Parabolic troughs to increase the geothermal wells flow enthalpy

This work investigates the feasibility of using parabolic trough solar field to increase the enthalpy from geothermal wells’ flow in order to increase the steam tons; in addition, it is possible to prevent silica deposition in the geothermal process. The high levels of irradiance in Northwestern Mexico make it possible to integrate a solar-geothermal hybrid system that uses two energy resources to provide steam for the geothermal cycle, like the Cerro Prieto geothermal field. The plant consists of a geothermal well, a parabolic trough solar field in series, flash separator, steam turbine and condenser. Well “408” of Cerro Prieto IV has enthalpy of 1566 kJ/kg and its quality must be increased by 10 points, which requires a Δh of 194.4 kJ/kg. Under these considerations the parabolic troughs area required will be 9250 m², with a flow of 92.4 tons per hour (25.67 kg/s). The solar field orientation is a N–S parabolic trough concentrator. The silica content in the Cerro Prieto geothermal brine causes problems for scaling at the power facility, so scale controls must be considered.     

Gases in steam from Cerro Prieto geothermal wells with a discussion of steam/gas ratio measurements

As part of a joint USGS-CFE geochemical study of Cerro Prieto, steam samples were collected for gas analyses in April, 1977. Analyses of the major gas components of the steam were made by wet chemistry (for H₂O,CO₂,H₂S and NH₃) and by gas chromatography (He,H₂,Ar,O₂,N₂ and hydrocarbons).The hydrocarbon gases in Cerro Prieto steam closely resemble hydrocarbons in steam from Larderello, Italy and The Geysers, California which, although they are vapor-dominated rather than hot-water geothermal systems, also have sedimentary aquifer rocks. These sedimentary geothermal hydrocarbons are characterized by the presence of branched C_4–6 compounds and a lack of unsaturated compounds other than benzene. Relatively large amounts of benzene may be characteristic of high-temperature geothermal systems. All hydrocarbons in these gases other than methane most probably originate from the thermal metamorphosis of organic matter contained in the sediments.     

Preliminary investigations of scaling and corrosion in high enthalpy geothermal wells in Hungary

A solubility equilibrium program GEOPROF was applied to the determination of the bubble-point depth, pressure and temperature, as well as the partial pressure profiles of the gases CO₂, CH₄ and N₂ between the bubble-point depth and the wellhead, in two high enthalpy geothermal wells, NSZ-2 and FAB-4 in southern Hungary. The pH, alkalinity, total carbonates, and equilibrium solubility for CaCO₃, CaSO₄, BaSO₄, and SrSO₄ along the well depth profile in the Na–K–Mg–Ca–H–Ba–Sr–Cl–Br–SO₄–OH–HCO₃–CO₃–CO₂–H₂O system were also determined and the concentrations of Ca²⁺, Ba²⁺, Sr²⁺, H⁺, OH⁻, HCO₃⁻, CO₃²⁻, and H₂CO₃* were computed at the actual temperature and CO₂ pressure using the Davies and Pitzer activity calculation methods. The calculated amounts of CaCO₃ scaling along the wells and at the surface were used in estimating service life. The results for well FAB-4 contain high uncertainties because of the estimated gas separation analysis data.     

Ammonia and boric acid in steam and water. Experimental data from geothermal wells in the phlegrean fields, Naples, Italy

In this study the variation in volatility with temperature of boric acid and ammonia is examined through experimental tests on fluids taken from Mofete 3 and Mofete 9 wells (Phlegrean Fields, Naples). Some samples of the brine and of the steam have been collected after separation of the two phases through a cyclone separator at different temperatures in the range 122–166°C. The distribution of the two species between liquid and steam agrees with known experimental data.     

A new liquid hold-up correlation for geothermal wells

A new liquid hold-up correlation is devised for cased wellbores using high-quality discharge and downhole pressure and temperature data from flowing geothermal wells. The latter dataset encompasses a wide range of wellbore diameters, discharge rates and flowing enthalpies. The measured wellhead pressures for wells in the dataset display excellent agreement with the pressures computed by using the new hold-up correlation. Good agreement between the computed and observed spinner responses provides additional verification of the hold-up correlation. Finally, an example illustrating the use of the hold-up correlation to match downhole pressure and temperature profiles and well characteristic data is given.     

