Geothermal Production Wells Research Articles

Prediction of clog time of amorphous silica in the pipes of the wells from Los Humeros geothermal field – A new geochemical model along with a computational algorithm

The primary objective of this article is to introduce a physicochemical model along with a computational algorithm that is valuable for quantifying the occurrence of silica scaling and fouling within geothermal production wells. This model utilizes two-phase flow parameters, temperature and pressure profiles, as well as brine composition to forecast the timing of production pipe constriction caused by amorphous silica deposition over time. The model is applicable to systems with temperatures ranging from 270°C to 305°C, vapor pressures ranging from 55 to 93 bars, and enthalpies ranging from 1189 kJ/kg to 1400 kJ/kg. The predictive model was calibrated using data gathered from six distinct wells within the Los Humeros geothermal field in Mexico, and subsequently, it was applied to forecast outcomes in two additional wells. When the conditions of temperature, pressure, and enthalpy remain relatively constant throughout the production process, the model developed in this study accurately predicts the times at which a certain percentage of pipe sealing occurs in wells with high volumetric fractions of liquid (εl>0.5) and temperatures between 270°C and 305°C. The prediction error is less than 3%.

The Exergy and Energy Analysis of Change Phase Steam in Patuha Geothermal Well Production

A Geothermal Power Plant Company in West Java Indonesia have capacity around 55MW. It company have increased the well production quality to keep up the Indonesia's energy demands. The power plant has been operated around ten years, it has ten geothermal production wells and two injection wells. The either of it wells has an abnormality. It wells has water dominated condition so, the separator equipment is required to separate the steam from water and vapor. Previous study indicated that subsurface water entering the broken reservoir pipe. That leaks in reservoir pipe was repaired in 2020. The impact after the pipe have been repaired it made the fluid characteristic of reservoir be vapor dominated. After that a separator as a steam separator is no longer required. The study compares the value energy and exergy power of the power plant. The calculations are performed from the well reservoir and all power plant equipment. According to the results of the energy study, the plant's energy efficiency after the transformation increased by 0.03%, from 19.48% to 19.52%. The exergy value of the brine in either wells (separated steam cycle) is also known to be 2.512,52 kW, beyond which the value will become zero.

Characterising and predicting low discharge pressure events in less permeable geothermal production wells

The heterogeneous nature of many geothermal resources can be observed in existing geothermal operations when an anomalously low permeability well is drilled amidst high permeability wells. This study looked at a low permeability, weak production well that shared a two-phase pipeline header and separator vessel with several stronger production wells on the same well pad. The utilisation history of the well under investigation had shown episodes wherein the well stopped producing due to its low discharge pressure, even at throttled conditions. Such episodes usually lead to additional costs for the power plant operator as the well has to be stimulated to reinitiate its discharge. Real-time wellhead pressure data was used in this work to develop classification models that would reliably predict the occurrence of such low discharge pressure events in the future. A good performing model would allow the operator to perform intervening measures to prevent the complete collapse of the well, thereby minimising well downtime and avoiding unnecessary costs.A workflow based on systematic time-series feature engineering was applied to characterise the dynamics of the wellhead pressure trend. Machine learning models built using the relevant time-series features were able to reliably predict the occurrence of the low discharge pressure events in the target well.A numerical reservoir model was developed and calibrated to match the transient pressure response and production history of the target well. The model results showed significant fluctuations in the fractional dimension parameter before and after the low discharge pressure events.

Modeling of two-phase flow of high temperature geothermal production wells in the Yangbajing geothermal field, Tibet

