The use of gas chemistry to evaluate boiling processes and initial steam fractions in geothermal reservoirs with an example from the olkaria field, Kenya

A method has been developed to evaluate boiling processes in the producing aquifer of “high-enthalpy” geothermal wells using data on the concentrations of CO₂, H₂S and H₂ in steam discharged. The extent to which water and steam are separated in the producing aquifer is evaluated as well as the amount of enhanced evaporation due to heat flow from the rock to the boiling water. Further, the initial steam fraction in the reservoir fluid is calculated. Results are presented for the Olkaria geothermal field, Kenya, to demonstrate the use of our method. They show that the initial steam fraction in the reservoir is very small: up to 0.25% of the mass, or about 10% by volume. Segregation of water and steam in the producing aquifers is rather extensive for some of the wells. Thus, water which has boiled and yielded steam into wells amounts to more than two times the mass of the fluid discharged from the well. The larger part of the exploited steam ( ) is generated by flow of heat from the rock to the boiling water.     

Production of superheated steam from vapor-dominated geothermal reservoirs

Vapor-dominated geothermal systems such as Larderello, Italy, The Geysers, California, and Matsukawa, Japan yield dry or superheated steam when exploited. Models for these systems are examined along with production data and the thermodynamic properties of water, steam and rock. It is concluded that these systems initially consist of a water and steam filled reservoir, a water-saturated cap rock, and a water or brine-saturated deep reservoir below a water table. Most liquid water in all parts of the system is relatively immobilized in small pores and crevices; steam dominates the large fractures and voids of the reservoir and is the continuous, pressure-controlling phase. With production, the pressure is lowered and the liquid water boils, causing massive transfer of heat from the rock and its eventual drying. Passage of steam through already dried rock produces superheating. After an initial vaporization of liquid water in the reservoir, the decrease in pressure produces increased boiling below the deep water table. With heavy exploitation, boiling extends deeper into hotter rock and the temperature of the steam increases. This model explains most features of the published production behavior of these systems and can be used to guide exploitation policies.     

Chemical and physical reservoir parameters at initial conditions in Berlingeothermal field,El Salvador: a first assessment

A study has been madeto obtain the main chemical and physical reservoir conditions of the Berlin field (El Salvador)before the commencement of large-scale exploitation of the geothermal resource The upflowzone and the main flow path within the geothermal system have been determined from the arealdistribution of chemical parameters such as Cl concentrations ratios such as Na/KK/Mg,K/Ca,and temperatures computed from silica concentrations and cation ratios Gas compositions havebeen used to calculate reservoir parameters such as temperature steam fraction and P_CO2 The computer code WATCH (new edition 1994) hasbeen used to evaluate the temperature of equilibration between the aqueous species and selectedalteration minerals in the reservoir The fluid in Berlin flows to the exploited reservoir from thesouth entering it in the vicinity of well TR-5 Along its flow-path (south–north direction) thefluid is cooled by boiling and conductive cooling The chloride-enthalpy diagram indicates theexistence of a parent water with a chemical composition similar to well TR-5 that boils and theresidual brine produces the fluid of well TR-3 which is very concentrated in salts The fluid ofTR-5 is probably produced from this parent water generating the fluids of wells TR-2 and TR-9by boiling and the fluids of wells TR-1 and TR-4 by conductive cooling The computed values forthe deep steam fraction clearly indicate that this is a liquid-dominated system with computedtemperature values decreasing from 310°C (upflow zone) to about 230°C from south to north© 1999 Published by Elsevier Science Ltd on behalf of CNR All rights reserved     

Aluto-langano geothermal field, ethiopian rift valley: physical characteristics and the effects of gas on well performance

This study, which focuses on the Aluto-Langano geothermal field, is part of the ongoing investigations of the geothermal systems in the Ethiopian Rift Valley. Aluto-Langano is a water-dominated gas-rich geothermal field, with a maximum temperature close to 360°C, in the Lakes District region of the Ethiopian Rift Valley. The upflow zone for the system lies along a deep, young NNE trending fault and is characterized by boiling. As a result, the deep upflow zone loses some water as steam and produces a cooler saline shallow aquifer. The high partial pressure of carbon dioxide (about 30 bar in the reservoir) depresses the water table and restricts boiling to deeper levels. The main aquifer for the system is in the Tertiary ignimbrite, which lies below 1400 m. The capacity of the existing wells is close to 7 MW_c; the energy potential of the area is estimated to be between 3000 and 6000 MW_t yr km⁻³, or 10–20 MW_c km⁻³ for over 30 years.     

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.     

