Water chemistry of San Marcos area,Guatemala

Two well-equilibrated NaCl geothermal liquids are recognizable in the San Marcos area. Both have the same Cl concentration (540 mg/kg) and the same isotopic composition (δD of −66.5‰ and δ¹⁸O of −9‰) under reservoir conditions, but they come from two distinct aquifers with different temperatures, i.e. 240°C below La Cimarrona and 185°C below La Castalia. The numerous thermal NaCl to NaClHCO₃ springs located in the San Marcos area originate through dilution and boiling of these two geothermal liquids and different degrees of re-equilibration at lower temperatures. Silica and K contents are useful in discriminating between dilution, boiling and re-equilibration phenomena. Thermal NaHCO₃ waters, generated through conductive heat transfer or input of geothermal vapor or gases from below, delineate the extent of the geothermal reservoir(s) at depth.     

Reservoir engineering assessment of Dubti geothermal field,Northern Tendaho Rift,Ethiopia

Following on from surface exploration surveys performed during the 1970s and early 1980s, exploration drilling was carried out in the Tendaho Rift, in Central Afar (Ethiopia), from October 1993 to June 1995. Three deep and one shallow well were drilled in the central part of the Northern Tendaho Rift to verify the existence of a geothermal reservoir and its possible utilisation for electric power generation. The project was jointly financed by the Ethiopian Ministry of Mines and Energy and the Italian Ministry for Foreign Affairs. Project activities were performed by the Ethiopian Institute of Geological Surveys and Aquater SpA. The main reservoir engineering data discussed in this paper were collected during drilling and testing of the above four wells, three of which are located inside the Dubti Cotton Plantation, in which a promising hydrothermal area was identified by surface exploration surveys. Drilling confirmed the existence of a liquid-dominated shallow reservoir inside the Dubti Plantation, characterised by a boiling-point-for-depth temperature distribution down to about 500 m depth. The main permeable zones in the Sedimentary Sequence, which is made up of lacustrine deposits, are located in correspondence to basalt lava flow interlayerings, or at the contact between volcanic and sedimentary rocks. At depth, the basaltic lava flows that characterise the Afar Stratoid Series seem to have low permeability, with the exception of fractured zones associated with sub-vertical faults. Two different upflows of geothermal fluids have been inferred: one flow connected to the Dubti fault feeds the shallow reservoir crossed by wells TD-2 and TD-4, where a maximum temperature of 245 °C was recorded; the second flow seems to be connected with a fault located east of well TD-1, where the maximum recorded temperature was 270 °C. A schematic conceptual model of the Dubti hydrothermal area, as derived from reservoir engineering studies integrated with geological, geophysical and geochemical data, has been tested by numerical simulation, using the TOUGH2/EWASG code. Preliminary simulations, using a simple 3-D numerical model of the Dubti fault area, showed that measured temperature and pressure distribution, as well as evaluated non-condensable gas pressure at reservoir conditions, are compatible with the rise of geothermal fluid, at about 290 °C, along the sub-vertical Dubti fault from beneath the surface manifestations DB1, DB2 and DB3 located at the south-eastern end of the fault. According to the proven shallow field potential, development of this field could meet the predicted electricity requirements of Central Afar until the year 2015.     

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.     

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.     

High-temperature measurements in well WD-1A and the thermal structure of the kakkonda geothermal system, Japan

The New Energy and Industrial Technology Development Organization (NEDO) drilled well WD-1a between 1994 and 1995 in the Kakkonda geothermal field as part of their Deep Seated Geothermal Resources Survey project. High-temperature measurements were carried out in WD-1a. Logging temperatures above 414°C were confirmed at 3600 m and 3690 m depth after 82 h standing time. Simple Horner extrapolations based on observed temperatures up to 82 h after shut-in suggested a temperature of about 500°C at 3500 m depth. Temperatures between 500°C and 510°C were also confirmed at 3720 m depth after 129–159 h standing time, using calibrated melting .tablets. These are the highest temperatures measured in a geothermal well. These results suggest a thermal structure consisting of three layers. Layer one is a shallow permeable zone of the reservoir, at less than 1500 m depth, at 230°C to 260°C. The second layer is a deep zone of the reservoir, which is less permeable and has a temperature of 350°C to 360°C from 1500 m to about 3100 m depth. The third layer is a zone of heat conduction. The transition between the hydrothermal-convection zone and the deeper heat-conduction zone is at 3100 m depth in well WD-1a.     

