Chemical variations in hydrothermal minerals of the Los Humeros geothermal system, Mexico

The Los Humeros geothermal system is composed of more than 2200 m of Quaternary altered volcanic rocks and an underlying Cretaceous sedimentary sequence. The low salinity of the fluids discharged at present (Na⁺ and Cl⁻ concentrations <500 ppm), and the excess steam, indicate that the reservoir contains a mixture of steam and dilute groundwater. Water-rock equilibrium is not attained. Hydrothermal minerals are present in veinlets, vugs, and replacing primary minerals. Three mineral zones are recognized: 1) a shallow argillic zone (<400 m depth), 2) a propylitic zone (ranging between 500 and 1800 m) and 3) a skarn zone (>1800 m). Petrographic examination of cuttings from five wells and temperature data indicate at least two stages of hydrothermal activity. Temperature is the main factor that affects the chemical composition of chlorite, epidote and biotite. Fe²⁺ and Al^IV increase in chlorite with temperature [from 1.4 formula position unit (fpu) to 2.8, and from 0.7 to 2.4 fpu, respectively]. The pistacite content of epidote varies from 18 to 33 mol% in high-temperature regions (>270 °C) and from 13 to 26 mol% in low-temperature regions (<250 °C). Biotite displays a slight increase in Al^IV contents (1.55–2.8) and octahedral occupancy (5.93–6.0 fpu) with temperature. Whole rock composition and variations in oxygen fugacity conditions are factors that also affect the concentrations of Fe, Al and Mg in the octahedral sites of chlorite, epidote, biotite and amphiboles. Chemical variations observed in alteration minerals at different depths in the Colapso Central-Xalapazco region could be used as indicators of relict physico-chemical conditions in the reservoir, before the present economic exploitation.     

Residual steam to energy: a project for Los Azufres geothermal field, Mexico

More than 500 t/h of residual steam are discharged into the atmosphere at Los Azufres geothermal field. Steam comes from nine back-pressure turbines that are at present generating 45 MW. A significant increase in output can be obtained if low pressure turbines are installed to expand residual steam from atmospheric pressure up to a vacuum pressure in condensing cycles. A net output optimization process for each unit in the condensing cycle is presented here. The exergy concept is also applied to compare efficiencies of back pressure cycles with new condensing schemes that include low pressure turbines. Results show that 27.8 MW of additional net output can be generated with the new schemes at competitive production costs and higher conversion efficiencies.     

Fluid chemistry and temperatures prior to exploitation at the Las Tres Vírgenes geothermal field,Mexico

Generation of electricity at the Las Tres Vírgenes (LTV) geothermal field, Mexico, began in 2001. There are currently nine geothermal wells in the field, which has an installed electricity generating capacity of 10 MW_e. The chemical and temperature conditions prevailing in the field prior to its exploitation have been estimated, including their central tendency and dispersion parameters. These conditions were computed on the basis of: (i) geochemical data on waters from springs and domestic wells, and on geothermal well fluids (waters and gases); most of the sampling took place between 1995 and 1999; (ii) fluid inclusion studies; (iii) geothermometric data; and (iv) static formation temperatures computed using a modified quadratic regression Horner method.Fluid inclusion homogenization temperatures (in the 100–290 °C range) suggest that there is a high-temperature fluid upflow zone near wells LV3 and LV4 in the southern part of the field. Computed average chemical equilibrium temperatures for the geothermal fluids are ∼260 °C, based on the Na/K and SiO₂ geothermometers, and ∼265 °C, based on the H₂/Ar, and CO₂/Ar geothermometers. In general, the fluid inclusion homogenization temperatures are consistent with geothermometric data, as well as with static formation temperatures. Some of the observed differences could be related to well interference effects and different fluid production/sampling depths. The deeper geothermal waters show higher concentrations of Cl, Na, K, B, Ba, but lower concentrations of SO₄, Ca, and Mg than the shallower waters. Fluid inclusion salinities are also higher in the deeper rocks. The measured Na/Cl ratios of the geothermal well waters are more or less uniform throughout the field and are very similar to that of seawater, strongly suggesting a seawater component in the fluid of the LTV system.The heat stored in the LTV geothermal system was estimated to be at least 9 × 10¹² MJ, of which some 4 × 10¹¹ MJ (equivalent to about 148 MW_e for 30 years of operation, assuming a conversion efficiency of ∼35%) might be extracted using wells. These results indicate that the installed capacity at LTV could be safely increased from the current 10 MW_e.     