Two-phase flow (flow of water in both liquid and gas phase, containing a non-condensable gas such as CO2) in the wellbore is one of the most important processes in the production performance and wellbore scaling evaluations of high temperature geothermal wells. This paper first describes the discharge tests in the Yangbajing geothermal field, Tibet. Next, a simple model for governing the two-phase flow in the presence of CO2 in the wellbore is constructed and a robust calculation method is presented. The model is applied to three production wells in the Yangbajing geothermal field. The results show that the velocity difference between the gas and liquid phase should be included in the model. Ignoring this difference (i.e., homogeneous model) would result in a significant deviation between measurements and calculations. A drift flux model (DFM) describes the velocity difference, where the specifics of the particular model can have significant effects on the results for the pressure and temperature profiles in the wellbore. Three commonly used DFMs are compared to estimate their performance. The calculated wellhead pressure and temperature are in the range of 2,3 bar and 125°C–135°C for all three wells at a production rate of about 20 kg/s. The estimated wellhead gas mass fraction is between 3% and 8%. Considering CO2 content, three different scenarios were evaluated, although the effect on the pressure and temperature profiles were limited. However, CO2 content has a much more significant effect on the flash depth, which is an important parameter for the estimation of calcite scaling.

A Practical Field Test and Simulation Procedure for Prediction of Scaling in Geothermal Wells Containing Noncondensable Gases

Scaling in a hydrothermal type of geothermal well reduces or interrupts the production of geothermal energy. Calcite is one of the most common scales in geothermal wells. The reason for its formation in geothermal production wells is clear. The flowing up of geothermal water causes a change in the pressure and temperature, which results in the escape of CO2 gas from the geothermal water, causing a rise in pH and the supersaturation of CaCO3 in the solution. To predict scaling in a new geothermal well, conditional data for geothermal well simulations are required. It is important to determine what field data are needed and how to obtain them. It is necessary to deal with some parameters that are hard to measure and that have not been described in detail in the existing literature. In this study, a two-phase flow model and a chemical reaction equilibrium model are integrated to simulate the scaling process in production wells. Based on the simulation, a comprehensive and practical approach, including a novel noncondensable gas content measurement method, is applied to predict the depth of the first gas bubble using simple field test data and does not require reservoir permeability and earth conductivity. The result shows good agreement with the location of scaling detected in the field.

Equivalent imputation methodology for handling missing data in compositional geochemical databases of geothermal fluids

Chemical, isotopic and thermal signatures, space-temporal concentration patterns, and geochemical conceptual models are used to gather insights of the internal structure of geothermal reservoirs. These investigations rely on historical and latest data dealing with the composition of the geothermal fluids discharged from either producer wells or surface manifestations. The creation of a compositional database is based on information compiled from the sample collection and handling, and the quantification results of chemical analyses. Sometimes, this integrated task may result incomplete due to some technical issues that might generate information gaps or missing data which are struggle to control in the field and lab works. To solve the low-quality data, the missing concentration values or the significant information gaps of geochemical databases, the use of machine learning techniques has been explored in the present study. Equivalent imputation models were proposed, for the first time, to rebuild a world solute-based geochemical database which exhibited some gaps and missing data in the records of the chemical composition of fluids from geothermal production wells. The application of these models enabled that missing records to be successfully recovered by using single and multiple imputation algorithms. The imputation models were efficiently evaluated by applying statistical equivalence metrics carried out between imputed and observed (measured) data. To avoid an arbitrary use of these models, and to keep the original geochemical signatures of the fluid composition patterns, the imputed results were assessed by conventional geochemistry tools (e.g., charge unbalances, fluid classification, and some water–rock interaction footprints). Results show that single imputation algorithms that model locally provide equivalent imputation values with acceptable confidence, not only in terms of statistical metrics but also improving charge unbalances in chemical analyses of anomalous samples without altering the original water–rock interaction signatures of the geothermal systems under evaluation. In contrast, multiple imputation algorithms only tend to obtain better charge unbalances by altering the original water–rock interaction signatures. The novel use of the imputation techniques for filling geochemical databases and recovering missing data will allow a better interpretation of hydrogeochemistry modeling in future geothermal exploration and exploitation studies.

