A helium isotope perspective on the Dixie Valley,Nevada, hydrothermal system

Fluids from springs, fumaroles, and wells throughout Dixie Valley, NV were analyzed for noble gas abundances and isotopic compositions. The helium isotopic compositions of fluids produced from the Dixie Valley geothermal field range from 0.70 to 0.76 Ra, are among the highest values in the valley, and indicate that ∼7.5% of the total helium is derived from the mantle. A lack of recent volcanics or other potential sources requires flow of mantle-derived helium up along the valley bounding Stillwater Range Front Fault, from which the geothermal fluids are produced. Using a one-dimensional flow model, a lower limit fluid flow rate up through the fault of 7 mm/yr is estimated, corresponding to a mantle ³He flux of ∼10⁴ atoms m⁻² s⁻¹.A comparison between the fluids from Dixie Valley springs, fumaroles, and wells and the fluids produced from the geothermal field reveals a mixing trend between the geothermal fluid and younger, cooler groundwaters. The exceptions are those features that either emanate directly from the Stillwater fault or wells that penetrate and extract fluids from the fault zone, all of which have helium isotopic compositions that are indistinguishable from the geothermal production fluids. The results of our study indicate that the Stillwater Range Front Fault system must act as a permeable conduit that can sustain high vertical fluid flow rates from deep within the crust and crust-mantle boundary and that high permeability may exist along most of its length. This suggests that the geothermal potential of the Stillwater fault may be significantly greater than the 6–8 km long system presently under production. Since all the numerous springs, wells, and fumaroles in the valley also contain a fluid component that is indistinguishable from the geothermal/Stillwater fault fluid, the potential for an additional deeper and more pervasive geothermal system also exists and should be further evaluated. Furthermore, we suggest that elevated helium isotope compositions in regions with little or no recent magmatism are an indicator of the deep crustal permeability that is required to drive and sustain extensional geothermal systems.     

Simulation of heat extraction from crystalline rocks: The influence of coupled processes on differential reservoir cooling

Processes operating during the extraction of heat from fractured rocks influence dynamically their fluid flow and heat transport characteristics. The incorporation of pressure- and temperature-dependent rock parameters, coupled with geomechanical deformation, is particularly important for predictive modelling of geothermal reservoirs hosted in crystalline rock masses. Changes in flow and transport parameters of fractures caused by variations in local effective stress are computed using an experimentally validated geomechanical model [McDermott, C.I., Kolditz, O., 2006. Geomechanical model for fracture deformation under hydraulic, mechanical and thermal loads. Hydrogeol. J. 14, 487–498]. Local effective stress changes are linked to alterations in reservoir fluid pressures, and to in situ stress conditions, including the build-up of thermal stresses resulting from the cooling of the rock mass. These processes are simulated using a finite-element model in order to study the behaviour of the Spa Urach (southwestern Germany) potential geothermal reservoir. The model couples mechanical deformation and alteration of fracture parameters with pressure-, temperature- and salinity-dependent fluid parameter functions. The effects of potential reservoir damage on reservoir productivity are investigated to help identify optimal heat recovery schemes for the long-term economical exploitation of geothermal systems. Simulation results indicate that preferential fluid flow paths and shortcuts may develop, depending on the mechanical and thermal stress releases that occur during intense exploitation of these systems.     

Differential attenuation of seismic waves at a dense array: A marker of the travale field from sources at regional distance

Dense arrays of three-component seismographs have been maintained in the Travale geothermal field during two periods of several weeks. The particular array configuration (20 stations on a 3 × 3 km area) was intended to allow a high horizontal resolution investigation of the field itself, by analysis of steep-incidence waves. Natural and artificial sources at 100 km distances provided phases reflected by deep crustal interfaces, from which differential attenuation factor estimates are obtained. Consequent signatures of the area around and within the productive field are discussed, with respect also to laboratory measurements on porous, fluid-filled rock samples.     

