The Tianjin geothermal field (north-eastern China): Water chemistry and possible reservoir permeability reduction phenomena

Injection of spent (cooled) thermal fluids began in the Tianjin geothermal district, north-eastern China, at the end of the 1990s. Well injectivities declined after 3–4 years because of self-sealing processes that reduced reservoir permeability. The study focuses on the factors that may have caused the observed decrease in permeability, using chemical and isotopic data on fluids (water and gas) and mineral phases collected from production and injection wells. The results of data processing and interpretation indicate that (1) it is very unlikely that calcite and silica precipitation is taking place in the reservoir; (2) the Fe- and Zn-rich mineral phases (e.g. sulfides, hydroxides and silicates) show positive saturation indexes; (3) SEM and XRD analyses of filtered material reveal that the latter mineral phases are common; (4) visual observation of casings and surface installations, and of corrosion products, suggests that a poor quality steel was used in their manufacture; (5) significant quantities of solids (e.g. quartz and feldspar crystals) are carried by the geothermal fluid; (6) seasonal changes in fluid composition lead to a reduction in casing corrosion during the summer.     

Modeling studies: The Dachny geothermal reservoir, Kamchatka, Russia

The spatial distribution of pre-exploitation conditions (i.e. temperature and pressure distribution, liquid and vapor saturations, circulation characteristics of high-temperature fluids) in the Dachny field of the Mutnovsky hydrothermal system, obtained using a three-dimensional (3-D) mapping method, are revised on the basis of natural-state simulations performed using the computer code TOUGH2. A 3-D model of the natural-state conditions at the Dachny site was developed. The fine-tuning of the model was achieved by comparing computer results to the observations made in geothermal wells tested during 1983–1988. Also studied was the behavior of the field in response to different exploitation scenarios, assuming production from existing and additional wells needed to supply sufficient steam to a proposed 80 MW_e power plant.     

Acoustic emission measurement of geothermal reservoir cracks in Takinoue (Kakkonda) field, Japan

Crack extension caused by a shut-down operation in production wells (build-up test) in Takinoue (Kakkonda) geothermal power plant, Iwate Prefecture, Japan, has been successfully measured by an acceleration-sensitive long-distance acoustic emission measurement technique. The distribution and activity of subsurface cracks in a geothermal reservoir can be determined by the combined procedure of the pressure build-up test and AE measurements. The feasibility and importance of the measurements are demonstrated by AE data and their interpretation.     

Radon in geothermal reservoir engineering

Radon has been shown to have potential as a tracer in the study of geothermal reservoirs. The combination of its physical and nuclear properties and its emanation and transport in geothermal fluids makes several types of time and concentration measurements useful for studies of fracture permeability and porosity and flow characteristics in different types of geothermal resources. Radon concentrations in several vapor-dominated and liquid-dominated producing reservoirs have been measured by short-period sampling of wellhead geofluid followed by radon separation on a charcoal trap and counting in a sensitive low background ZnS detector. The results show significant variations not only among different fields, but also between neighbouring wells in a single field. Evidences for transport differences between the two major types of hydrothermal systems are noted.     

Production capacity estimation by reservoir numerical simulation of northwest (NW) Sabalan geothermal field, Iran

A three dimensional numerical model of the northwest (NW) Sabalan geothermal system was developed on the basis of the designed conceptual model from available field data. A numerical model of the reservoir was expressed with a grid system of a rectangular prism of 12 km × 8 km with 4.6 km height, giving a total area of 96 km². The model has 14 horizontal layers ranging in thickness between 100 m to 1000 m extending from a maximum of 3600 to −1000 m a.s.l. Fifteen rock types were used in the model to assign different horizontal permeabilities from 5.0 × 10⁻¹⁸ to 4.0 × 10⁻¹³ m² based on the conceptual model.Natural state modeling of the reservoir was performed, and the results indicated good agreements with measured temperature and pressure in wells. Numerical simulations were conducted for predicting reservoir performances by allocating production and reinjection wells at specified locations. Three different exploitation scenarios were examined for sustainability of reservoir for the next 30 years. Effects of reinjection location and required number of makeup wells to maintain the specified fluid production were evaluated. The results showed that reinjecting at Site B, immediate adjacent to production zone, is most effective for pressure maintenance of the system.On the base of existing data and assumptions the reservoir can sustain producing fluid equivalent to 50 MW_e of electricity for more than 30 years. The reservoir can produce the maximum amount of fluids equivalent to 90-100 MW_e for only 5 years, but the production capacity decreases to 50 MW_e after 20 years of operation because of pressure and enthalpy drop. The reservoir can sustain 50 MW_e over 100 years that can be defined as a sustainable production level of the field.     

