anomaly (temperatures up to ~83°C) of the area. The thick argillic clay-cap (~300m) located above the geothermal reservoir, has a pivotal role in preventing the resurgence of fluids, whereas an efficient interaction between the rising magmatic fluids and the liquid-dominated reservoir favours acidic gas scrubbing processes, thus allowing the ascent of a steam depleted in magmatic compounds. This steam interacts with the shallower, discontinuous aquifer(s) below Pampa Apacheta, with the consequent nearly complete dissolution of CO2 and H2S, and heating of the shallow groundwater. This results in low CO2 fluxes and extended slight thermal anomaly at the surface. Finally, an integrated conceptual model using our results and available geological, mineralogical, geochemical and geophysical data of the Cerro Pabellón hidden geothermal system is presented, with the aim of providing criteria for geothermal exploration of similar high-enthalpy geothermal systems in the Andes that also exhibit a lack of thermal surface features.

Three dimensional inversion of magnetotelluric (MT) resistivity data from Reykjanes high temperature field in SW Iceland

The resistivity structure of Reykjanes is presented, derived from a three dimensional (3D) model developed in 2016. The model is the final results of a 3D inversion of MT data, corrected for static shift by joint inversion with TEM soundings at the same location as the MT soundings. In the model presented here (REYSAND), a total of 136 sounding pairs are used for the inversion. The model covers Reykjanes area and Sandvík potential geothermal area. The resistivity structure of a high enthalpy system reflects the thermal alteration of the rock. The Reykjanes models reveal a conventional resistivity structure for a high enthalpy geothermal system, i.e. a low resistivity cap underlain by a high resistivity core. The low resistivity cap reaches surface in the hot spring area, Gunnuhver, and dips down in all directions forming an elongated area of 2.5 km × 3 km in strike direction down to 1 km depth bounded by the low resistivity. The resistivity of the low resistivity cap is less 3 Ωm and is underlain by a high resistivity core with resistivity of 10–30 Ωm down to 3 km depth. At greater depth, high resistivity anomalies are prominent with a zone of lower resistivity, in strike direction, under the main field. This zone coincides with an aseismic zone at depth and may indicate a fracture zone with higher temperature and/or permeability within the geothermal system. Comparison to data from the IDDP-2 drilling as well as seismic monitoring confirms that suggestion.

Emplacement of metamorphic core complexes and associated geothermal systems controlled by slab dynamics

Slab rollback results in the development of low-angle normal faults (detachments) and metamorphic core complexes (MCCs) in back-arc domains. Although the mechanical consequences of slab dynamics on lithospheric and crustal behaviors have already been studied, thermal effects have not been investigated yet. This study shows that slab rollback produces lithospheric-scale thermal perturbations intrinsically associated with emplacement of amagmatic high-enthalpy geothermal systems. Using a multi-scale numerical modeling approach, with lithospheric-scale 3-D thermo-mechanical models of subduction, and 2-D models of fluid flow at the scale of detachments, we demonstrate that subduction-induced extensional tectonics controls the genesis and distribution of crustal-scale thermal domes from the base of the crust, and the location of high-energy geothermal systems. We find that when slab tearing occurs, Moho temperatures can temporarily increase by up to 250 °C due to significant shear heating in the flowing upper mantle. Associated thermal anomalies (with characteristic width and spacing of tens and hundreds of km, for crustal and lithospheric scales, respectively) then migrate systematically toward the retreating trench. These thermal domes weaken the crust, localize deformation and enhance the development of crustal-scale detachments. These thermo-mechanical instabilities mimic genesis of high-temperature MCCs with migmatitic cores in the back-arc domain, such as those of the Menderes (western Anatolia, Turkey) and Larderello (southern Tuscany) provinces in the Mediterranean realm, and those in the Basin and Range (western United States), where detachments control the bulk of the heat transport. At the scale of MCCs, the bulk fluid flow pattern is controlled by topography-driven flow while buoyancy-driven flow dominates within the permeable detachments, focusing reservoir location of high-energy geothermal systems at shallow depth beneath the detachments.