Isotope systematics of C-bearing gas compounds in the geothermal fluids of Larderello, Italy

The origin of carbon-bearing compounds (CO₂, CH₄, C₂–C₄ saturated hydrocarbons) and helium in the geothermal fluid of Larderello is investigated by means of the variations in concentration and isotopic composition. The CO₂ (δ¹³C from −1.4 to −7.1‰ versus V-PDB) is mainly of crustal origin. The carbon isotopes of methane (δ¹³C from −20.9 to −31.7‰) and other hydrocarbons indicate a complex thermogenic origin. The temperatures obtained with the CH₄–CO₂ isotope geothermometer are in rough agreement with those observed in deeper geothermal wells. The CH₄/C₂H₆ ratios show a tendency towards partial equilibrium with increasing temperature. He isotopes (R/R_A from 0.5 to 3) indicate that although the major part of helium derives from crustal sources, a significant fraction of mantle helium is also present. Helium contamination by air, deducted from He/Ne ratios, is generally negligible.     

Modeling heat extraction from hot dry rock in a multi-well system

A mathematical model is developed for describing the heat energy extracted from a hot dry rock in a multi-well system. The solutions for the water temperature, accounting for a geothermal gradient in a geothermal reservoir, are given in the Laplace domain and computed by numerical inversion, the modified Crump method. The results show that the heat extraction effectiveness is affected significantly by the well spacing, well radius, reservoir thickness, and pumped flow rate in a multi-well system. The water temperature decreases with increasing pumping rate and increases with the well spacing, well radius, and reservoir thickness. The geothermal gradient affects only the early time heat extraction effectiveness significantly and has direct impact on the water temperature all the time if the vertical thickness of geothermal reservoir is large. The present solution is useful for designing and simulating the heat extraction project of geothermal energy exploitation in a multi-well system.     

Positive heat flow anomaly in the Carpathian basin

Intensive geothermal investigations in the central Hungarian Tertiary sedimentary basin show uniform high temperature gradients between 45 and 70°C/km. New temperature measurements between 3000–5800 m depth confirm previous values between 400–2000 meters. Sixteen heat flow measurements showed values between 2.0 and 3.3 μcal/cm² s (84 and 138 mW/m² respectively). Sporadic measurements outside the Carpathian basin have shown invariably average or low heat flow and low temperature gradients.The investigation of the crustal structure in Hungary along 5 profiles indicates the depth of the Moho as being between 24.5 and 30.4 km. Comparing the isobaths of the Moho with the temperature gradient map there is an evident relation between high temperature gradients, the consequent high heat flow and the elevated position of the Moho. Areas with high heat flow are found where the Moho is 24.5–26 km deep. The Carpathian basin can be compared with the Black Sea depression where the Moho is 20 km deep.An interesting geothermal similarity exists between the Carpathian basin and the marginal basins of the western Pacific. The Okhotsk, Japan, Shikoku, Parace-Vela basins have a high mean heat flow above 2 μcal/cm² s. In the southwestern Pacific, the Fiji plateau and the Lau basin are also characterized by high heat flow. The Japan and Okhotsk seas may represent a subsidence similar to the Carpathian basin, caused by the uplift of the surrounding mountain ranges e.g., Kurili island arc.     

Parabolic troughs to increase the geothermal wells flow enthalpy

This work investigates the feasibility of using parabolic trough solar field to increase the enthalpy from geothermal wells’ flow in order to increase the steam tons; in addition, it is possible to prevent silica deposition in the geothermal process. The high levels of irradiance in Northwestern Mexico make it possible to integrate a solar-geothermal hybrid system that uses two energy resources to provide steam for the geothermal cycle, like the Cerro Prieto geothermal field. The plant consists of a geothermal well, a parabolic trough solar field in series, flash separator, steam turbine and condenser. Well “408” of Cerro Prieto IV has enthalpy of 1566 kJ/kg and its quality must be increased by 10 points, which requires a Δh of 194.4 kJ/kg. Under these considerations the parabolic troughs area required will be 9250 m², with a flow of 92.4 tons per hour (25.67 kg/s). The solar field orientation is a N–S parabolic trough concentrator. The silica content in the Cerro Prieto geothermal brine causes problems for scaling at the power facility, so scale controls must be considered.     