Characterising and Predicting Low Discharge Pressure Events in Less Permeable Geothermal Production Wells

Characterising and Predicting Low Discharge Pressure Events in Less Permeable Geothermal Production Wells

Numerical analysis and optimization of the performance of CO2-Plume Geothermal (CPG) production wells and implications for electric power generation

CO2-Plume Geothermal (CPG) power plants can produce heat and/or electric power. One of the most important parameters for the design of a CPG system is the CO2 mass flowrate. Firstly, the flowrate determines the power generated. Secondly, the flowrate has a significant effect on the fluid pressure drawdown in the geologic reservoir at the production well inlet. This pressure drawdown is important because it can lead to water flow in the reservoir towards and into the borehole. Thirdly, the CO2 flowrate directly affects the two-phase (CO2 and water) flow regime within the production well. An annular flow regime, dominated by the flow of the CO2 phase in the well, is favorable to increase CPG efficiency. Thus, flowrate optimizations of CPG systems need to honor all of the above processes. We investigate the effects of various operational parameters (maximum flowrate, admissible reservoir-pressure drawdown, borehole diameter) and reservoir parameters (permeability anisotropy and relative permeability curves) on the CO2 and water flow regime in the production well and on the power generation of a CPG system. We use a numerical modeling approach that couples the reservoir processes with the well and power plant systems. Our results show that water accumulation in the CPG vertical production well can occur. However, with proper CPG system design, it is possible to prevent such water accumulation in the production well and to maximize CPG electric power output.

Geothermal Boreholes in Poland—Overview of the Current State of Knowledge

Geothermal energy can be useful after extraction from geothermal wells, borehole heat exchangers and/or natural sources. Types of geothermal boreholes are geothermal wells (for geothermal water production and injection) and borehole heat exchangers (for heat exchange with the ground without mass transfer). The purpose of geothermal production wells is to harvest the geothermal water present in the aquifer. They often involve a pumping chamber. Geothermal injection wells are used for injecting back the produced geothermal water into the aquifer, having harvested the energy contained within. The paper presents the parameters of geothermal boreholes in Poland (geothermal wells and borehole heat exchangers). The definitions of geothermal boreholes, geothermal wells and borehole heat exchangers were ordered. The dates of construction, depth, purposes, spatial orientation, materials used in the construction of geothermal boreholes for casing pipes, method of water production and type of closure for the boreholes are presented. Additionally, production boreholes are presented along with their efficiency and the temperature of produced water measured at the head. Borehole heat exchangers of different designs are presented in the paper. Only 19 boreholes were created at the Laboratory of Geoenergetics at the Faculty of Drilling, Oil and Gas, AGH University of Science and Technology in Krakow; however, it is a globally unique collection of borehole heat exchangers, each of which has a different design for identical geological conditions: heat exchanger pipe configuration, seal/filling and shank spacing are variable. Using these boreholes, the operating parameters for different designs are tested. The laboratory system is also used to provide heat and cold for two university buildings. Two coefficients, which separately characterize geothermal boreholes (wells and borehole heat exchangers) are described in the paper.

Design of Electrical Submersible Pump system in geothermal wells: A case study from West Anatolia, Turkey

Geothermal is defined as one of the renewable and sustainable energy sources. The sustainability of geothermal wells is highly dependent on the pressure drive mechanism and non-condensable gas (NCG) content of the produced geothermal fluid. Pressure decline and decline of non-condensable gases are commonly observed in geothermal production wells, which is unfavorable for wells’ lifetime. Electrical Submersible Pumps (ESP) are one of the solutions for extending the lifetime of geothermal wells. In this study, the application of ESP in a geothermal well was designed and simulated. The case study well is located in one of the most exploited geothermal fields in Western Turkey: the Alasehir geothermal field. ESP design is performed by using the codes constructed in PYTHON in this study. The sensitivity of production profiles of the well is simulated by using a wellbore simulation program called WELBOR. Sensitivity studies are conducted for different sizes of production tubing (5, 6, 65/8, and 7 inches), setting depths (500, 600, 700 m), and flow rates (85, 150, 180, 250, and 275 ton/hour). The optimum ESP design conditions are determined by considering pump consumption, flashing depth, wellhead flowing pressure, and production rate. Finally, it was found that ESP design will increase the production rate of the case study well by 165 tons/hour. Furthermore, the proposed ESP will make a profit for at least 8 months, according to the economic analysis in this study.