Element mobility during the hydrothermal alteration of rhyolitic rocks of the Los Azufres geothermal field, Mexico

The effects of hydrothermal alteration on major, rare-earth, and other trace-element concentrations in rhyolitic rocks of the Los Azufres geothermal field, Mexico, were investigated by statistically comparing the chemical compositions of altered drill cuttings (taken above 450 m depth) with those of fresh rock outcrop samples. Altered rhyolitic rocks show predominantly vitreous and fluidal textures, with alteration products (mainly clay minerals, chlorites and, less commonly, sericite) comprising up to 40% of the rock mass; cryptocrystalline quartz, chlorite, and zeolites fill fractures and vesicles. In altered rocks the greater statistical variances of several elements (Y, Ce, Pr, Nd, Sm, Lu, and Pb) are probably due to alteration effects, whereas smaller variances for CaO, Sr, Rb/Sr, and Rb/Ba suggest that alteration processes have resulted in more uniform chemical compositions. Only MnO, P₂O₅, Ta, Zr, and Nb have significantly different concentrations in hydrothermal altered rocks as compared to fresh rocks. MnO, P₂O₅, Ta, Rb/Zr, and Rb/Nb decrease, whereas Zr, Nb, and Nb/Y increase in the altered rocks. The present study stresses that caution should be taken when using these chemical parameters for petrogenetic studies of old hydrothermally altered areas, particularly with rhyolitic rocks. Rare-earth element (REE) concentrations were not significantly different between fresh and altered rhyolitic rocks. This may indicate that these elements were relatively immobile during the hydrothermal alteration processes affecting the rhyolites at Los Azufres, or more likely that they were reincorporated into hydrothermal minerals after being mobilized from the primary phases.     

Mineralogy and distribution of hydrothermal mineral zones in Los Azufres (Mexico) geothermal field

The general features of the geometry of Los Azufres reservoir have been defined through the mapping of hydrothermal mineral alteration zones. Hydrothermal alteration has been studied in cuttings and drill cores from most of the active wells. X-ray diffraction microprobe analysis and classical optical methods have been employed for the identification of primary and authigenic minerals in fresh and altered samples. Observed patterns of alteration have been correlated with temperature and patterns of fluid circulation. The resulting model depicts a body of geothermal fluid at depth, which ascends and discharges through two main fracture systems. These two circulation zones are characterized by concentric aureoles of increasing hydrothermal alteration towards quasivertical axes. The overall pattern could be described as a dome structure produced by the abnormal thermal gradient, distorted by the effects of active upward circulation of the fluids.     

Basement structure, lithology and permeability at Kawerau and Ohaaki geothermal fields, New Zealand

Poorly permeable basement rocks commonly occur in geothermal regions around the world, and the Quaternary Taupo Volcanic Zone (TVZ) of New Zealand is no exception. Production from basement terrane requires detailed knowledge of its geological and geophysical parameters, as shown by the history of Kawerau and Ohaaki, the only geothermal fields in the TVZ where Mesozoic Torlesse terrane greywacke (litharenite) basement is commonly penetrated at drilled depths of 1–2.5 km. In both fields the basement is step-faulted down into the TVZ. Although hot and hydrothermally altered, the greywackes have little permeability. Some production wells feed from elusive basement faults at Kawerau, but rarely at Ohaaki. Greywackes at Ohaaki are of “granite-rhyolite” provenance, and have more interbedded argillite than the “andesite-dacite” derived Kawerau greywackes. In consequence, the Kawerau basement may sustain brittle fracture at higher temperatures and depths than the more ductile Ohaaki basement, allowing convective circulation of higher enthalpy fluids into permeable Quaternary aquifers.     

Main aspects of geothermal energy in Mexico

With an installed geothermal electric capacity of 853 MW_e, Mexico is currently the third largest producer of geothermal power worldwide, after the USA and the Philippines. There are four geothermal fields now under exploitation: Cerro Prieto, Los Azufres, Los Humeros and Las Tres Vírgenes. Cerro Prieto is the second largest field in the world, with 720 MW_e and 138 production wells in operation; sedimentary (sandstone) rocks host its geothermal fluids. Los Azufres (88 MW_e), Los Humeros (35 MW_e) and Las Tres Vírgenes (10 MW_e) are volcanic fields, with fluids hosted by volcanic (andesites) and intrusive (granodiorite) rocks. Four additional units, 25 MW_e each, are under construction in Los Azufres and due to go into operation in April 2003. One small (300 kW) binary-cycle unit is operating in Maguarichi, a small village in an isolated area with no link to the national grid. The geothermal power installed in Mexico represents 2% of the total installed electric capacity, but the electricity generated from geothermal accounts for almost 3% of the national total.     

