Long-term changes associated with exploitation of the Beppu hydrothermal system, Japan

Exploitation of the Beppu hydrothermal system started mainly in the lowland areas as early as the 1880s, and by the 1920s the number of wells had increased to about 1000. This caused the piezometric head of the thermal groundwater to draw down, and seawater to intrude into the thermal groundwater aquifer near the coast. A second flurry of exploitation occurred during the 1960s–70s, by which time there were over 2300 wells, and the mass and heat flows had increased due mainly to the discharge of high-temperature chloride waters in the highland areas. This caused a decline in piezometric head of the deep chloride water, a decline in the subsurface flow of chloride water towards the lowlands, and intrusion of steam-heated shallow water into the chloride water layer.     

An updated numerical model of the Larderello–Travale geothermal system, Italy

Larderello–Travale is one of the few geothermal systems in the world that is characterized by a reservoir pressure much lower than hydrostatic. This is a consequence of its natural evolution from an initial liquid-dominated to the current steam-dominated system. Beneath a nearly impermeable cover, the geothermal reservoir consists of carbonate-anhydrite formations and, at greater depth, by metamorphic rocks. The shallow reservoir has temperatures in the range of 220–250 °C, and pressures of about 20 bar at a depth of 1000 m, while the deep metamorphic reservoir has temperatures of 300–350 °C, and pressures of about 70 bar at a depth of 3000 m. The 3D numerical code “TOUGH2” has been used to conduct a regional modeling study to investigate the production mechanism of superheated steam, the interactions between the geothermal field and the surrounding deep aquifers, and the field sustainability. All the available geoscientific data collected in about one century of exploration and exploitation have been used to provide the necessary input parameters for the model, which covers an area (4900 km²) about 10 times wider than the Larderello–Travale geothermal field (400 km²). The numerical model explains the origin of the steam extracted in about one century of exploitation and shows that, at the current level, the production is sustainable at least for the next 100 years.     

A Simulation Study of CO2 Sequestration in the Arno River Plain (Tuscany,Italy)

We present a geochemical model for the sequestration of anthropogenic CO₂ (from hypothetical hydrocarbon-based H₂ production) consequent to the theoretical injection of gas beneath the River Arno plain, near the city of Pisa, Italy. The potential reservoirs for gas disposal are of two different types: dolostones or sandstones. The model predicts the sandstones to be the best reservoirs because CO₂ sequestration in these rocks can occur via the deposition of solid carbonates. On the contrary, the injection of gas into the dolostone could generate gas accumulation and high gas pressures (50–100 atm) that could possibly promote rock fracturing and gas leakage in these shallow (500–1000 m depth) reservoirs. The best sites for gas disposal are therefore located west of the town of Pisa, along the coastal plain.     

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.     

Application of fluid inclusions to the study of Bagnore geothermal field (Tuscany, Italy)

A fluid inclusion study of the hydrothermal minerals in two breccias from two wells in the Bagnore geothermal field (Italy) has provided information on the evolution of the fluids, and has also demonstrated that fluid inclusions can be utilized as geothermometers in this geothermal field. Both breccias come from reservoir zones: one (Bagnore 3bis (Bg 3bis)) was cored at a depth of 3111 m below ground level (b.g.l.), whereas the other (Bagnore 22 (Bg 22)) was ejected during a blow-out, probably from a fractured zone present between 2200 and 2300 m b.g.l. The hydrothermal cement of the breccias is mostly made up of quartz, K-feldspar, Na-rich plagioclase, calcite, chlorite and illite. Fluid inclusion studies were carried out on quartz (Bg 3bis and Bg 22 breccias) and adularia (Bg 22 breccia). Three types of fluid inclusions were recognized in the Bg 3bis breccia. Type I (liquid-rich) inclusions trapped an aqueous fluid with a CO₂ concentration (1.7–2.7 mol/kg) that is significantly higher than present-day fluids (0.5 mol/kg). Type II (liquid-rich) inclusions formed after type I, and trapped a fluid with less CO₂ (0.6–1.0 mol/kg). Type III (vapor-rich) coexist with type I inclusions, and record an early fluid circulation under boiling conditions. The decrease of the CO₂ (and total gas) concentrations from type I inclusions to type II inclusions, and on to present-day conditions can be related to boiling with gas loss and/or mixing. Only one type of fluid inclusion (type II), with moderate CO₂ concentration (0.7–0.3 mol/kg), was found in the Bg 22 breccia. Boiling and/or mixing explain the variation of the CO₂ content in the Bg 22 reservoir fluid from inclusion formation to modern CO₂ concentration (0.3 mol/kg). The absence of any type I inclusions in Bg 22 breccia may be related to non-uniform CO₂ concentrations in different parts of the field. Present-day temperatures (295±10 °C for Bg 3bis and 320±10 °C for Bg 22) are close or equal to fluid inclusion average total homogenization temperatures (around 290 °C for Bg 3bis and 320 °C for Bg 22), suggesting that fluid inclusions can be useful for estimating local temperatures when direct measurements are not available or dubious.     