Processes controlling the isotopic composition of steam and water discharges from steam vents and steam-heated pools in geothermal areas

On the basis of isotopic analyses of steam and water discharges from the Wairakei, El Tatio and The Geysers geothermal areas, underground steam separation from the rising geothermal fluid appears to be adequately described in terms of a single-step process at temperatures of around 230°C. Absorption of this steam into nearly stagnant pools gives rise to the formation of isotopically enriched waters with compositions following a line with slope σ = ε′_D / (Δ_rw + ε′_¹⁸O − ε_{¹⁸O .230°C}), where ε′_D and ε′_¹⁸O are the effective kinetic isotope fractionation factors (50‰ and 16‰) for steam heated pools, ε_{¹⁸O .230°C} is the equilibrium fractionation factor for oxygen-18 at 230°C (2‰) and δ_rw is the difference in ¹⁸O-content of deep chloride and local groundwater (oxygen shift) respectively. The sulfate content of these pools is a function of the proportion of steam absorbed and its H₂S-content.     

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.     

A finite element model for high enthalpy two-phase flow in geothermal wellbores

This paper introduces a computational model for transient high enthalpy fluid flow through geothermal wellbores. The drift-flux model is utilized to formulate the physical behavior of fluid, and the constitutive relationships are described using relevant equations of state and empirical relationships. The governing equations are solved using the finite element method. All important physical phenomena and processes occurring along the wellbore, including buoyancy, phase change, compressibility, thermal interaction, wall friction and slip between phases are considered. Airlifting of water and air, initially existing in the wellbore before production, is also considered. During airlifting and early stages of production, two fluids exist along the wellbore: airlifted water-dry air fluid, and reservoir water-vapor fluid; giving rise to a discontinuity in thermodynamic properties between the two fluids. The discontinuity is modeled using the level-set method. Two numerical examples illustrating the computational capability and accuracy of the model are presented. The physical phenomena occurring during airlifting and production along the wellbore are highlighted.     

A novel method for measuring the coarse water droplets in wet steam flow in steam turbines

IntroductionWetoess in last stages Of LP steam ~es has beenlmown for a long time as a cause of efficiency loss anderosion of bladeslll. It comprises fine dIDPlets (also fogdrOPletS or Primal droplets) less than about 2 11 m indiameter and coarse water (big drOPlets or secondsdropletS), with diameter ~ about 10 11 m up tO sevedhundreds inicmns.For the fine droplet, the global method, known aSthe extinction method has been chosen tO Obtain itS sizedistribution and concentalon (weiness). …     

A vessel for collecting subsurface water samples from geothermal drillholes

A vessel, featuring a break-off tube sealed in series with a non-return valve, for the collection of subsurface water and gas samples from geothermal drillholes is described. During downhole tests the vessel was successfully operated at temperatures up to 300°C and pressures up to 150 atm. It remained uncorroded by waters ranging in salinity from 1500 to 300,000 mg/kg sodium chloride, and with pH's as low as 2.5.     

Prediction of enthalpy production from fractured geothermal reservoirs using partitioning tracers

The goal of geothermal reservoir management is to economically recover as much energy as possible from the reservoir. This paper presents the development of improved techniques for monitoring and predicting two-phase mass and heat transport in fractures. Partitioning tracers can yield valuable, early information about fracture properties used within a semi-analytical approach for calculating enthalpy production from the fractured system. This is demonstrated for a synthetic model with a fracture network. The comparisons of the semi-analytical solutions with the simulation results show that the model predicts enthalpy production is easy, fast and ideally suited for sensitivity studies.     