Hydrothermal fluid evolution in the ‘Botro ai Marmi’ quartz-monzonitic intrusion, Campiglia Marittima, Tuscany, Italy. Evidence from a fluid-inclusion investigation

ABSTRACTThe quartz-monzonitic intrusion of ‘Botro ai Marmi’ in Tuscany, Italy, can be considered to be a typical example of an intrusion-centred magmatic hydrothermal system. The evolution of hydrothermal fluids in the ‘Botro ai Marmi’ intrusion was investigated using fluid-inclusion analyses to provide suitable physico-chemical constraints on the fluids involved in the late- to post-magmatic hydrothermal activity that affected the intrusion, providing inferences on their origin and variations of temperature and pressure with time.This work demonstrates that the earliest fluids circulating in the ‘Botro ai Marmi’ intrusion were high-temperature brines exsolved directly from the crystallizing magma. This fluid circulated in the intrusion under lithostatic conditions (P &gt; 90 MPa, T &gt; 540°C). A second evolutionary stage of the magmatic hydrothermal system is marked by the transition from lithostatic (&gt;90 MPa) to hydrostatic dominated conditions (50 to 10 MPa). In this stage the fluids are also interpreted to be mainly orthomagmatic in origin but unmixed in a high-salinity brine and in a low-salinity vapour aqueous phase, at a temperature ranging from ~500 to 300°C. These fluids were responsible for the potassic alteration facies. At a later stage of hydrothermal evolution, abundant meteoric dominated fluids entered the system and are associated with propylitic alteration. This event marks the transition from a magmatic-hydrothermal system to a typical hydrothermal (‘geothermal’) system, which can be assumed to be similar to some extent to the nearby active high-enthalpy geothermal system of Larderello. Low-temperature and low-salinity meteoric water-dominated fluids characterize the latest stage of the ‘Botro ai Marmi’ hydrothermal system.

Application and reliability of calcic amphibole thermobarometry as inferred from calc-alkaline products of active geothermal areas in the Andes

Igneous thermobarometry may contribute to locate the upper-crust heat source/s representing one of the main factors required to establish the potential of high enthalpy geothermal areas through models of the heat flux. To test this possibility, we applied amphibole thermobarometry to some of the youngest lavas and pyroclastics (andesite to rhyolite) of Apacheta and La Torta (Chile) and Chachimbiro (Ecuador) areas representing medium to high enthalpy geothermal systems located in two different crustal contexts along the Andean active continental margins of South America. We used the robust application of Ridolfi and Renzulli (2012) allowing to estimate the intrinsic physico-chemical parameters with reasonably low uncertainties (T ± 24 °C, P ± 12%, fO2 ± 0.4 log units) and the composition of the melt in equilibrium with Mg-rich calcic amphibole in a wide range of conditions, up to 1130 °C and 2.2 GPa. In addition, analyses of the intra-crystalline compositional and textural variations allowed us to propose a method to distinguish between chemically homogeneous and heterogeneous amphiboles, avoiding some incorrect applications of the Ridolfi and Renzulli (2012)'s equations and thus unreliable results.Thermobarometric constraints indicate a polybaric crystallization of magmas below the Chilean geothermal areas of Apacheta and La Torta. In the last 150 ka, the feeding systems of these areas mainly consisted of shallow silicic magma chambers at depths of 4–8 km (740–840 °C), giving rise to dacite domes (tortas). Amphiboles in the rhyolite lava flows, erupted earlier (Pliocene-Pleistocene) during the activity of the nearby Apacheta-Aguilucho Volcanic Complex, show a similar T-depth range (780–810 °C, 4–6 km). At deeper levels (10–20 km), the depths estimated using amphiboles from the magmatic enclaves (in the dacite domes) and from the andesite lavas are approximately aligned along the upper morphologic margins of the well geophysically constrained Altiplano-Puna Magma Body (APMB; a partially melted zone in the upper crust where the S-waves velocities are <2.9 km/s), with estimated temperatures of 850–1010 °C. Heat source contributions for the investigated Chilean geothermal areas should therefore be represented by both the shallower magma bodies and the huge APMB.The Holocene products of the Ecuadorian Chachimbiro geothermal area indicate amphibole crystallization from 7 to 17 km depth at temperature conditions in the range of 800–910 °C. The occurrence of two separated regions of magma storage at approximately 230 ± 30 MPa and 380 ± 60 MPa is discussed. They should be taken into account as heat sources for this Ecuadorian geothermal area.

