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.     

Study on the flow production characteristics of deep geothermal wells

This paper describes a study on the potential flow production characteristics of three non-producing, deep (average depth 4000 m) geothermal wells in the Cerro Prieto geothermal field. The expected production characteristics of these wells were computed in order to determine whether their inability to sustain flow was due to: (1) heat loss effects in the well; (2) the influence of casing diameters; (3) transient temperature effects during the first days of well discharge, and/or (4) the effects of secondary low-enthalpy inflows. For the study, the conservation equations of mass, momentum and energy for two-phase homogeneous flow were solved for the wellbore, since homogeneous flow provides the simplest technique for analyzing two-phase flows when the flow patterns are not well established. The formation temperature distribution was computed assuming radial transient heat conduction. The numerical model was validated by comparison with analytical solutions and with measured pressure and temperature profiles of well H-17 from the Los Humeros geothermal field, Mexico. It was found that the wells should have sustained production. The early heat losses were so large that the flow needed to be induced, and flow will be sustained only after a few days of induced discharge. For well M-202, the analysis suggests that the inflow of secondary colder fluids was responsible for stopping the flow in this well.     

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 chloride method for the determination of the enthalpy of steam/water mixtures discharged from geothermal wells

Chloride concentrations and pressures (or temperatures) in two points of a geothermal test line have been used to determine the enthalpy of the steam/water mixture discharged from Nisyros I geothermal well (Greece). The results are in agreement with those obtained by the Russell James method. The chloride method described in this paper is fast, requires cheap, portable equipment and is thus particularly suitable for short-term well testing during drilling or immediately after well completion, especially in steam-rich mixtures.     

Low enthalpy geothermal project in Zambia

The project, financed by the Italian Ministry of Foreign Affairs (MAE), is to implement the installation of two organic Rankine cycle (ORC) turbogenerators in remote, rural areas of Zambia.The Italian Government grant amounts to 2,000,000 US$. The Government of Zambia will bear all costs of the Zambian counterpart and will provide the low voltage transmission line and distribution grid.     

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.     

Flow in geothermal hot water wells

One of the basic conditions for planning the production of free flowing and airlifted wells is the ability to model correctly the pressure losses incurred in wells with single- and two-phase flows. Based on theories developed in the petroleum industry, as well as the data obtained in the course of Hungarian air-lift water pump tests, the present paper gives a survey of the laws governing vertical flow in high-yield hot water wells of small gas and air content. An explanation is given for the fundamental disagreement between the relationships used to simulate pressure losses in short and long tubing. Modes for calculating the pressure profiles of wells are included.     

Sulfide scaling in low enthalpy geothermal environments: A survey

A review of the sulfide scaling phenomena in low-temperature environments is presented. While high-temperature fluids tend to deposit metal sulfides because of their high concentrations of dissolved metals and variations of temperature, pressure and fluid chemistry, low temperature media are characterized by very low metal content but much higher dissolved sulfide.In the case of the geothermal wells of the Paris Basin, detailed studies demonstrate that the relatively large concentrations of chloride and dissolved sulfide are responsible for corrosion and consequent formation of iron sulfide scale composed of mackinawite, pyrite and pyrrhotite. The effects of the exploitation scheme are far less important than the corrosion of the casings. The low-enthalpy fluids that do not originate from sedimentary aquifers (such as in Iceland and Bulgaria), have a limited corrosion potential, and the thin sulfide film that appears may prevent the progress of corrosion.     

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.     

An instrument for measuring the gas concentrations in geothermal wells

An instrument and technique for rapid measurement and monitoring of gas concentrations in steam discharged from geothermal wells is described. The instrument directly measures the partial pressure of non-condensible gases present in a sample of the total discharge, at a given temperature and a given total pressure. This is achieved by deducting the vapour pressure of distilled water, at the same temperature. from the combined pressure of the water vapour and gases present. The volume of gas in the discharge can be obtained from the differential pressure by application of the ideal gas law with appropriate data from the steam tables. The percentage by weight of gas in the fluids can also be obtained if the chemical composition of the gases present is known. The results compare favourably with other methods.     

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.     

Characterization of clay scales forming in Philippine geothermal wells

Smectite scales occur in 24 out of 36 blocked wells located in Tongonan, Palinpinon and Bacon-Manito. These comprise 2–85% of the well scales and form at depths of 36–2620 m. where measured and fluid inclusion temperatures are 41–320°C. Most, however, occur below the production casing shoe where temperatures are 230°C, often at depths coinciding with aquifers. The clay scales are compositionally and structurally different from the bentonite used in drilling, which is essentially a sodium-rich montmorillonite. The clay deposits are expanding, generally disordered, and combine the characteristics of a montmorillonite, saponite and vermiculite in terms of reaction to cationic exchange treatments, structure and composition. Six types of clay scales were identified, but the predominant one, comprising 60–100% of the clay deposits in a well, is Mg- and Fe-rich and referred to as a vermiculitic species. The crystallinity, degree of disorder, textures, optical characteristics, structure and relative amounts of structural Al, Mg and Fe vary with time, temperature and fluid composition, but not with depth and measured pressure. Despite its variance from bentonite characteristics one of the dominant suggested mechanisms for clay scale formation uses the drilling mud in the well as a substrate, from which the Mg and Fe-rich clay evolves. Another important possible mechanism for formation of the clay scale is the precipitation of the clays from hydrothermal fluids, wherein Mg and Al appear to be the limiting elements. Other, less important, mechanisms of formation are: alteration of chloritized cuttings in the well to the vermiculitic species and the probable introduction of smectite from the formation.     

Pressure-transient behavior during cold water injection into geothermal wells

During injection testing, the pressures in geothermal wells used for reinjection sometimes initially increase but then decline as injection continues. Injection tests carried out at the Yutsubo geothermal field in Kyushu, Japan, exhibit this peculiar behavior. During injection testing of Yutsubo well YT-2, the observed downhole pressures eventually began to decline despite sustained injection rates. We have carried out numerical simulation studies using a radial flow model to examine this behavior. Double porosity (MINC) models are adopted, in which the fracture porosity increases as a result of both cooling and pressure build-up, and the permeability is very sensitive to porosity changes. This extreme sensitivity of fracture permeability to porosity appears to be necessary to reproduce the late-time pressure decline, and suggests that fractures were opened by injection-induced cooling near the well.     

Analysis of casing failures of deep geothermal wells in Greece

The specific characteristics of the Greek geothermal fields on the volcanic islands of Milos and Nisyros are described, with emphasis on the characteristics affecting well casing. Overpressure, high temperatures and highly saline brines are typical features of the Greek fields. A new treatment of the different load conditions along the casing string in proposed that takes into consideration the possible formation overpressure, the axial thermal stresses and their effect on burst and collapse resistance. This treatment is further applied to some typical wells that have experienced casing failure.     

Hydrothermal alteration minerals in Aluto Langano geothermal wells, Ethiopia

Aluto Langano geothermal field is characterized by alteration mineral assemblages of calcite, quartz, chlorite, undifferentiated clays, hematite, biotite and epidote. The presence of garnet and sphene is also reported for one of the wells. Measured temperature for the reservoir is above 300°C. Permeability of the reservoir is highly influenced by the deposition of hydrothermal minerals.