Isotopic and fluid inclusion study of hydrothermal and metamorphic carbonates in the Larderello geothermal field and surrounding areas, Italy

Fluid inclusions have been studied on six calcite veins from the shallow part (480 to 1515 m below ground level) of the Larderello geothermal field and outcropping in peripheral zones of the geothermal area. Oxygen and carbon isotopic analyses have been carried out on these carbonate veins, as well as on the dolostone layers found inside the Paleozoic metamorphic units of the deep part of the field (from 1939 to 3177 m below ground level). Fluid inclusion observations suggest that boiling processes probably occurred during calcite precipitation in most of the veins. The fluids that formed or interacted with the calcite veins below the uppermost reservoir (made up of Mesozoic marine carbonates), and with the calcite hydrothermal veins of Sassa, were characterised by an apparent salinity from 1.3 to 5.3 wt.% NaCl eq. and a homogenisation temperature from 137 to 245°C. The fluid inclusions related to the calcite veins hosted above the uppermost reservoir show a wide range of apparent salinity (from 1.7 to 22.2 wt.% NaCl eq.) and homogenisation temperatures from 224 to 296°C. Apparent salinity/homogenisation temperature covariations of the latter veins are interpreted as being the result of a mixing process between a low-temperature, high-salinity fluid and a higher-temperature, moderate-salinity fluid. The oxygen isotopic compositions of the calcite veins (δ¹⁸O from 10.34 to 11.45‰) located below the Mesozoic carbonates and in the outcrops (δ¹⁸O from 9.42 to 17.07‰) indicate that the vapour in equilibrium with these veins was isotopically similar to the present-day discharge steam. The aqueous fluids in equilibrium with these veins could be meteoric water that interacted with the Mesozoic carbonates of the upper reservoir. The δ¹³C values of the CO₂ produced at Larderello and the constant concentration of this gas over time are, however, indicative of a deep source inside the reservoir that is probably related to the decarbonation reaction within the metamorphic units that form the present-day deep reservoir. Fluid inclusion salinities (up to 22.2 wt.% NaCl eq.) and isotopic results (δ¹⁸O from 13.43 to 21.99‰, δ¹³C between −1.26 and −0.18‰) on the calcite veins hosted above the uppermost reservoir suggest that the water circulating in these veins has strongly interacted with Mesozoic carbonates or Neogene sediments containing evaporite layers. The isotopic values (δ¹⁸O from 14.09 to 19.91‰, δ¹³C from −4.09 to 1.90‰) of dolomite samples present in the Paleozoic metamorphic rocks indicate a reaction with fluid of variable temperatures under different water/rock ratios. The isotopic composition of one sample reveals equilibrium with present-day discharge fluids. This fact aside, the remaining data indicate that the Paleozoic dolomitic layers do not seem to contribute significantly to the production of CO₂.     

Mixing of thermal and non-thermal waters in the Steamboat Hills area, Nevada, USA

Groundwater monitoring began in 1985 at two geothermal facilities in the Steamboat Hills area, Nevada. Wells representing non-thermal, thermal, and mixed waters are evaluated by assessing temporal variations in B and Cl concentrations, water levels, and temperature. The objective is to assess the hydrologic and geochemical connection between the fractured bedrock geothermal reservoir and the alluvial aquifer. Results suggest that fault-controlled groundwater flow between the geothermal system and the alluvial aquifer is the dominant hydrologic process. Temporal trends suggest that the thermal water component in the alluvial aquifer has increased in most areas but decreased in at least one area.     

Volcano–tectonic structures, gravity and helium in geothermal areas of Tuscany and Latium (Vulsini volcanic district), Italy

Since the early 1980s, geological and structural mapping, gravity, and helium soil–gas studies have been performed in the eastern sector of the Vulsini Volcanic District (Roman Magmatic Province) in an attempt to locate potential geothermal reservoirs. This area is characterised by an anomalous geothermal gradient of >100°C/km, and by widespread hydrothermal mineralization, thermal springs, high gas fluxes, and fossil and current travertine deposits. The results of these surveys indicate the existence of a number of fault systems, with N–S and E–W structures that appear to be superimposed on older NW–SE and NE–SW features. Comparison of the results of the various studies also reveals differences in permeability and potential reservoir structures at depth.     