Effect of the water–steam phase transition on the electrical conductivity of porous rocks

The effect of the water–steam phase transition on electrical conductivity was experimentally investigated in volcanic and sandstone samples to support the interpretation of resistivity data to determine changes in steam saturation in geothermal reservoirs. The measurements were performed at simulated in situ conditions with controlled pore fluid chemistry, temperature, and confining and pore pressures. At constant temperature (150 °C) and confining pressure, pore fluid was withdrawn from the sample by steadily increasing the volume of the pore fluid system. At the vapor saturation pressure, the pore water progressively boiled to steam, resulting in a continuous conductivity decrease by a factor of approximately 20. The study showed that: (1) for rocks in which conduction is controlled by the pore fluid, the concurrent changes in both electrical conductivity and pore (vapor) pressure are defined by the pore size distribution; the changes in liquid–steam saturation are approximately proportional to those in conductivity and can thus be quantified; and (2) for rocks in which surface conduction is predominant there is no direct relation between conductivity, pore pressure and drained fluid volume; this implies that the conduction mechanism controls the pattern of electrical conductivity variations as steam saturation changes.     

高效利用地热水的系统型式及其经济分析

讨论了解决当前地热水严重浪费问题的几种技术方案。分析了三种系统型式：用于普通热水锅炉给水的系统；有蒸气锅炉和溴化锂吸收式热泵的复合系统；有电动蒸汽压缩式热泵的系统。证明地热水供热系统中使用热泵具有节能与经济意义。     

Hydrofluorocarbons as geothermal vapor-phase tracers

Two hydrofluorocarbons, R-134a and R-23, have been developed for use as a vapor-phase tracer in geothermal systems. These low molecular weight compounds are volatile, electrically neutral, nontoxic, relatively inexpensive, and have detection limits as low as 10⁻⁵ ppm. Data from laboratory and field tests indicate that they are stable enough to be used as tracers in vapor-dominated systems such as The Geysers. However, these compounds have a higher volatility than water, which affects the rate of transfer of the tracer from the liquid injectate to reservoir steam during boiling. Simple analytic models of boiling were used to estimate the effects of the high tracer volatilities on the outcome of tracer tests in vapor-dominated systems. The results imply that the effects of volatility are exaggerated under conditions of high superheat, which promote the continuous removal of steam from the vicinity of the boiling interface. In contrast, low to moderate superheat reduces the effects of volatility to the extent that the volatile-tracer test results qualitatively resemble those in which tritiated water is used as a tracer. Thus, volatile tracers can be used with confidence to qualitatively describe the distribution of injected water in vapor-dominated systems where superheat is low to moderate.     

Isotopic (δO, δD) patterns in Los Azufres (Mexico) geothermal fluids related to reservoir exploitation

Isotopic patterns for the year 2000 in the Los Azufres geothermal reservoir were related to injection of a condensed steam–water–air mixture as well as to the occurrence of reservoir physical processes resulting from exploitation. Reservoir boiling and mixing of reservoir fluids with cooler fluids were the most important processes identified. Boiling takes place in two zones of the field. In the north, the boiling area includes wells AZ-13, AZ-28, AZ-48, AZ-43 and AZ-32, while, in the south, boiling affects wells AZ-16AD, AZ-22, AZ-18, AZ-26 and AZ-36. Mixing of reservoir fluids with cooler waters was identified in wells AZ-2, AZ-33, AZ-16 and AZ-46 located in the southern zone and in well AZ-4 in the north. The isotopic (oxygen-18 and deuterium) patterns of fluids collected in September 2000 show that the original convective process found in the unperturbed reservoir is still taking place, although mixing of reservoir and reinjected fluids is also indicated. According to N₂ data, the effects of reinjection on the physical and chemical characteristics of the reservoir fluids can be observed in the northern part of the field. Until now, however, only a steam phase, resulting from boiling and steam separation of the re-injected mixture, is evident in the fluids discharged by the northern wells.     

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.     

Simulation of Steam Injection Process for Horizontal Well with Heavy Oil Recovery

Horizontal well technology is widely used in the production of heavy oil. Steady-state model is used as main research method and assume constant wet steam parameters in wellbore, ignoring the impact of heat and mass transfers of steam from wellbore to the reservoir. Numerical calculation is used to analyze steam-water-oil three-phase on flow and heat transfer rule in reservoir and wellbore in startup phase. The influence rule on diffusion process of vapor and water hindered by oil stockpile in wellbore was analyzed, as well as vapor and water parameters change rule along the well. Result indicated that wet steam moving forward was hindered by oil stockpile in wellbore, which lead reservoir suction steam to be not uniform; dryness and temperature of steam gradually reduce, resulting in high temperature at the heel and low temperature at the toe of reservoir; reservoir suction steam effect was improved and reservoir heated range was expanded gradually with the increasing of steam injection volume and dryness; variation of reservoir porosity and permeability have a similar effect on reservoir suction steam, comparing with steam injection volume and dryness. When porosity and permeability were enlarged, reservoir suction steam effect and reservoir heated range would become better.     