Geochemistry of the high-salinity geothermal field of Asal,Republic of Djibouti,Africa

In 1987–1988 deep exploratory geothermal wells were drilled in the Asal Rift in the Republic of Djibouti. The salinity of the brines produced in the Asal field exceeded 100 000mg/kg, with a low gas content of the order of 0.6 mmol/mol. Measured temperatures in the producing zones and at wellbottom vary from about 260 to 359°C. The sphalerite and galena scaling observed in producing wells starts at the flash level located at about 850 m depth. The calculated geochemical temperatures are close to 260°C, which is the average temperature of the main reservoir located at depths between 1000 and 1300 m. As a result of sulphide precipitation, about 90% of H₂S is removed from the original fluid before discharge. The computed composition of the brine at reservoir conditions is compatible with the presence of stable phases of epidote, (MgFe)-chlorite, albite, K-feldspar, K-mica, pyrite and anhydrite. The main recharge water is seawater.     

Spatial distribution of temperature in the low-temperature geothermal Euganean field (NE Italy): a simulated annealing approach

The spatial distribution of groundwater temperatures in the low-temperature (60–86 °C) geothermal Euganean field of northeastern Italy has been studied using a geostatistical approach. The data set consists of 186 temperatures measured in a fractured limestone reservoir, over an area of 8 km². Investigation of the spatial continuity by means of variographic analysis revealed the presence of anisotropies that are apparently related to the particular geologic structure of the area. After inference of variogram models, a simulated annealing procedure was used to perform conditional simulations of temperature in the domain being studied. These simulations honor the data values and reproduce the spatial continuity inferred from the data. Post-processing of the simulations permits an assessment of temperature uncertainties. Maps of estimated temperatures, interquartile range, and of the probability of exceeding a prescribed 80 °C threshold were also computed. The methodology described could prove useful when siting new wells in a geothermal area.     

Geochemistry of thermal waters in the Tengchong volcanic geothermal area, West Yunnan Province, China

The Tengchong volcanic geothermal area is one of the areas in China which has powerful geothermal energy potential. The chemical compositions of the thermal waters discharged in this area were studied to obtain information on boiling and mixing relationships and average reservoir temperatures. Then a conceptual model of the Tengchong volcanic geothermal area was formulated. Hydrothermal areas have reservoir temperatures ranging from 90 to 150°C; such temperatures can be found in up to 60% of the 58 hydrothermal areas. Five hydrothermal areas have high temperatures, with an average reservoir temperature of more than 150°C, and occupy less than 10% of the total. The Hot Sea geothermal field is one of the five high temperature hydrothermal areas where a more detailed investigation was made.     

Hot springs and geothermal gradients in northern Thailand

Chemical geothermometry of hot springs in northern Thailand indicates that many have reservoir temperatures in excess of 150°C and some in excess of 180°C. Measurements of temperatures in abandoned oil wells in Fang Basin indicate geothermal gradients of 70 – 130 mK/m. The high geothermal gradient may be the result of extensional tectonics in northern Thailand, caused indirectly by sea-floor spreading in the Andaman Sea. Relatively high reservoir temperatures and shallow reservoir depths suggest that hot spring areas in northern Thailand may be potential sources of geothermal energy.     