An oxygen isotope study of silicates in the larderello geothermal field, Italy

Stable-isotope analyses were carried out on hydrothermal minerals sampled from the deep metamorphic units at Larderello, Italy. The ∂¹⁸O values obtained for the most retentive minerals, quartz and tourmaline, are from + 12.0‰ to + 14.7‰ and 9.9‰, respectively, and indicate deposition from an ¹⁸O-rich fluid. Calculated ∂¹⁸O values for these fluids range from + 5.3‰ to + 13.4‰. These values, combined with available fluid inclusion and petrographic data, are consistent with the proposed existence of an early thermal fluid of probable magmatic origin and a late meteoric water. Mixing between these two fluids occurred locally.     

A tentative correlation between seismic activity and changes in the composition of thermal waters on Vulcano Island, Italy

On Vulcano Island, Italy, the thermal water in the shallow water well W2 is a mixture of reservoir water and shallow steam-heated groundwater of meteoric origin. In the period 1986–1988, the composition of the W2 water changed just before two sequences of tectonic earthquakes, from the almost pure reservoir component to the almost pure steam-heated meteoric component. After the seismic activity, the W2 water returned to its pre-earthquake reservoir composition. These changes in composition could be explained by the dilatancy-fluid diffusion model. According to this model, the observed variations in W2 water composition could be the consequence of stress build-up prior to the seismic events, and stress reduction afterwards.     

The oxygen isotope exchange between carbon dioxide and water in the Larderello geothermal field (Italy) during fluid reinjection

The oxygen isotope compositions of CO₂ and water vapor samples collected from Larderello geothermal wells after the start of the fluid reinjection program suggest that if the oxygen isotope exchange in the vapor phase does, in fact, exist, it is a very slow process when compared with the residence time of the fluids in the geothermal reservoir. This is because carbon dioxide and water vapor phases could not have equilibrated significantly in the vapor-dominated reservoir. This conclusion implies that the oxygen isotope composition of carbon dioxide may possibly be used as a tool in geothermal exploration for revealing the presence of liquid water in deep geothermal systems. Based on the interpretation of the oxygen isotope data of the CO₂, I propose that the origin of the low oxygen isotope ratios of carbon dioxide at Larderello is the high-temperature exchange with liquid water in the lower reservoir. In Larderello, the liquid water–rock interaction in the lower reservoir may have increased the ¹⁸O/¹⁶O ratio of the recharge meteoric component. By contrast, lack of high-temperature liquid water in the upper reservoir suggests that the large “δ¹⁸O shift” described for the upper-reservoir steam during the last decades reflects varying degrees of dilution of the lower-reservoir fluid by the low-¹⁸O vaporized liquid water of meteoric origin that recharges the field at shallow depth, with local contribution from still deeper high-¹⁸O water vapor of magmatic origin. The low oxygen isotope composition of the Mesozoic carbonaceous rocks that form the upper reservoir, consequently, likely represents a “fossil” record of the past hot-water geothermal stage.     

Results of a 3D seismic survey at the Travale (Italy) test site

In the last 15 years geothermal exploration in Tuscany, Italy, has addressed deep reservoirs (depth ≥ 3000 m), hosted within complex geological systems, such as metamorphic formations and/or intrusive bodies. Reservoir productivity is linked to fractured and permeable zones that are rather confined and not uniformly distributed. In this context, the seismic methods represent one of the most reliable geophysical techniques for locating potential drilling targets. A 3D seismic survey has been acquired at the Travale test site, and its results have been used to develop a geological and structural model of the site, and to identify and characterize fractured zones inside the deep geothermal reservoir. A correlation between a high-amplitude reflector (H marker) and fractured contact-metamorphic rocks has been highlighted. More than 70% of the total geothermal fluid production at the Travale area comes from this seismic marker.     

Geothermal prospecting with the magneto-telluric method (M.T.-5-E.X.) in the Travale area (Tuscany, Italy)

The application of a new method, namely the M.T.-5-E.X. (Musé 1973), to geothermal prospecting in the region of Travale (Tuscany, Italy) is described here.The aim is to test the efficacy of this method in revealing, on the inside of the geothermal area, the most fractured zones and possibly to localize where the fluids rise from the deeper layers. A first discussion of results is given and a model is proposed which fits the recorded facts. Although a full interpretation of the results will only be possible when the research foreseen in this zone is at a more advanced stage, it is already evident that the method is destined to be widely applied in the future in geothermal research.     