Improvement of loads calculation of the perforated liner in a geothermal production well

The integrity of perforated liners is crucial in geothermal production wells and this should be well designed according to correct standards. The available geothermal standards, such as the New Zealand Standard, NZS2403:2015, provides a general but less detailed discussion of geothermal perforated liner system design considerations.This research aimed to improve the limitations of NZS2403:2015 by calculating and evaluating the dynamic loads of perforated liner during four phases: borehole drilling and liner installation, flow testing, stand-alone production, and production during the injection operation. The data from production well HCE29 and injection well HCE29A, Dieng Field, Indonesia, were used in this research. In every phase, perforated liner yield strength changed, and load mechanisms varied between 1.3-2.7 and 1.6–4.6 times of the actual liner burst and collapse strength, respectively, and these lead to perforated liner failure and its lifetime reduction. The calculation results, combined with the significant production decline and production casing failure, indicated the perforated liner failure. The results also showed when it was mandatory to revise operation at later phases and to conduct appropriate maintenance according to actual perforated liner condition. Furthermore, this study can be used as a basis of future wells design and operation related to perforated liner.

Effect of high salinity and temperature on water–volcanic rock interaction

In order to understand the processes occurring in natural hydrothermal systems, it was carried out a series of water–volcanic rock interaction studies in the laboratory and an intermediate volcanic rock samples from geothermal production wells in Tuzla geothermal field (TGF) in western Turkey. A high-pressure autoclave was used to conduct water–rock interaction experiments under similar conditions of the field. Rainwater and seawater were treated with volcanic rocks at 140 °C (reservoir temperature) and 4.5 bar pressure. The change in the ionic content of the resulting fluids was examined in terms of the type of volcanic rocks and mineral saturation index. The results indicate that talc and diopside minerals in geothermal systems may cause scaling at high temperatures depending on the geothermal fluid and pH.

Evaluation of the interactions of arsenic (As), boron (B), and lead (Pb) from geothermal production wells with agricultural soils

Geothermal energy is a low-pollution energy source. However, air, soil, and water near geothermal plants may be affected by their operation. One of the largest geothermal energy sources in the world, Cerro Prieto, has a capacity of 720 MW and is located in northwest Mexico near an agricultural area. The abstracted geothermal fluids, which are enriched with arsenic (As), boron (B), lead (Pb), cadmium (Cd), and other heavy metals, are either reinjected into the aquifer or sent to an evaporation pond located in the geothermal plant. Because spills have occurred in other geothermal zones, it is important to evaluate the effect of those contaminants on the soils of the surrounding area and their possible infiltration into shallow groundwater. To that aim, soils (one chromic Vertisol and two calcic Regosols) from three sites near the Cerro Prieto Geothermal Plant were sampled to evaluate their behavior regarding As, Pb, and B retention. Batch experiments were carried out using the soils as the sorbent and geothermal water from three geothermal production wells as the sorbate. Raw water concentrations in each well were as follows: As: 0.2442 mg/L, 0.2774 mg/L, and mg/L; B: 18.409 mg/L, 13.5075 mg/L, and 16.646 mg/L; and Pb: 0.22 mg/L, 0.13 mg/L, and 0.26 mg/L. The physicochemical characteristics of the soils were determined and compared to the experimental results. A good adjustment of the chromic Vertisol sample to Freundlich isotherms was observed for As (r2 > 0.9), followed by Pb (r2 = 0.61), and B (r2 > 0.5). As retention also showed a good adjustment to the Langmuir model (r2 > 0.9). The retention followed the order Pb >As ≫B in one of the two calcic Regosols, while the other only retained Pb ≫ As. Cationic exchange capacity; clay minerals; carbonate; organic matter; and iron, aluminum, and manganese amorphous and crystalline oxides influenced the soils’ retention capacities. Irrigation with geothermal water could not imply a toxicity risk to plants grown in the chromic Vertisol soil due to its high Pb and As sorption capacity. Pb concentration could not be a toxicity issue in the calcic Regosols for the same reason, but As and B could be. B would be a hazard to vegetables and water due to its low or lack of retention in the three soils and also for its possible infiltration into shallow groundwater used for irrigation in the area. This study highlights the importance of maintaining adequate operation and control of the disposal of geothermal fluids in geothermal plants.