Comparative analysis of natural and conventional working fluids for use in transcritical Rankine cycle using low‐temperature geothermal source

Theoretical analyses of natural and conventional working fluids‐based transcritical Rankine power cycles driven by low‐temperature geothermal sources have been carried out with the methodology of pinch point analysis using computer models. The regenerator has been introduced and analyzed with a modified methodology considering the considerable variation of specific heat with temperature near the critical state. The evaluations of transcritical Rankine cycles have been performed based on equal thermodynamic mean heat rejection temperature and optimized gas heater pressures at various geothermal source temperature levels ranging from 80 to 120°C. The performances of CO₂, a natural working fluid most commonly used in a transcritical power cycle, have been indicated as baselines. The results obtained show: optimum thermodynamic mean heat injection temperatures of transcritical Rankine cycles are distributed in the range of 60 to 70% of given geothermal source temperature level; optimum gas heater pressures of working fluids considered are lower than baselines; thermal efficiencies and expansion ratios (Exp_r) are higher than baselines while net power output, volume flow rate at turbine inlet (V₁) and heat transfer capacity curves are distributed at both sides of baselines. From thermodynamic and techno‐economic point of view, R125 presents the best performances. It shows 10% higher net power output, 3% lower V₁, 1.0 time higher Exp_r, and 22% reduction of total heat transfer areas compared with baselines given geothermal source temperature of 90°C. With the geothermal source temperature above 100°C, R32 and R143a also show better performances. R170 shows nearly the same performances with baselines except for the higher V₁ value. It also shows that better temperature gliding match between fluids in the gas heater can lead to more net power output. Copyright © 2010 John Wiley & Sons, Ltd.     

Research and development of geothermal energy production in Hungary

The basement of the Pannonian (Carpathian) basin is represented by Paleozoic metamorphic and Mesozoic dolomite and limestone formations. The Tertiary basin gradually subsided during the Alpine orogeny down to 6000 m and was filled by elastic sediments with several water horizons.A heat flow of 2.0 to 3.4 μcal/cm²s gives temperature gradients between 45 and 70 °C/km in the basin. At 2000 m depth the virgin rock temperature is between 110 and 150°C. 80 geothermal wells about 2000 m deep have shown the great geothermal potential of the basin.The main hot water reservoir is the Upper Pliocene (Pannonian) sandstone formation. Hot water is produced by wells from the blanket or sheet sand and sandstone, intercalated frequently by siltstone. Between a 100–300 m interval, 3 to 8 permeable layers are exploited resulting in 1–3 m³/min hot water at 80–99°C temperature.Wells at present are overflowing with shut-in pressures of 3–5 atm.The Pannonian basin is a conduction-dominated reservoir. Convection systems are negligible, hot igneous systems do not exist. The assessment of geothermal resources revealed that the content of the water-bearing rocks down to 3000 m amounts to 12,600 × 10¹⁸cal. In the Tertiary sediments 10,560 × 10¹⁸cal and in the Upper Pannonian, 1938 × 10¹⁸cal are stored. In the Upper Pannonian geothermal reservoir, below 1000 m, where the virgin rock temperature is between 70 and 140°C, the stored heat is 768 × 10⁸cal. A 10¹⁸ cal is equivalent to the combustion heat of 100 million tons of oil. The amount of recoverable geothermal energy from 768 × 10⁸cal is 7.42 × 10¹⁸cal, i.e. about 10,000 MW century, not considering reinjection.At present the Pannonian geothermal reservoir stores the greatest amount of identified heat which can be mobilized and used. Hungary has 496 geothermal wells with a nominal capacity of 428 m³/min, producing 1342 MW heat. 147 wells have an outflow temperature of more than 60°C producing 190 m³/min, that is, 845 MW. In 1974 290 MWyear of geothermal energy was utilized in agriculture, district heating and industry.     