Application of active audiomagnetotellurics (AAMT) in the geothermal field of Travale, Tuscany

In October 1981 the AAMT method was tested in the geothermal field of Travale. This method is based on the MT method, but uses artificial EM fields excited by a transmitter some kilometres from the receiving station. The transmitter consists of a switch mode amplifier for the lower frequency band (< 300 Hz) and six stacked linear amplifiers for the high frequency band. Maximum output is about 5 kW. For measurement of the very small EM field at the receiver the correlation technique is used to obtain best noise rejection. Interpretation of measured data is done by model calculations with the help of a near field theory. Cagniard's theory is found to be applicable only for high frequencies (i.e. short periods). The final result is a three-layer model in good agreement with known geology and resistivity models from other authors.     

Hydrogeochemistry and geothermometry of thermal groundwaters from the Birdsville Track Ridge, Great Artesian Basin, South Australia

The hydrogeochemistry of thermal artesian groundwaters flowing from 12 stock bores along the Birdsville Track Ridge in northeast South Australia has been examined. The Na-HCO3-Cl type groundwater composition has provided a basis for the application of chemical geothermometers to estimating aquifer temperatures and has allowed comparisons of various silica and cation geothermometers. Aquifer and bore penetration depth decrease between Birdsville and Marree from 1220 m to 170 m. A corresponding decrease in measured emergence temperature is also observed (94–31 °C). Chalcedony geotemperature estimates ranging from 110 °C to 41 °C between Birdsville and Marree are considered the most accurate of the various geothermometers tested. Log(Q/K) versus T diagrams have also been evaluated to determine likely aquifer mineral assemblages and reservoir temperatures (111–39 °C). The Birdsville Track Ridge acts as a conduit for low salinity groundwater (total dissolved solids range from 640 mg/l at Birdsville to 1900 mg/l at Marree) in the Great Artesian Basin. Old, slowly migrating groundwater from the deeper basins on either side of the ridge is characterised by higher emergence temperature and higher total dissolved solids. This old groundwater is inferred to have mixed with the younger, lower temperature, lower salinity groundwater that is migrating relatively rapidly along the Birdsville Track Ridge axis. Since the various geothermometers provide distinct types of information, evidence for the mixing is provided by the interpretation of the different temperature estimates. Silica equilibration temperatures reflect aquifer temperatures along the ridge axis, whereas cation geotemperatures partly preserve higher temperatures from the deeper, flanking basins. This study demonstrates how the thermal regime and hydrodynamics of an area can be characterised using a sparse dataset, thus representing a novel and effective methodology for regions anomalous to this central Australian example.     

Occurrence and significance of anomalous chloride waters at the Orakei Korako geothermal field, Taupo Volcanic Zone, New Zealand

Water with Cl concentrations from 15 to almost 10,000 mg/kg, and molecular SO₄/Cl ratios ranging from 0.003 to 1.87, drips periodically from the roof of Ruatapu cave and a side chamber, Rahu Rahu, located in the Orakei Korako geothermal field, Taupo Volcanic Zone, New Zealand. Pools in the bottom of both Ruatapu and Rahu Rahu contain sulfate–chloride waters with pH values ranging from 2.5 to 3.0; their Cl contents have varied temporally from 120 to 240 mg/kg and their molecular SO₄/Cl ratios from 0.86 to 1.30. The Cl in the water dripping from the cave roof cannot come directly from the alkali chloride–bicarbonate water that circulates in the reservoir at Orakei Korako since the modern and historic piezometric surfaces are several meters below the cave roof. Nor does all the Cl in the cave pool waters derive from the reservoir fluid as the volume input required is incompatible with their Na and K contents. A more likely source for the Cl is one whereby rain water, percolating through the fractured rhyolitic country rock, dissolves Cl present either in glass shards or halite deposited in prehistoric times when trapped alkali chloride water boiled to dryness. Given that Cl in the cave pool waters is therefore supplied from a source above rather than, as previously assumed, below, the axiom that Cl present in acid sulfate–chloride ± bicarbonate waters is necessarily a signature of a deep water or magmatic input needs qualification.     