Pinpointing Deep Geothermal Upflow in Zones of Complex Tectono‐Volcanic Degassing: New Insights from Aluto Volcano, Main Ethiopian Rift

AbstractActive rifts release large amounts of gases from deep sources to the atmosphere by advection and diffusion processes along permeable fracture zones. The objective of this study is to develop geothermal exploration concepts for areas with little or no hydrothermal surface expressions suitable for fluid sampling and analyses (e.g., hot springs, geysers, and fumaroles). In such areas, soil gas surveys can complement established geophysical and geochemical exploration. We report CO2, 222Rn (radon), and 220Rn (thoron) emission data and ground temperatures from the Aluto volcanic complex in the Main Ethiopian Rift to improve understanding of tectonic and volcanic controls on the existing geothermal system. This suite of gas emission measurements allows us to identify major, deep‐rooted permeable structures with active fluid circulation and identify suitable drilling targets for geothermal production wells on Aluto. We show that significant differences in gas signatures (i.e., efflux and spatial pattern) can be used to identify predominantly volcanically and/or tectonically influenced compartments. Major gas emissions indicate significant fluid circulation at depth, which is typical for magmatic systems. Such high gas emissions have been observed in areas affected by major tectonic structures interacting with magmatic bodies at depth (tectono‐volcanic). Predominantly fault‐controlled sectors also show hydrothermal fluid circulation, but to a lower extent compared to tectono‐volcanic sectors. Within the Aluto volcanic complex, geothermal production wells mainly target such fault‐controlled domains, whereas results of the study indicate strongest fluid circulation in tectono‐volcanic sectors. This result should be considered for the future exploration and development strategy of the site.

Heat losses associated with the upward flow of air, water, CO2 in geothermal production wells

This present work reports on the heat losses associated with the upward flow of hot air, water, and CO2 in a production well. The heat losses considered here are the frictional heat loss, the gravitational heat loss, the Joule-Thomson effect, and the conduction heat loss to the surrounding rock formation. These heat losses were characterized using the mass flow rate, the operation time as variable parameters while the surrounding formation was assumed to have constant properties with a linear geothermal gradient. The results show that, the frictional heat loss was small while the influences of the others depended on the flow conditions and the operation time. For water, the Joule-Thomson effect was a heating effect and its magnitude was comparable to that due to the gravitational effect. The conduction heat loss was dominant for all situations. For air as well as CO2, the Joule-Thomson effects were the cooling effects. For low mass flow rates, the conduction heat loss was the dominant heat loss during the initial stages of the operation and the combined heat losses due to the gravitational and the Joule-Thomson effects became dominant during the later times. For high mass flow rates, throughout the operation life time, the gravitational and the Joule-Thomson effects were the dominant heat losses that control the temperature of air and CO2 in the production well.

The study on transient behaviors of two-phase flow in a geothermal production well for a short period of continuous measurement

Deliverability of geothermal production wells is evaluated by measuring wellhead pressure versus mass flow rate at several wellhead pressures. Controlling a main valve of the well results in the changes in wellhead pressure and mass flow rate. However, duration required for these variables to stabilize depends on flowing behavior of steam-water two-phase fluid in the wellbore. Fluid stabilization seems to be an important parameter, mainly for short period of measurement. In addition, the manner of controlling valve operation whether it starts from the well in full-closed condition or in full-opened condition seems to affect this flow stabilization. Thus, the experiment was carried out for a production well 2H-21 at Hatchobaru geothermal field, Japan to verify these hypotheses. The measurement apparatus consisted of pressure sensors, water level sensor and a PC to store the measured data. The data were recorded every second for the pressures at upstream of the orifices, the pressure drops at the orifices, the pressures at the separators, and the water level at weir box. The degree of valve opening was changed three times during the measurement. Then, the duration of respective wellhead valve operation for controlling flow rate affecting the flow stabilization in wellbore was observed. It was found that quick valve operation caused longer period for stabilization and produced impulse response of flow rate soon after the valve operation stopped. The fluid flow required about 7min to stabilize for closing valve operation whereas it needed longer than 20min to reach the stable condition for opening valve operation. After changing the wellhead pressure, the steam flow rate stabilized quickly while the water flow rate showed a gradual change.