A two-phase flow model for geothermal wells in the presence of non-condensable gas

Quantitative information on the phenomena occuring during the upward flow of a geothermal fluid in water-dominated wells is a requisite for designing the wellhead system and optimizing resource exploitation. The geothermal fluid consists, for the most part, of a two-phase mixture of water containing dissolved salts, steam and non-condensable gases. Various, closely interrelated effects must therefore be taken into consideration: pressure drop of the rising fluid; heat and mass transfer between the phases (due to evaporation and desorption); heat exchange with rock formations. Simultaneous application of the mass, energy and momentum equations results in a rather complex model that can be solved by a numerical computer program. The model described here accounts for the effects of: the presence of salts, when computing all the thermodynamic properties of the fluids, especially enthalpy, density, vapour pressure of the brine and superheated steam enthalpy; the presence of non-condensable gases, considering their deviations from ideal behaviour and their contribution to density; the heat exchange with the surrounding rock formations; variation in salt concentration along the flow-path; possible variation in pipe diameter and surface roughness with height. The simplified hypotheses adopted are: fluid flow is stationary; thermodynamic equilibrium conditions exist between the phases in each point along the well; the non-condensable gases are assumed to be CO₂; Henry's law is assumed valid and the quantity dissolved chemically is assumed negligible; the salts are assumed to be NaCl; the activity coefficients are unitary; liquid surface tension and viscosity values are assumed equal to those of pure water. Comparison of the results of the computer program and the experimental pressure and temperature profiles shows that these are in satisfactory agreement within a rather wide range of operative conditions. The noncondensable gases, even in very low concentrations, were shown to be of importance to these calculations. Once the experimental temperature and pressure profiles are known, the model will also permit calculation of the concentration of non-condensable gases. The most efficient of the two correlations used to compute pressure drop in two-phase regimes seems to be that devised by CISE^‡, which is based on global parameters not correlated to the different flow regimes.     

A simple method to compute hydrogen chloride abatement in geothermal power plants

One of the most important problems affecting geothermal fields is the abatement of hydrogen chloride contained in the vapor phase. If the chloride concentration exceeds a few ppmw, steam scrubbing must be provided in order to prevent corrosion of the gathering system and turbine failure. In some fields at Larderello, one of the most important geothermal areas in Italy, steam scrubbing is performed by injecting a caustic solution directly into the steam pipeline. In particular, the abatement system depends on absorption with chemical reaction of hydrogen chloride by a sodium hydroxide solution. This paper describes some of the Larderello power plant abatement systems and presents analyses of the different solutions adopted for this purpose. Finally, some simplified models for computing abatement efficiency in sprays, pipelines, static mixers, cyclones and vane type demisters—the equipment generally used in these plants—are proposed. The proposed models are able to predict the data measured in these power plants with good accuracy, and so they can be regarded as useful tools for designing new abatement systems or optimizing the existing ones.     

An upstream reboiler design for removal of noncondensable gases from geothermal steam for Kizildere geothermal power plant, Turkey

Kizildere geothermal power plant, Turkey, has an installed capacity of 20.4 MW_e. The field contains a high level of noncondensable gases (NCGs), changing from well to well, in amounts as high as 10–20% (with an average of 13% at the inlet of the turbine) by weight of steam. This amount of NCGs is being extracted from the condenser by gas compressors that consume about 17% of the total power production of the plant.An upstream reboiler process could be adopted to remove the NCGs from geothermal steam before they enter the turbine. Upstream reboilers therefore provide a cleaner and less corrosive steam supply to the turbine and condenser, increasing power generation performance for very high NCG contents.In this paper, upstream reboiler systems are investigated as an alternative to conventional gas extraction systems for Kizildere geothermal power plant. A vertical tube type reboiler has been designed and it is found that, as NCG content increases, the condensation heat transfer coefficient reduces steeply. It is concluded that vertical tube type reboilers are not efficient for fields that contain high levels of NCG (>15% by weight of steam). It is recommended that the use of direct contact reboilers be further investigated for this application.     

A preliminary study on boron removal from Kizildere/Turkey geothermal waste water

A study is made of boron removal from Kizildere/Turkey geothermal waste water using the boron selective resin Amberlite IRA 743. The resin in salt form has no boron removal capacity if the solution is unable to neutralize the released acid during the exhaustion period. In the case of Kizildere waste water, with a pH value of 8.9 and high HCO₃⁻ content, single stage regeneration is feasible and the exhausted resin can be regenerated economically. According to a preliminary estimate, the electricity production cost would rise by 1 ¢/kWh.