Cold groundwater temperatures and conductive heat flow in the Mt. Amiata geothermal area, Tuscany, Italy

The hydrogeochemical study of the cold springs present in the Mt. Amiata geothermal area, where the Bagnore and Piancastagnaio geothermal fields are situated, has defined the different shallow groundwater systems.The cold groundwater temperature of the volcanic phreatic aquifer is largely correlated with the geothermal heat flow. Through the analysis of the temperature of cold groundwaters, a possible method for geothermal prospection has been developed, supported by the results obtained in the studied area. Through the enthalpic balance of the aquifer and in agreement with available data, a geothermal flow of about 200 mW m^-2 has been calculated.     

Heat flow and deep temperatures in the Chaves geothermal system,Northern Portugal

The geological, geoelectrical, geochemical and temperature data related to the Chaves geothermal system have been integrated to obtain a better understanding of the Chaves basin. Geoelectrical surveys carried out in the basin reveal a low-resistivity zone (10 ohm m), associated with a shallow geothermal reservoir, in the central part of the graben, bounded by higher-resistivity rocks. The top of this zone varies between 400 and 200 m and its maximum thickness (1600 m) is located at the centre of the basin. Thermal models for the Chaves basin and for the region are presented using the structure obtained by geoelectrical methods and a mean heat flow value of 95 mW m^-2 derived from borehole measurements. The heat transfer takes place mainly by conduction, except near the faults, where convective flow is important. The medium is considered dishomogeneous and there is a great thermal conductivity contrast between the sediments in the basin and the surrounding rocks. The results obtained for the Chaves basin show that the mean temperature value in the shallow geothermal reservoir is 62°C. The maximum temperature value predicted to the bottom of this reservoir is 95°C. A regional forced convective-circulation model is suggested based on geomorphological, geochemical, isotopic data and to rmal models.     

Flashing point compressibility of geothermal fluids with low CO2 content and its use in estimating reservoir volume

In this study, the bubble point pressure effect or, as it is more commonly known, the flashing point effect of CO₂ on geothermal fluids is shown, and the compressibility of the geothermal fluids containing low concentrations of dissolved CO₂ at the flashing point is formulated for isoenthalpic phase change. The compressibility, termed the isoenthalpic flashing point compressibility, can be calculated with well-known parameters. New, easy-to-use graphs are presented to estimate the compressibility for such systems. Correction for ionic strength of the geothermal brine is also considered. The practical use of the compressibility is illustrated to estimate the fluid content of a geothermal reservoir. A material balance method incorporating the isoenthalpic flashing point compressibility in geothermal modeling is investigated. The material balance method presented in the paper is of primary importance in the evaluation of geothermal reservoirs since both the areal extent and the vertical thickness can not be accurately determined, particularly in the initial development stages of the reservoirs. Applications of the method to geothermal fields are discussed.     

Geothermal fluid chemistry and human health

Several effects on the environment arising from the utilization of geothermal resources may affect human health. (1) Human surroundings: changes in noise level, local climate, and landscape. (2) Water pollution: hot water geothermal fields emit large quantities of saline fluids which contain fluoride, boron, arsenic, and minor amounts of other heavy metals. Heavy metal sulphide precipitates form from some geothermal waters and collect in river sediments. Processing of geothermal waters to remove heavy metals may be necessary. Sulphide, boron, ammonia, and mercury may also enter local waters from geothermal steam condensates. (3) Air pollution: emission of CO₂ and H₂S from geothermal power stations in some extreme cases may approach the outputs from similar-sized coal-fired stations. Sulphide emission can be controlled by chemical processing of condensate and waste gases. Mercury from geothermal steam is unlikely to cause local levels in air to rise by more than 1 ppb.The significant environmental problems from geothermal development are capable of technical solution but add significantly to the cost of geothermal power.     