Los campos geotérmicos de Centroamérica: influencia del proceso de subducción sobre su composición volátil

Se recolectaron gases de pozos de produccion geotermica en Costa Rica, Nicaragua y El Salvador, y se analizo su contenido de gases mayores y gases nobles, con el fin de determinar las fuentes de los vo-latiles dentro de los sistemas volcanicos activos de America Central. Las razones de 3He/4He indican que elmanto superior es la fuente principal de helio, con cantidades menores provenientes de la corteza en el campode Berlin en El Salvador. El campo de Miravalles en Costa Rica se caracteriza por altas relaciones de CO2/3He,las cuales probablemente son producto del aumento de devolatilizacion en la parte suroeste de la placa del Coco. El campo de Momotombo en Nicaragua demuestra un alto contenido de nitrogeno y metano, ambos probablemente el producto de contribuciones locales dentro de la depresion de Nicaragua. El arco volcanico de America Central se caracteriza, por lo general, por altas razones de N2/3He y 3He/4He, una combinacion poco usualpara los sistemas de arco de islas. Los resultados demuestran que un buen entendimiento de la sistematica delos gases en los sistemas geotermicos, durante su inicio y su produccion comercial, es esencial para la planificacion y mantenimiento de este importante recurso.

Theoretical Study on Steam and Water Flow Rates from Geothermal Production Wells

Theoretical Study on Steam and Water Flow Rates from Geothermal Production Wells

Elevated carbon dioxide flux at the Dixie Valley geothermal field, Nevada; relations between surface phenomena and the geothermal reservoir

In the later part of the 1990s, a large die-off of desert shrubs occurred over an approximately 1 km 2 area in the northwestern section of the Dixie Valley (DV) geothermal field. This paper reports results from accumulation-chamber measurements of soil CO 2 flux from locations in the dead zone and stable isotope and chemical data on fluids from fumaroles, shallow wells, and geothermal production wells within and adjacent to the dead zone. A cumulative probability plot shows three types of flux sites within the dead zone: locations with a normal background CO 2 flux (7 g m −2 day −1); moderate flux sites displaying “excess” geothermal flux; and high flux sites near young vents and fumaroles. A maximum CO 2 flux of 570 g m −2 day −1 was measured at a location adjacent to a fumarole. Using statistical methods appropriate for lognormally distributed populations of data, estimates of the geothermal flux range from 7.5 t day −1 from a 0.14-km 2 site near the Stillwater Fault to 0.1 t day −1 from a 0.01-km 2 location of steaming ground on the valley floor. Anomalous CO 2 flux is positively correlated with shallow temperature anomalies. The anomalous flux associated with the entire dead zone area declined about 35% over a 6-month period. The decline was most notable at a hot zone located on an alluvial fan and in the SG located on the valley floor. Gas geochemistry indicates that older established fumaroles along the Stillwater Fault and a 2-year-old vent in the lower section of the dead zone discharge a mixture of geothermal gases and air or gases from air-saturated meteoric water (ASMW). Stable isotope data indicate that steam from the smaller fumaroles is produced by ≈100°C boiling of these mixed fluids and reservoir fluid. Steam from the Senator fumarole (SF) and from shallow wells penetrating the dead zone are probably derived by 140°C to 160°C boiling of reservoir fluid. Carbon-13 isotope data suggest that the reservoir CO 2 is produced mainly by thermal decarbonation of hydrothermal calcite in veins that cut reservoir rocks. Formation of the dead zone is linked to the reservoir pressure decline caused by continuous reservoir drawdown from 1986 to present. These reservoir changes have restricted flow and induced boiling in a subsurface hydrothermal outflow plume extending from the Stillwater Fault southeast toward the DV floor. We estimate that maximum CO 2 flux in the upflow zone along the Stillwater Fault in 1998 was roughly seven to eight times greater than the pre-production flux in 1986. The eventual decline in CO 2 flux reflects the drying out of the outflow plume.