Revision,calibration, and application of the volume method to evaluate the geothermal potential of some recent volcanic areas of Latium,Italy

The volume method is used to evaluate the productive potential of unexploited and minimally exploited geothermal fields. The distribution of P_CO2 in shallow groundwaters delimits the geothermal fields. This approach is substantiated by the good correspondence between zones of high CO₂ flux, and the areal extension of explored geothermal systems of high enthalpy (Monte Amiata and Latera), medium enthalpy (Torre Alfina) and low enthalpy (Viterbo). Based on the data available for geothermal fields either under exploitation or investigated by long-term production tests, a specific productivity of 40 t h⁻¹ km⁻³ is assumed. The total potential productivity for the recent volcanic areas of Latium is about 28 × 10³ t h⁻¹, with 75% from low-enthalpy geothermal fields, 17% from medium-enthalpy systems, and 8% from high-enthalpy reservoirs. The total extractable thermal power is estimated to be 2220–2920 MW, 49–53% from low-enthalpy geothermal fields, 28–32% from medium-enthalpy systems, and 19–20% from high-enthalpy reservoirs.     

Integration of 2D printing technologies for AV2O6 (A=Ca,Sr, Ba)-MO (M=Cu,Ni, Zn) photocatalyst manufacturing to solar fuels production using seawater

The non-polluting nature of photocatalytic H₂ production makes of interest the study of semiconductors for this process. Scale-up of the photocatalytic hydrogen process to a pilot plant requires the photocatalyst's immobilization to enhance the charge transfer and facilitate its recovery. In this work, screen-printed films from the AV₂O₆ (A = Ca, Sr, Ba) semiconductor family were fabricated and evaluated in photocatalytic water splitting for H₂ production in distilled water and seawater under UVA light. The films exhibited ∼3.1 eV band gaps, high crystallinity, and heterogeneous morphologies. BaV₂O₆ film exhibited the highest H₂ production in distilled water (691 μmol/g), related to the synergistic effect between a higher crystallinity and traces of V⁺⁴ species that decrease the recombination of the photogenerated charges. Also, to take advantage of the dissolved species in seawater that could act as sacrificial agents, the BaV₂O₆ film was evaluated in seawater, in which H₂ production was up to 6 times higher (4374 μmol/g) than in distilled water. The BaV₂O₆ film was decorated with simple oxides (CuO, NiO, and ZnO) by the ink-jet printing technology to increase its photocatalytic performance for H₂ production. The highest efficiency with distilled water was obtained with the BaV₂O₆-CuO film, which reached an H₂ production up to 30 times higher than the bare BaV₂O₆, own to the n-p heterostructure formation that enhances the charge transport in the photocatalytic process. When the BaV₂O₆-CuO film was evaluated in seawater, a more constant H₂ production was observed; moreover, the efficiency was similar compared to the production in distilled water (20,563 μmol/g). To elucidate the seawater compounds that most influence the H₂ production, a two levels Plackett–Burman experiments design was carried out in simulated seawater. The analysis revealed that the SO₄²⁻ ions from the CaSO₄ could be decreasing the H₂ production by acting as Lewis's acid sites that trap the photogenerated e⁻ competing for its usage with the H⁺. Additionally, the Cl⁻ ions and the HCO₃⁻ reduction improved the HCOOH production from simulated seawater, reaching 26 times a higher production (23,333 μmol/g) than in distilled water.     

Investigation of the geothermal anomaly of Travale (Tuscany) by telluric, magnetotelluric and geomagnetic deep sounding measurements

As part of the European Community research programme telluric, magnetotelluric and geomagnetic deep sounding measurements were undertaken at 40 sites within the geothermal area of Travale. In the period range of 6–10,000 s the telluric field inside the Travale graben is strongly polarized and directed, independent of the period, about parallel to the graben strike. The lateral variation of the telluric field amplitude is determined mainly by the distribution of the rocks (e.g. the central part of the geothermal anomaly inside the graben is correlated with a horst structure of resistive rocks) and an influence of the geothermal anomaly on the telluric field distribution cannot be observed. The apparent resistivity, as well as the phase curves, are rather similar at all sites within the graben, exhibiting 4–40 ohm · m for periods of 10 s and 50–500 ohm · m for periods of 10,000 s in E-polarization. In the period range of 10–100 s the E- and B-polarization of magnetotelluric measurements can be interpreted by the 2-D effect of the Travale graben, while with increasing period the induced current system becomes more and more 3-D below all sites. This limits the determination of the sedimentary cover thickness (max. 2500 m) by 1-D and 2-D model calculations to periods of less than 100 s.