Modeling studies for production potential of Chingshui geothermal reservoir

The Chingshui geothermal power plant was decommissioned in 1993 due to a continued decline in production. Although some geothermal exploration and field investigation had been exercised, the production potential of the reservoir is still not well understood. In this paper, numerical modeling approaches for characterization of the geothermal reservoir, investigation of reservoir production performance, and evaluation of exploitation scheme design are presented. At first, a site-scale refined grid numerical model was developed for simulating the natural state of Chingshui geothermal reservoir. Through the model, the production potential of the geothermal reservoir was estimated and the availability of water resources was assessed. We further built production model to simulate the production history during 1981–1993. From the production model, we can conclude that the abnormal drop of the reservoir production capacity is mainly caused by carbonate scaling. Potential production schemes with different reinjection designs were evaluated through the model. Simulation results indicated that a sustainable hot water production capacity of Chingshui geothermal reservoir is about 200 t/h without reinjection, and 300 t/h or even higher with reinjection which is enough for a 3 MWe power plant. The simulation results indicate that reinjection provides an effective approach for maintaining reservoir pressure during hot water/steam production.     

Design of geothermal steam supply systems in Iceland

The role of the geothermal steam supply system is to receive the geothermal fluid from the geothermal wells, separate the steam from the water and to deliver steam and/or water to a user of the thermal energy. It may be for direct use in any kind of an industrial process, such as drying, heating, cooling, etc., or it may be intended for electric power generation. The steam supply system delivers the fluid at a specified temperature, pressure and quality to the user.The steam supply system consists of wellheads, steam collection pipelines, nowadays in Iceland normally designed for two-phase flow of water and steam, steam-water separators, main steam/water lines, moisture separators, control valves, exhaust system, and effluent disposal equipment as needed and may include compressors and/or pumps for long distance transportation.Design criteria for the system depend on one hand upon the characteristics of the geothermal field, and on the other upon the intended use and required steam quality and economy. High enthalpy fields, for example, are capable of producing high pressure steam which is relatively economical when electric power generation alone is being considered. For such systems, high quality of the steam is of utmost importance.The paper gives a general overview of the steam supply systems in Iceland and describes the main features of the Nesjavellir steam supply system where the main emphasis was laid on high steam quality in order to prevent scaling in turbines, control valves and heat exchangers. New systems or systems needing restoration should be based on the same features.     

Downhole measurements and fluid chemistry of a castle rock steam well,the Geysers,Lake County,California

Wellbore and reservoir processes in a steam well in the Castle Rock field of The Geysers have been studied by means of down-hole pressure and temperature measurements and analyses of ejected water and steam produced under bleed and full flow. Down-hole measurements show that below a vapor zone there is liquid water in the well in pressure equilibrium with reservoir steam at a depth of 2290 m. The progressive decreases, from 1973 to 1977, of pressure and temperature in the vapor zone indicate that wellbore heat loss is high enough to condense a large fraction of the steam inflow. The chemical composition of water ejected from the well is consistent with an origin from wellbore condensation of steam. Calculations using the differences in gas and isotopic compositions between bleed and full-flow steam show that about half of the full-flow steam originated as liquid water in the reservoir and that about 30% of the steam entering the well under bleed was condensed in the wellbore and drained downward. Heat loss calculations are also consistent with this amount of condensation.     

System dynamics modelling of spring behaviour in the Orakeikorako geothermal field,New Zealand

System dynamics software STELLA is used to obtain mass and thermal balances of a spring in the Orakeikorako geothermal field, New Zealand, based on field measurements of water level, barometric pressure, rainfall and spring temperature. The model identifies the interactions of the principal influences on spring behaviour of rainfall, groundwater, geothermal steam and barometric pressure. The geothermal steam inflow estimated from the model, of about 0.022 kg/s, confirms the existence of a weak hydraulic connection with a deeper geothermal reservoir.     