Predicting subsidence at Wairakei and Ohaaki geothermal fields, New Zealand

A finite-element model coupling compaction and fluid flow processes in porous materials has been applied to the subsidence bowls at Wairakei and Ohaaki geothermal fields to provide a basis for predicting subsidence. Most of the subsidence is due to slow drainage of relatively impermeable (0.05–0.3 mD), compressible (15–45 kbar⁻¹) mudstone at less than 300 m depth. Maximum subsidence rates at both fields peaked at close to 500 mm/year, before declining to between 200 and 300 mm/year today. However, it took over 20 years for maximum subsidence rates to start to decrease at Wairakei, compared to 8 years at Ohaaki. This difference is due to the relatively rapid stabilisation of pressure beneath the compacting mudstone at Ohaaki compared to that at Wairakei. Predictions of future subsidence at both fields are made assuming that the pressure beneath the mudstone remains constant. At Wairakei, the present total maximum subsidence of 14 m is predicted to increase to 20 ± 2 m by the year 2050. At Ohaaki, the short history of subsidence makes predictions less certain, and the present maximum subsidence of 2.5 m is predicted to be 3–4 m by the year 2006.     

Hydrothermal alteration and fluid inclusion geothermometry of los humeros geothermal field, Mexico

The Los Humeros geothermal field, located in Puebla State, Mexico, occurs in a caldera; drillholes to 3000 m depth encountered a sequence of Quaternary lavas and pyroclastic rocks that range in composition from rhyolite to basalt but are dominantly andesitic. These rest upon the local basement comprising limestone and siltstone of Cretaceous age, which was encountered below 2500 m in the northern part of the field and 1000 m in its southern part.Examination of 29 cores, mostly from below 900 m depth, from 14 wells show that the hydrothermal minerals that occur in the volcanic host rocks include quartz, calcite, epidote, amphibole, sericite, smectite, illite, chlorite, biotite, pyrite and hematite. Their distribution mainly reflects the prevailing hydrological and thermal regime where temperatures locally exceed 300°C. The limestone basement rocks, however, have altered to an assemblage that includes calcite, quartz, wairakite, garnet, wollastonite, parawollastonite, sericite and fluorite.The homogenization temperatures of 356 fluid inclusions were measured and the freezing temperatures of 200 determined. All except two sets of inclusions homogenized into the liquid phase and neither daughter minerals nor a clathrate phase were seen. The homogenization temperatures mostly match measured bore temperatures that range from 250 to 360°C and the apparent salinities are from 0.2 to 2.7 weight per cent NaCl equivalent, but some contribution to freezing point depression by CO₂ is likely.A preliminary model for the hydrology of the field based upon the hydrothermal alteration mineralogy and fluid inclusion data suggests that dilute hot water ascends via faults in the Central Caldera collapse area of the field and moves laterally outward to elsewhere within the caldera.     

Hydrology,hydrochemistry and geothermal potential of El Chichón volcano-hydrothermal system,Mexico

Fluid and heat discharge rates of thermal springs of El Chichón volcano were measured using the chloride inventory method. Four of the five known groups of hot springs discharge near-neutral Na–Ca–Cl–SO₄ waters with a similar composition (Cl ∼ 1500–2000 mg kg⁻¹ and Cl/SO₄ ∼ 3) and temperatures in the 50–74 °C range. The other group discharges acidic (pH 2.2–2.7) Na–Cl water of high salinity (>15 g/L). All five groups are located on the volcano slopes, 2–3 km in a straight line from the bottom of the volcano crater. They are in the upper parts of canyons where thermal waters mix with surface meteoric waters and form thermal streams. All these streams flow into the Río Magdalena, which is the only drainage of all thermal waters coming from the volcano. The total Cl and SO₄ discharges measured in the Río Magdalena downstream from its junction with all the thermal streams are very close to the sum of the transported Cl and SO₄ by each of these streams, indicating that the infiltration through the river bed is low. The net discharge rate of hydrothermal Cl measured for all thermal springs is about 468 g s⁻¹, which corresponds to 234 kg s⁻¹ of hot water with Cl = 2000 mg kg⁻¹. Together with earlier calculations of the hydrothermal steam output from the volcano crater, the total natural heat output from El Chichón is estimated to be about 160 MWt. Such a high and concentrated discharge of thermal waters from a hydrothermal system is not common and may indicate the high geothermal potential of the system. For the deep water temperatures in the 200–250 °C range (based on geothermometry), and a mass flow rate of 234 kg s⁻¹, the total heat being discharged by the upflowing hot waters may be 175–210 MWt.     