Geothermal development in Nicaragua

In order to meet national energy requirements, Nicaragua has had to direct its attention towards sources of “alternative energy”, such as geothermal. Excellent geothermal prospects exist in this country, for which reason the Revolutionary Government has deemed it convenient to direct its energy policy towards this alternative source. Studies carried out during past years have led to the selection of nine areas in western Nicaragua, four of which were earmarked Very High Priority because they contain high enthalpy geothermal fields, three areas were earmarked High Priority and only two Low Priority. The positive results obtained in the Momotombo geothermal project have become an incentive to continue research and development nation-wide.At present, a power plant is operating with 35 MW at the Momotombo geothermal field; another 35 MW power plant is under construction in the same field; the El Hoyo-Monte Galan project is in the pre-feasibility phase and surveys are under way throughout national territory.     

Thermodynamic analysis and performance optimization of organic rankine cycles for the conversion of low‐to‐moderate grade geothermal heat

The present study considers a thermodynamic analysis and performance optimization of geothermal power cycles. The proposed binary‐cycles operate with moderately low temperature and liquid‐dominated geothermal resources in the range of 110°C to 160°C, and cooling air at ambient conditions of 25°C and 101.3 kPa reference temperature and atmospheric pressure, respectively. A thermodynamic optimization process and an irreversibility analysis were performed to maximize the power output while minimizing the overall exergy destruction and improving the First‐law and Second‐law efficiencies of the cycle. Maximum net power output was observed to increase exponentially with the geothermal resource temperature to yield 16–49 kW per unit mass flow rate of the geothermal fluid for the non‐regenerative organic Rankine cycles (ORCs), as compared with 8–34 kW for the regenerative cycles. The cycle First‐law efficiency was determined in the range of 8–15% for the investigated geothermal binary power cycles. Maximum Second‐law efficiency of approximately 56% was achieved by the ORC with an internal heat exchanger. In addition, a performance analysis of selected pure organic fluids such as R123, R152a, isobutane and n‐pentane, with boiling points in the range of ?24°C to 36°C, was conducted under saturation temperature and subcritical pressure operating conditions of the turbine. Organic fluids with higher boiling point temperature, such as n‐pentane, were recommended for non‐regenerative cycles. The regenerative ORCs, however, require organic fluids with lower vapour specific heat capacity (i.e. isobutane) for an optimal operation of the binary‐cycle. Copyright © 2015 John Wiley & Sons, Ltd.     

Numerical modeling of Basin and Range geothermal systems

Basic qualitative relationships for extensional geothermal systems that include structure, heat input, and permeability distribution have been established using numerical models. Extensional geothermal systems, as described in this paper, rely on deep circulation of groundwater rather than on cooling igneous bodies for heat, and rely on extensional fracture systems to provide permeable upflow paths. A series of steady-state, two-dimensional simulation models is used to evaluate the effect of permeability and structural variations on an idealized, generic Basin and Range geothermal system of the western U.S.Extensional geothermal systems can only exist in a relatively narrow range of basement (bulk) permeability (10⁻¹⁵ m² to 10⁻¹⁶ m²). Outside of this window, shallow subsurface fault zone temperatures decrease rapidly. Mineral self-sealing does not significantly affect the flow system until the flow path is almost completely sealed off. While topography gives an extra “kick” to convective circulation, it is not a requirement for geothermal system development. Flow from the ranges to the fault dominates the circulation, while secondary flow systems exist on the range front slopes. A permeable fault in one valley can also induce cross-range flow if there are no equally good upflow paths in the adjacent valleys. When bulk permeability is high enough, additional deep circulation cells develop in adjacent valleys, diverting heat and fluid from the fault and consequently reducing temperatures in the fault itself. Qualitative comparison between temperature–depth logs from actual geothermal systems and from the generic models is a significant aid to understanding real-world geothermal fluid flow, and suggests new or better interpretations of existing systems.     