Hydrothermal alteration mineralogy as an indicator of hydrology at the Ngawha geothermal field,New Zealand

Interaction between geothermal fluids and the rocks through which they migrate alters many earlier formed minerals and produces others. The minerals thus formed preserved evidence of hydrological conditions prevailing within an active geothermal system; in particular, they can reflect the range of temperatures under which they formed. This feature was tested at the Ngawha geothermal system, which is different from others in New Zealand in that its reservoir comprises fractured basement rocks covered by a 500–600m thick sequence of sedimentary rocks. Petrographic examination of cores and cuttings recovered from drillholes at Ngawha shows that the secondary minerals present within the rock matrices and veins are of different ages. The thermally sensitive minerals include epidote, titanite, biotite and clays, including some that are interlayered. Comparison of the measured downwell temperatures with those deduced from the secondary mineralogy and by homogenizing fluid inclusions, shows that the central part of the field has remained thermally stable since the youngest secondary minerals deposited there but its southern margin has cooled by 20–40°C or perhaps more. A likely cause of this is an inflow of cooler water from the east, which also causes the temperature inversion clearly evident in hole Ng8. By contrast, some fluid inclusion geothermometry results suggest that the northern part of the drilled field has heated since their host hydrothermal quartz crystals formed.     

Thermal anomalies in lanzarote (Canary Islands)

About a hundred thermometers have been installed 3 m deep in an area of about 20 km² in the ? Montañas de Fuego ? (island of Lanzarote). The temperatures measured oscillate between 16°C and 350°C. In other wells 150 to 250 m deep a gradient of 0.2°C/m has been measured. This gradient is always found in a region of about 200 km² around the zones of the highest superficial anomalies.The local surface anomalies are related to the outflow of hot fluids through tectovolcanic fractures. At great depths these fluids probably form a convective system under impervious layers (hyaloclastites?). The heat transfer takes place by means of convection in a deep reservoir and in the superficial levels, but an intermediate zone with impervious layers must exist where heat is transmitted by conduction.     

Oxygen isotopic ratios of sulfate ions-water pairs as a possible geothermometer

In this paper a brief review is given of the dependence of the oxygen isotopic fractionation of the sulfate ions-water system on temperature and the pH. From the available experimental data some relationships have been elaborated, which show that the isotopic exchange time is strongly temperature and pH dependent. The times for 97 per cent of isotopic exchange (near equilibrium conditions) at pH 7.0 are about 9 years at 200°C and 0.6 years at 330°C, while at pH 3.8 and at the same temperatures the times of exchange are 1.5 years and 0.08 years respectively. Thus, at the temperatures and pH of geothermal reservoirs the sulfate could be in isotopic equilibrium with environmental water, and the oxygen isotopic fractionation factors of sulfate-water geothermal pairs, being temperature dependent, can be used as geothermometers.Also reported here are some results on the O¹⁸ content of sulfate-water pairs from some wells on the edge of and outside the Larderello geothermal basin. The estimated isotopic temperatures are not very significant for the deep reservoir temperatures due to the geological features of the Larderello area which show important outcropping and deep anhydrite layers. Furthermore, as regards the wells outside the Larderello basin (Travale wells) some mixing of the geothermal water with colder underground water has been proved. However, the isotopic temperatures are generally higher than those measured at the well-head, and the highest ones are close to those estimated for the geothermal reservoir.In other geothermal areas more convenient from a geological point of view, the O¹⁸ content of the sulfate-water pair can be a useful and accurate thermometer.The O¹⁸/O¹⁶ ratios of several other sulfates (surface and deep anhydrite samples, sulfate ions in thermal springs) from the same area were also determined and differ substantially from borehole sulfate values.     

High-temperature measurements of water adsorption in geothermal rocks

Water adsorption phenomena are a rich and new field of study in geothermal research, particularly at very high temperatures. ENEL is interested in the exploitation of geothermal regions with superheated steam, so it is important to understand the behaviour of water-rock interaction. We have analysed four samples of Monteverdi cuttings (southwest of Larderello field, Tuscany) in the 170–200°C temperature range and the results are presented in this paper. The presence of adsorbed water is very important, and it is able to multiply the estimated reserves of superheated steam by a factor of 14 in the Monteverdi geothermal field. There is only a slight influence of the reservoir temperature on the amount of adsorbed water. © 1999 CNR. Published by Elsevier Science Ltd. All rights reserved.     