Resistivity imaging of Aluto-Langano geothermal field using 3-D magnetotelluric inversion

Magnetotelluric (MT) method is a widely used geophysical method in geothermal exploration. It is used to image subsurface resistivity structures from shallow depths up to several kilometers of depth. Resistivity imaging using MT method in high-enthalpy geothermal systems is an effective tool to identify conductive clay layers that cover the geothermal systems and to detect a potential reservoir. A resistivity model is vital for deciding the location of pilot and production sites at the early stages of a geothermal project. In this study, a 3-D resistivity model of Aluto-Langano geothermal field was constructed to map structures related to a geothermal resource. The inversion program, ModEM was used to recover the 3-D resistivity model of the study area. The 3-D inversion result revealed the three main resistivity structures: a high-resistivity surface layer related to unaltered volcanic rocks at shallow depth, underlain by a conductive zone associated with the presence of conductive clay minerals, predominantly smectite. Beneath the conductive layer, the resistivity increases gradually to higher values related to the formation of high-temperature alteration minerals such as chlorite and epidote. The resistivity model recovered from 3-D inversion in Aluto-Langano corresponds very well to the conceptual model for high-enthalpy volcanic geothermal systems. The conductive clay cap is overlying the resistive propylitic upflow zone as confirmed by the geothermal wells in the area.

NATURAL DEGASSING OF CARBON DIOXIDE AND HYDROGEN SULPHIDE AND ITS ENVIRONMENTAL IMPACT AT MILOS ISLAND, GREECE

The Aegean region represents an active convergent zone, where continental micro-plates exhibit a complex interaction between the African and the Eurasian plates. The calc-alkaline volcanic activity of the Southern Aegean region developed in various volcanic centers from Soussaki to Nisyros through Methana-Poros, Milos and Santorini. Milos Island has been an active volcano till the middle of Quaternary and is at present characterized by a high enthalpy geothermal system. The volcanism started 3.5 Ma ago and still continues up today in the form of post-volcanic manifestations. Most quiescent volcanoes released large amounts of CO2 and H2S through fumarolic activity and soil diffuse degassing. Numerous small fumaroles occur in various places, mainly at Kalamos and Adamas volcanic areas. Also along the southern coast of the island there are volcanic gas manifestations in the sea. Gases were sampled from fumaroles at Kalamos area as well as from north east part of Adamas village. Furthermore many soil gases were sampled at 50 cm depth and analyzed for their chemical composition. Apart from atmospheric gases (N2 and O2), which sometimes contaminate the samples, the main gas phase is CO2. Sometimes also H2S, CH4 and H2 are present in high amounts while CO and He are always present in trace amounts. The He isotopic composition highlights a significant mantle component. CO2 and H2S concentrations higher than in the normal atmosphere can be stimulating for plant growth until certain levels and detrimental above them. As for many active geothermal areas of the world also H2S and CO2 concentrations measured in the area of Milos could be of concern for human health.

Ngatamariki Geothermal System: Magmatic to Epithermal Transition in the Taupo Volcanic Zone, New Zealand