Hydrogeochemical exploration of geothermal prospects in the Tecuamburro Volcano region, Guatemala

Chemical and isotopic analyses of thermal and nonthermal waters and of gases from springs and fumaroles are used to evaluate the geothermal potential of the Tecuamburro Volcano region, Guatemala. Chemically distinct geothermal surface manifestations generally occur in separate hydrogeologic areas within this 400 km² region: low-pressure fumaroles with temperatures near local boiling occur at 1470 m elevation in a sulfur mine near the summit of Tecuamburro Volcano; non-boiling acid-sulfate hot springs and mud pots are restricted to the Laguna Ixpaco area, about 5 km NNW of the sulfur mine and 350–400 m lower in elevation; steam-heated and thermal-meteoric waters are found on the flanks of Tecuamburro Volcano and several kilometers to the north in the andesitic highland, where the Infernitos fumarole (97°C at 1180 m) is the primary feature; neutral-chloride hot springs discharge along Rio Los Esclavos, principally near Colmenares at 490 m elevation, about 8–10 km SE of Infernitos. Maximum geothermometer temperatures calculated from Colmenares neutral-chloride spring compositions are 180°C, whereas maximum subsurface temperatures based on Laguna Ixpaco gas compositions are 310°C. An exploration core hole drilled to a depth of 808 m about 0.3 km south of Laguna Ixpaco had a bottom-hole temperature of 238°C but did not produce sufficient fluids to confirm or chemically characterize a geothermal reservoir. Hydrogeochemical data combined with regional geologic interpretations indicate that there are probably two hydrothermal-convection systems, which are separated by a major NW-trending structural boundary, the Ixpaco fault. One system with reservoir temperatures near 300°C lies beneath Tecuamburro Volcano and consists of a large vapor zone that feeds steam to the Laguna Ixpaco area, with underlying hot water that flows laterally to feed a small group of warm, chloriderich springs SE of Tecuamburro Volcano. The other system is located beneath the Infernitos area in the andesitic highland and consists of a lower-temperature (150–190°C) reservoir with a large natural discharge that feeds the Colmenares hot springs.     

拉德瑞罗地热电站可持续开发经验——记拉德瑞罗地热发电100周年

意大利地热发电的世界霸主地位持续了60多年,直至20世纪70年代被美国超越。位于意大利中部托斯卡纳区的拉德瑞罗地热电站是世界第一座地热电站,1913年11月13日开始运行,到1957年仍是世界唯一的地热发电。截至2013年11月,拉德瑞罗地热电站已发电运行100周年,实现了可再生地热能的可持续开发。拉德瑞罗地热电站稳定发展和持续运行100年的经验,一是发电系统设备更新换代,2013年拉德瑞罗地热电站运行机组22台,总装机容量594MW,全部是20世纪90年代以来的新建机组;二是发电废汽回收实行冷凝水回灌,拉德瑞罗地热田现有23口回灌井,将发电废蒸汽回收后的冷凝水回灌到地下热储中,电站因此在2005~2009年增加了4台机组、共100MW的新增装机容量;三是地热田的勘探扩展。拉德瑞罗地热发电的百年经验也是对世界地热开发的重要贡献。     

The United Nations' approach to geothermal resource assessment

Although the emphasis of United Nations' assisted geothermal projects has been on demonstrating the feasibility of producing geothermal fluids, the potential capacity of individual fields has been estimated by both the energy in place and decline curve methods. The energy in place method has been applied to three geothermal fields resulting in total resource estimates ranging from 380 to 16,800 MW-yr. The results of these studies must be considered highly tentative, however, due to inadequate reservoir data and a poor knowledge of producing mechanisms. The decline curve method has not given quantitative results concerning ultimate field potential because of the relatively short duration of well tests (several weeks to a maximum of 11 months). In all cases, however, the decline of flowing wellhead pressure, field pressure, and flow rate has continued to decrease with time.A new method for making regional assessment of geothermal potential is described, which is based, in part, on an assessment of the probable range of the power potential of geothermal fields as inferred from a frequency distribution analysis of fields already under development throughout the world. Depending on the reservoir containing dry steam or water, and its location in a region of groundwater recharge or discharge, average power potentials can be expected to range from 36 to 3360 MW.     

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.     

Thermodynamic evolution of the Los Azufres,Mexico, geothermal reservoir from 1982 to 2002

An investigation has been made of the response of the Los Azufres geothermal reservoir to 20 years of development, beginning in 1982. The simulator WELFLO was used to characterize the thermodynamic conditions of the reservoir fluids. The first response to exploitation consisted of a decrease in pressure and an increase in enthalpy. Small decreases in reservoir pressure associated with large increases in fluid enthalpy characterize the long-term response in the northern production area. In the southern production area, long-term changes include decreases in pressure and mass flow rate, increases in steam production and, in wells affected by injection, increases in both pressure and total mass flow rate. These changes reflect the effects of boiling, cooling and fluid mixing, processes resulting from large-scale fluid production.