Hydrothermal Minerals and Precious Metals in the Broadlands-Ohaaki Geothermal System: Implications for Understanding Low-Sulfidation Epithermal Environments

The Broadlands-Ohaaki geothermal system is a boiling hydrothermal system hosted by a sequence of Quaternary felsic volcanic rocks and Mesozoic metasediments. More than 50 wells have been drilled (400 to >2,600 m deep) to assess the geothermal potential for the production of electricity. Fluids and precipitates sampled from wells, along with descriptions of the alteration minerals in more than 500 drill cores, provide a three-dimensional picture of the distribution of fluid types and secondary minerals. Interpretation of these features and the distribution of gold and silver highlight the relationship between alteration and mineralization in an active, low-sulfidation epithermal environment. Quartz, illite, K feldspar (adularia), albite, chlorite, calcite, and pyrite are the main hydrothermal minerals that occur in the deep central upflow zone at >250°C and> 600 m depth. These minerals form through recrystallization of the volcanic host rocks and incorporation of H 2 O, CO 2 , and H 2 S in the presence of a deeply derived chloride water containing ~1,000 mg/kg Cl and ~26,400 mg/kg CO 2 . At the same time, and on the periphery of the upflow zone, illite, smectite, calcite, and siderite form through hydrolitic alteration in the presence of CO 2 -rich steam-heated waters that contain 2 . Upward and outward from the deep central upflow zone, mineral patterns reflect the shift from rock-dominated to fluid-dominated alteration and the prevailing influence of boiling, mixing, and cooling on fluid-mineral equilibria. Accordingly, the abundance of quartz and K feldspar increase toward the upflow zone, whereas clay abundance increases toward the margin of the upflow zone (with smectite dominating at 200°C); the abundance of chlorite, pyrite, and calcite varies here, but albite is absent. Geothermal production wells with high fluid fluxes are the main sites of precious-metal mineralization. The deep chloride water (with or without minor amounts of vapor) enters the well at depths >500 m and undergoes a pressure drop that causes boiling. As a result, precious metals precipitate and accumulate as scales on back-pressure plates or as detritus in surface weir boxes; these deposits contain 1,000 mg/kg Au, 10,000 mg/kg Ag and ~10 to ~1,000 mg/kg As and Sb, each. Within production wells, platy calcite deposits as a scale at the site of first boiling near the fluid feed point, while crustiform-colloform-banded amorphous silica deposits in surface pipe work. By contrast, the hydrothermally altered host rocks contain low concentrations of gold, ranging from <0.01 to 1.0 mg/kg Au (68 analyses), and these correlate positively with arsenic (<100 to ~5,000 mg/kg) and antimony ( Reaction path modeling using SOLVEQ and CHILLER shows that calcite, K feldspar, gold, and amorphous silica deposit in sequence from a chloride water that cools along an adiabatic boiling path (300° to 100°C), analogous to fluid flow in a production well. By contrast, calcite, quartz, K mica, and pyrite deposit from a chloride water that cools due to mixing with CO 2 -rich steam-heated waters; dilution prevents precipitation of precious metals. Thus field observations and reaction path modeling demonstrate that boiling is the main process influencing the deposition of precious metals. The results of this study show how peripheral hydrolytic alteration by CO 2 -rich steam-heated waters relate to propylitic and potassic alteration by chloride waters in the epithermal environment of a hydrothermal system. Both the distribution of alteration mineral assemblages associated with the different water types and the broad-scale distribution of temperature-sensitive smectite and illite reflect the location of the upflow zone. On a local scale, the occurrence of platy calcite, crustiform-colloform silica, and K feldspar in veins indicates the existence of boiling conditions conducive to precious-metal deposition.