The Ngatamariki geothermal system is one of more than 20 high enthalpy (>225°C) geothermal systems in the Taupo Volcanic Zone, North Island, New Zealand. At shallow levels (<2 km), they are analogous to low-intermediate sulfidation state epithermal ore-forming environments. Unique to Ngatamariki is the occurrence of an intrusive complex with an associated magmatic-hydrothermal alteration halo that was intersected by deep geothermal drilling (~3 km depth) and that resembles hydrothermal alteration associated with high-intermediate sulfidation state epithermal mineralization. This paper describes the results of a study involving alteration petrography, X-ray diffraction, shortwave infrared (SWIR) reflectance spectroscopy, backscattered electron scanning electron microscopy/energy dispersive spectroscopy (BSE-SEM/EDS), plus whole-rock and trace element geochemistry to document and characterize distinct hydrothermal alteration found nowhere else in the Taupo Volcanic Zone. Two separate phases of hydrothermal activity are distinguished, old and modern, as defined by a paleosurface that is dated at 0.68 Ma, which occurs ~500 m below sea level. A composite plutonic body comprising intrusions of diorite to tonalite was encountered in three adjacent drill holes (NM4, NM8, and NM9) between 2,000 and 3,200 m below sea level (~2,300–3,500 m depth below the surface) in the northern part of the Ngatamariki system. The associated hydrothermal alteration is zoned and made up of potassic, advanced argillic, phyllic, and propylitic mineral assemblages that occur between 500 and 2,500 m below sea level. Subtle potassic alteration consisting of biotite + magnetite ± K-feldspar mantles the intrusive complex. It is crosscut by a hypogene advanced argillic alteration containing pyrophyllite ± minor andalusite ± topaz ± anhydrite ± rare aluminophosphates (AP) and fluorine-bearing minerals, but lacks alunite. The deep-formed advanced argillic alteration appears in some samples to be overprinted by phyllic alteration, made up of quartz + muscovite + pyrite. Between 500 and 1,000 m below sea level, the intense phyllic alteration is less pervasive, and the hydrothermal alteration is dominated by kaolinite, rare dickite, and localized occurrences of highly silicified rocks that resemble vuggy quartz, which is bounded at the top by the paleosurface (defined as the unconformity at the base of the overlying Whakamaru group ignimbrite). In the central and southern part of the system below the paleosurface, propylitic hydrothermal alteration consisting of chlorite + calcite + epidote ± wairakite ± actinolite (along with ± albite and ± illite) is widespread, and could equally have formed during old hydrothermal activity associated with emplacement of the intrusive complex or in the deep hot parts of the modern hydrothermal system. The mineralogy and geochemistry of advanced argillic altered rock indicates that acidic magmatic-hydrothermal fluids leached base cations, resulting in the loss of elements typically considered immobile (Al, Ti, Y, Zr, Nb, as well as rare earth elements) to form cation-depleted minerals such as pyrophyllite, andalusite, and topaz. The strongest enrichments in Au (0.6 g/t), Ag (4.6 g/t), Te, As, Sb, and Bi coincide with intense acid alteration at <250-m depth beneath the paleosurface. The results of this study reveal a complex history of intrusion and hydrothermal activity that provides a modern example of successive development (~600,000 years apart) of acid and neutral pH hydrothermal alteration assemblages, which are associated with the two end member types of epithermal mineralization.

Blowout mechanism of Alasehir (Turkey) geothermal field and its effects on groundwater chemistry

Anatolia region is one of the most seismically active regions in the world and has a considerably high level of geothermal energy potential. Some of these geothermal resources have been used for power generation and direct heating. Most of the high enthalpy geothermal systems are located in western part of Turkey. Alasehir is the most important geothermal site in western part of Turkey. Many geothermal wells have been drilled in Alasehir Plain to produce the geothermal fluid from the deep reservoir in the last 10 years. A blowout accident happened during a geothermal well drilling operation in Alasehir Plain, and significant amount of geothermal fluid surfaced out along the fault zone in three locations. When drilling string entered the reservoir rock about 1000 m, blowout occurred. As the well head preventer system was closed because of the blowout, high-pressure fluid surfaced out along the fault zone cutting the Neogene formation. In order to understand the geothermal fluid effects on groundwater chemistry, physical and chemical compositions of local cold groundwater were monitored from May 2012 to September 2014 in the study area. The geothermal fluid was found to be of Na–HCO3 water type, and especially, arsenic and boron concentrations reached levels as high as 3 and 127 mg/L, respectively. The concentrations of arsenic and boron in the geothermal fluid and groundwater exceeded the maximum allowable limits given in the national and international standards for drinking water quality. According to temporally monitored results, geothermal fluid has extremely high mineral content which influenced the quality of groundwater resources of the area where water resource is commonly used for agricultural irrigation.