Geothermal reinjection experience

Reinjection into geothermal reservoirs is discussed and the experience gained by reinjection experiments in 44 geothermal fields is reviewed. Reinjection started purely as a disposal method, but has more recently been recognised as an essential and important part of reservoir management. Only a small part of the thermal energy in place in geothermal reservoirs can be recovered if reinjection is not applied. Thermal breakthrough has been observed in few geothermal reservoirs but has in all cases been found to be a manageable part of field operation. Silica scaling in surface equipment and injection wells is a delicate aspect of the reinjection process in most high-temperature geothermal fields, but silica scaling in the reservoir has not been considered a problem. Reinjection of low-enthalpy geothermal fluid into sandstone has not been successful, for reasons that are poorly understood. The location of injection wells in relation to production wells influences the ratio of injected fluid recovered in production wells. For peripheral injection, about one third of the injected fluid is commonly recovered, whereas injection within the production area results in a higher ratio of recovered fluid. Subsidence is in general of small concern in geothermal operations and micro-gravity has proved a valuable tool to estimate the recharge to geothermal reservoirs.     

Exploration of Lahendong geothermal field in North Sulawesi, Indonesia

This paper describes the progress made in developing the geothermal resources at Lahendong, North Sulawesi, Indonesia for utilization in power generation. Exploration of the whole region included a geophysical survey undertaken exclusively by the Volcanological Survey of Indonesia (VSI). A temperature survey at various depths was conducted through gradient boreholes. The results show that the area of anomalous temperature corresponds to the area of low resistivity revealed by the seismic survey. Two shallow exploratory boreholes (300–400 m) drilled by VSI confirmed the existence of the resources. The deep reservoir in Lahendong field extends over an area of 10 km²; the upper parts of the reservoir are presumed to be water dominated (temperatures in excess of 200°C) and to overlie a zone of hot chloride water at an undetermined depth. The potential of Lahendong field is estimated to about 90 MW.In Pelita IV (1984–1989), the fourth 5-year plan, the State Electricity Public Corporation plans to construct a 30 MW geothermal power-plant in the Lahendong field.     

Chemical description of spring waters from the Tutupaca and Rio Calientes (Peru) geothermal zones

Analysis of chemical data from 34 spring samples from the Tutupaca and Río Calientes (Republic of Perú) geothermal zones is presented in this paper. The main objective of the work was to characterize geothermal resources to be exploited in the future. The investigated zones were: Tutupaca and Río Calientes‐Ancocollo‐Challapalca. Thermal waters from Tutupaca showed low pH values and they were classified as sulphate type waters. Thermal springs from the Río Calientes zone showed almost neutral pH values and they were classified as sodium chloride type waters with a probably geothermal origin. Reservoir temperatures were estimated and the results indicated that water–rock equilibrium in the hydrothermal system was not completely attained. Taking into account the chemical composition of some equilibrated or ‘mature’ waters from the Río Calientes zone, reservoir temperatures in the range of 210–240°C were estimated. Mixing models based on silica, chloride and specific enthalpy of the samples were used to estimate the composition of the reservoir liquid. Isotopic data showed oxygen‐18 shift for the waters from Río Calientes‐Ancocollo regarding the waters from Tutupaca zone which were found on the world meteoric line. Copyright © 1999 John Wiley & Sons, Ltd.     

Geochemical study of fluid inclusions from the western upflow zone of the Matsukawa geothermal system,Japan

Prior to development, the Matsukawa geothermal field was partially vapor-dominated. The youngest mineral assemblage consists of early pyrophyllite, diaspore and pyrite, and later anhydrite and quartz, implying deposition from an acidic, high-temperature fluid. Fluid inclusions in anhydrite and quartz from core and cutting samples collected in wells drilled in the western upflow zone of the field were studied to characterize the temperatures and compositions of these late fluids.The results of fluid inclusion studies indicate that the temperatures during the deposition of anhydrite and quartz were up to several tens of degrees lower than the reservoir temperatures at the time of exploitation. Fluids trapped in anhydrite had temperatures of up to 257 °C, CO₂ concentrations in the 0.4–2.6 mol% range and salinities of 1.9–11.3 wt.% NaCl. This compositional variation is related to vapor loss occurring during boiling. The data suggest that the geothermal reservoir is currently being reheated by subvolcanic intrusions.     