Effect of recent temperature change on shallow geothermal measurements

The measured underground temperatures of four boreholes (Rapagnano, Giulianova, Imola and San Marino) drilled into clayey formations in the central regions of Italy, as part of a geothermal research program, have shown that they are affected by more or less regular disturbances. The absence of groundwater movements in these formations, the relative flatness of the topography around the holes and the shape of the disturbances have led them to be considered as the effects of some recent local climatic variations.An inverse theory approach has been used to construct the local surface thermal history. It was not possible to compare it with the temperatures observed by meteorological instruments over the areas close to the boreholes because these data are not available. The results have been compared successfully with the climatic variations over the Northern Hemisphere during the last century. Among several models of the surface forcing temperature, the most reliable one for Rapagnano is represented by a sinusoidal wave, while a parabolic decrease holds good for Giulianova.Suggestions are given for geothermal prospecting.     

The thermal and geochemical structure of the broadlands-ohaaki geothermal system, new zealand

Fifty two wells have been drilled into the Broadlands-Ohaaki geothermal system, New Zealand, in the course of its development. Fluid samples collected from these wells and measured temperatures indicate that boiling is common within the East and West Bank production zones, separated at the surface by the Waikato River. Steam-heated waters form over the top of the system, above zones of boiling, and are also present on the margins of the system. They are C0₂-rich, and are responsible for dilution of the deep chloride fluids, particularly on the margins of the system. Thermal inversions are common on the margins of the system, associated with the steam-heated waters. The eastern portion of the East Bank and margins of the West Bank have cooled since peak thermal conditions, possibly due to dilution, as indicated by comparing fluid inclusion data with temperatures now present. However, fluid inclusion Th and Tm data indicate that boiling and dilution patterns similar to those now present have existedsince inclusion formation. The hydrothermal alteration of the silicic volcanics comprises an assemblage of quartz—albite—illite—adularia—calcite—chlorite—pyrite; epitode and wairakite are rare, and pyrrhotite, sphalerite and galena are generally confined to the margins of the system. Kaolin, Camontmorillonite, cristobalite and siderite are also present on the margins of the system to depths of 600–1200 m, and are related to the presence of the C0₂-rich, steam-heated waters. The deep production fluids originate from a parent (preboiled) fluid with a temperature of 300°C and CO₂ content of 0.6 mol. Excess enthalpy (i.e. two phase feed zone) discharges are not suitable for the calculation of activity ratios in the reservoir liquid and assessment of mineral—fluid equilibria; this is probably due to non-equilibrium distribution of gas species between liquid and vapor. However, an assessment of mineral—fluid equilibria is possible from the compositions of liquid feed wells. Based on these data, the reservoir fluids are now slightly undersaturated with respect to calcite and are in equilibrium with K-mica, pyrite and chlorite. The common presence of adularia and calcite in veins and open spaces may be due to a shift in mineral—fluid equilibria caused by extensive boiling and gas loss in fractures as compared to formation fluid. In contrast, the marginal steam-heated waters are in equilibrium with pyrite-pyrrhotite. Their lower pH values make them more undersaturated with respect to calcite and K-feldspar than the chloride fluids, due mainly to the lower temperatures and concentration of CO₂, resulting in interstratified illite-smectite and even kaolinite ± siderite stability. Dilution and cooling of the boiling fluids by the steam-heated waters has caused their shift to K-mica stability; the resulting deposition of illite in fractures of the East Bank may be responsible for the lower permeabilities here, causing excess enthalpy conditions.Steam-heated waters are common in geothermal systems throughout the world; recognition of dilution patterns helps in deducing the overall geochemical structure of each system. Knowledge of the distribution of steam-heated waters will also assist in locating upflow zones, and also allows their potential for casing corrosion and production-induced incursion to be assessed.     

Rotorua地热田水文地质特征及变化趋势

地热能是新能源家庭中重要一员,我国有丰富的地热资源,北京2008年绿色奥运将充分利用地热资源。为了让大家了解地热田的有关知识,我们以新西兰ROTORUA市地热田为例介绍相关情况,以飨读者。——编者     

榆林市浅层地热能资源量调查评价研究

浅层地热能作为新能源,具有分布范围广、可重复利用、对环境无污染等优势。在调查榆林市工程地质条件、水文地质条件的基础上,利用层次分析法,建立了地埋管热泵适宜分区评价体系,对榆林市浅层地热资源开发利用的适宜性进行了分区与评价。计算了浅层地热能适宜区的热容量、换热功率以及潜力评价。为榆林市科学合理开发利用浅层地热资源提供了理论和数据支撑。     