Utilization of existing deep geological wells for acquisitions of geothermal energy

The aim of this work is to assess the possibility and usefulness of accessing geothermal energy from the existing production well, Jachowka K-2. Discussions of both, a heat flow transferred between a deposit and a heat carrier and a heat flow permeated through the barrier are presented. A computational model, was designed to determine the volume of a gained geothermal heat flux with the use of a double-pipe geothermal heat exchanger with the dead centre [12]. Lastly, the article there are the results of calculations of available heat flux in the investigated well at the depth of L=3950 m are analyzed.     

A geothermal field model based on geophysical and thermal prospectings in Nea Kessani (NE Greece)

The present study completes a study by Thanassoulas et al. (1986) Geophys. Prosp.34, 83–97 and deals with geophysical exploration for geothermal resources in Nea Kessani area, NE Greece. The results of some deep electrical soundings (AB = 6000 m) with the interpretation of a gravity profile crossing the investigated area are considered together with thermal investigations. All subsequent information, along with the conclusions of an earlier paper dealing with a reconnaissance geophysical survey of the same area, are used to highlight a subsurface geothermal field model.     

A computational method for determining segmental and overall geothermal gradients and geothermal heat flow values

A new computational method is presented which calculates geothermal heat flow values and geothermal gradients with more precision than permitted by previously published techniques. The data required are: geothermal temperature at a known depth, mean surface temperature, the rock types in the stratigraphic column and the thermal resistivity values for the different types of rocks. This method is valuable in areas that have no measured gradient values. Basic equation used was the Fourier heat transfer equation where is heat flux in μcal/(cm² s), ρ_i is thermal resistivity (°C s cm/μcal) and ∂T/∂x is the x component of the temperature gradient (°C/cm). The thermal resistivity was allowed to vary linearly with temperature ρ_i = ρ_i^o [1 + K_i (T − 30)] where ρ_i is thermal resistivity of the lithographic segment «iå at a temperature T, ρ_i^o is thermal resistivity at 30°C and K_i is the temperature coefficient of thermal resistivity. The procedure consisted of integrating the combined equation for heat flux in terms of temperature dependent resistivity.Two iterative solutions were used to simplify the calculations: exact and approximate. The heat flux for each well was assumed to be 1.0 HFU and segmental temperatures were calculated from the bottom (arbitrarily) up, until a surface temperature was obtained. The calculated surface temperature could then be compared with the mean surface temperature (MST). Correction in the heat flux value was made until the calculated surface temperature and MST agreed. An analysis of three deep Appalachian test wells was made and the results showed the critical importance of lithographic ordering and the temperature dependence of thermal resistivity upon calculated geothermal quantities.     

Geochemistry of thermal waters on the island of Ischia (Campania, Italy)

The stratigraphic and structural situation on the island of Ischia (southern Italy), the recent volcanic activity and the presence of hot springs and fumaroles, suggest the existence of a geothermal field. The chemical and isotopic compositions of the waters from several springs and wells were examined to obtain information on deep temperatures and to formulate a geothermal model of the island. δD values range from −33.60 to −12.50‰ and δ¹⁸O from −7.10 to −1.71‰, relative to SMOW. These variations have mainly been attributed to the presence of seawater, as confirmed by the general shift to more positive values with the increase of Cl content. Water-rock reactions, evaporation and subsurface boiling also contribute to the δ¹⁸O−δD trend. The chemical analyses reveal the presence of alkaline sulphate chloride water (seawater), bicarbonate waters and waters interpreted as the result of mixing. The chemical and isotopic composition of the latter are dependent on water-rock interactions, water circulation rates and eventual evaporation and condensation phenomena. The silica geothermometer, which seems to be the most suitable for determining the deep temperatures of these waters, gave values of about 200°C, even for mixing models. Our data suggest the following geothermal model: the heat flow heats up a deep reservoir, causing steam to rise through faults and fractures and transfer heat to a shallower aquifer. The temperatures of 200°C obtained by the geothermometers are not the maximum reservoir temperatures, but are probably water-rock equilibrium temperatures for the shallower aquifers. The high boron contents and the isotopic data confirm the presence of steam in the system.