The geothermal system of Ischia Island (southern Italy): Critical review and sustainability analysis of geothermal resource for electricity generation

In this paper we analyze the main available data related to the geothermal system of Ischia Island, starting from the first geothermal exploration in 1939. Our aim is to define a conceptual model of the geothermal reservoir, according to geological, geochemical, geophysical and stratigraphic data. In recent times, the interest on geothermal exploitation for electricity generation in Italy is rapidly increasing and the Ischia Island is one of the main targets for future geothermal exploitation. Nowadays, one of the main economic resources of the island is the tourism, mainly driven by the famous thermal springs; so, it is crucial to study the possible interaction between geothermal exploitation and thermal spring activities. To this aim, we also analyze the possible disturbance on temperature and pressure in the shallow geothermal reservoir, due to the heat withdrawal for electric production related to small power plant size (1–5 MWe). Such analysis has been performed by using numerical simulations based on a well known thermofluid-dynamical code (TOUGH2^®). Obtained results show that such geothermal exploitation generates a perturbation of temperature and pressure field which, however, is confined in a small volume around the well. At shallow level (0–100 m) the exploitation does not produce any appreciable disturbance, and can be made compatible with thermal spring exploitation. Moreover, such results are crucial both for the evaluation of volcanological processes in the island and for the general assessment of geothermal resource sustainability.     

Geoelectric study of some indian geothermal areas

The electrical resistivity technique has been used extensively in the Indian sub-continent for the exploration of geothermal areas. The first systematic application of the resistivity method for locating the geothermal reservoir was made in the Puga area, which is situated very close to the collision junction of the Indian and the Asian plates and has numerous hot springs with temperatures varying from 30 to 84°C (boiling point at that altitude). The resistivity depth probes indicated the presence of a conductive zone, with a value of 10–25 ohm·m and a thickness varying from 50 to 300 m over an area of 3 km², which was inferred to correspond to a shallow thermal reservoir. Thermal surveys also revealed a significant anomaly corresponding to this zone, which, when drilled, encountered a reservoir of wet steam with a temperature of up to 135°C, thus confirming the results of the resistivity surveys. Somewhat similar results have been obtained in the adjoining area, where much thicker zones with moderate electrical conductivity have been mapped.Another significant application of the electrical resistivity method has been made in the NNW-SSE extending West Coast geothermal belt of India, which is covered by Traps (Basalts) of the Cretaceous-Eocene. The area is characterized by the existence of a number of hot springs, with temperature up to 70°C, along a 400 km long alignment, associated with steep gravity gradients and an isolated occurrence of native mercury in the zone of a gravity “high”. The enigmatic geology of this area has been mapped, giving quantitative estimates of the thickness of the Traps and inferring the structural features. In addition, the electrical resistivity depth probes have also been used to identify the pre-Trappean geology, thereby locating the probable areas which could act as geothermal reservoirs.This paper presents the results of the electrical resistivity surveys in the form of geoelectric sections for some of the geothemal fields in the Indian sub-continent.     

Temperature–depth relationships based on log data from the Los Azufres geothermal field,Mexico

Equilibrium temperatures based on log data acquired during drilling stops in the Los Azufres geothermal field were used to study the relationship between temperature, depth and conductive heat flow that differentiate production from non-production areas. Temperature and thermal conductivity data from 62 geothermal wells were analyzed, displaying temperature–depth, gradient–depth, and ternary temperature–gradient–depth plots. In the ternary plot, the production wells of Los Azufres are located near the temperature vertex, where normalized temperatures are over 0.50 units, or where the temperature gradient is over 165°C/km. In addition, the temperature data were used to estimate the depth at which 600°C could be reached (5–9 km) and the regional background conductive heat flow (≈ 106 mW/m²). Estimates are also given for the conductive heat flow associated with the conductive cooling of an intrusive body (≈ 295 mW/m²), and the conductive heat flow component in low-permeability blocks inside the reservoir associated with convection in limiting open faults (from 69 to 667 mW/m²). The method applied in this study may be useful to interpret data from new geothermal areas still under exploration by comparing with the results obtained from Los Azufres.