International legal status of the use of shallow geothermal energy

Shallow geothermal energy (<400 m depth) is used in many countries worldwide, with a rising number of installations over the last decades. The use of ground source heat pump (GSHP) and groundwater heat pump (GWHP) systems results in local temperature anomalies (cold or heat plumes). Since groundwater is used in many countries as source for drinking water a balance between its use and protection has to be found. Therefore, to avoid detrimental environmental impacts it is necessary to define groundwater temperature limits for heating and cooling and minimum distances between such geothermal systems. The aim of the present study is to provide a comprehensive overview of the current international legal status for the use of shallow geothermal energy. Therefore, an international survey was performed using a questionnaire, which was sent to more than 60 countries worldwide. The questionnaire requested information on the corresponding national legislation, temperature limits and minimum distances for GSHP and GWHP systems. The answers to the inquiry showed an extremely heterogeneous outcome. Until now national and legally binding regulations only exist in few countries such as Denmark or Sweden. However, all existing regulations show a wide range for minimum distances (5–300 m) and temperature limits for groundwater. The highest inconsistency was observed for the acceptable temperature change with 3 K in Switzerland to 11 K in France. However, most countries have no legally binding regulations or even guidelines, which highlight the urgent need for further research on the environmental impact and legal management of shallow geothermal installations.     

Fluid inclusions in minerals from the geothermal fields of Tuscany,Italy

A reconnaissance study on fluid inclusions from the geothermal fields of Tuscany indicates that the hydrothermal minerals were formed by fluids which were, at least in part, boiling. Four types of aqueous inclusions were recognized: (A) two-phase (liquid + vapor) liquid rich, (B) two-phase (vapor + liquid) vapor rich, (C) polyphase hypersaline liquid rich and (D) three phase—H₂O liquid + CO₂ liquid + CO₂-rich vapor. Freezing and heating microthermometric determinations are reported for 230 inclusions from samples from six wells. It is suggested that boiling of an originally homogeneous, moderately saline, CO₂-bearing liquid phase produced a residual hypersaline brine and a CO₂-rich vapor phase. There are indications of a temperature decrease in the geothermal field of Larderello, especially in its peripheral zones.     

Hydrogeological and isotopic survey of geothermal fields in the Buyuk Menderes graben, Turkey

The main high and low enthalpy geothermal fields in the Buyuk Menderes graben (Western Anatolia) and their reservoir temperatures are as follows: Kizildere (242 °C), Germencik (232 °C), Aydin-Ilicabasi (101 °C), Yılmazkoy (142 °C), Salavatli (171 °C), Soke (26 °C), Denizli -Pamukkale (36 °C), Karahayit (59 °C), Golemezli (101 °C) and Yenice (70 °C). The geothermal systems are controlled by active graben faults. The reservoir rocks in the geothermal fields are the limestone and conglomerate units within Neogene sediments and the marble-quartzite units within Paleozoic metamorphic formations. There are clear δ¹⁸O shifts from the Mediterranean Meteoric Water Line (MMWL) in the Kizildere, Germencik and Aydin fields, where a good relation between high temperatures and δ¹⁸O shift has also been observed, indicating deep circulation and water rock interactions. In the Pamukkale, Karahayit, Golemezli and Yenice fields and in Soke region, low temperatures, small isotope shifts, shallow circulations and mixing with shallow cold water have been noted.     

Thermal and geochemical structure of the Uenotai geothermal system, Japan

The Uenotai geothermal area is located in southern Akita prefecture of northern Honshu Island. The Uenotai geothermal system is a liquid-dominated system with a central zone of aquifer boiling. The two-phase reservoir has evolved from liquid in the natural state due to exploitation. Gas composition of the vapor phase in the reservoir is nearly in equilibrium and correlates with the vapor fraction in the reservoir and with discharging steam quality. The marginal part of the Uenotai system has cooled with the drop in ground-water level. The chemical characteristics of the geothermal water indicate mixing of the immature high Cl source water with conductively heated or steam-heated shallow water or surface water, as well as boiling and steam gain.     

苏州市浅层地热能应用潜力评估与研究

介绍了浅层地热能应用潜力评估的方法,从影响浅层地热能开发的各项因素中确立浅层地热能适宜区分区的关键指标,通过GIS技术实现浅层地热能开发适宜性分区;并对苏州市进行了地埋管换热系统适宜性分区,计算了苏州各行政区浅层地热能单位温差储量和适宜区与较适宜区的总换热功率,为苏州市浅层地热能合理开发利用提供参考依据。。