Sustainability evaluation of limestone geothermal reservoirs with extended production histories in Beijing and Tianjin, China

Sustainable development of geothermal resources is the key to prolong the commercial life of geothermal fields. This paper uses two well-known geothermal fields in China, with long histories of production, to demonstrate how sustainability can be quantitatively evaluated, and to highlight the essential differences between sustainable and non-sustainable development schemes. After examining the complex nature of sustainability evaluation, fuzzy synthetic evaluation is applied as a tool for the quantitative sustainability rating of geothermal reservoirs. The evaluation procedures include systemic criteria selection, weighing, individual criterion evaluation and finally final multi-criterion decision analysis evaluation. The sustainability of a limestone reservoir in the urban sector of Tianjin is rated rather low because of insufficient levels of water injection. On the contrary, the carbonate rock reservoirs of the Xiaotangshan geothermal field in suburban Beijing have a strong sustainability rating, mainly due to high injection rates.     

The Podhale geothermal reservoir simulation for long-term sustainable production

The Podhale geothermal system, located in the southern, mountainous part of Poland, is the most valuable reservoir of geothermal waters discovered in the country to date and the one with the highest capacities in Central and Eastern Europe. Over 20 years of continuous operation has proved its stable operating parameters – a small drop in pressure and an unnoticeable temperature change. Production of over 500 m³/h of geothermal water with an 86 °C wellhead temperature is current practise, while drilling a new production well and reconstruction of an injection well allows for production that may significantly exceed 600 m³/h. To utilize these vast resources, a binary power cycle for electricity and heat production is considered by group of researchers. The results of numerical modelling of heat extraction from the Podhale reservoir are presented in the article as a preliminary step to the detailed analysis of combined heat and power production through a binary power cycle.     

A survey of geothermal reservoir properties

This paper presents results of a literature survey on thermal, hydrological and chemical characteristics of geothermal reservoirs. The data are presented in a table summarizing important fluid and rock parameters. The primary parameters of interest are the permeability, permeability-thickness, porosity, reservoir temperature and concentration of dissolved solids and non-condensible gases. Some preliminary correlations between these parameters are given.     

Management of the Balcova–Narlidere geothermal reservoir,Turkey

The 2000–2005 management and field monitoring procedures at the Balcova–Narlidere geothermal field, Turkey are described. During that period, fluid production increased from 140 to 300 kg/s and the living space being heated grew from 0.64 to 1.6 million m². The shallow (depth <160 m) injection done between 1996 and 2002 cooled the fluids being produced; the hydraulic connection between shallow production and injection wells was confirmed by tracer tests. Two deep injection wells were drilled to mitigate the problem and to increase injection capacity. Because net fluid extraction was reduced, reservoir pressure drawdown was controlled. Wells drilled after 2000 indicated that the eastern portion of the field had greater potential and yielded higher temperature fluids. After testing and establishing well flow performance, pump capacities were matched to production well capacities. Mineral scaling in wells and surface installations was brought under control reducing the annual cost of inhibitors by about US$100,000. Since all production and injection wells are located near the Agamemnon fault zone and because the capacity of the district heating system is being continuously increased, there is the risk of thermal breakthrough in the production wells.     

天津大学地热开发利用结硕果

由天津大学地热中心承担的国家“九五”攻关课题“地热新技术示范与新产品开发”于２００１年４月中旬通过科研成果鉴定。该课题所包含的３个子项目：“地热热泵应用技术研究”、“地热高温潜水电泵研制（１００℃～２００℃）和“地热利用新技术研究”,为我国地热能源的利用与开发提供了２项新产品、１项新技术。地热作为一种新能源,近十几年来发展很快,我国在地热直接利用方面的发展更是迅速,目前已居世界前列,除地热发电外,地热利用面涉及采暖、工农业利用、医疗康复等。北方城镇在地热利用上以发展采暖为中心,在节约燃料、减轻城…     

Scaling in the reservoir in Kizildere geothermal field, Turkey

At Kizildere geothermal field, the fluid in the reservoir is hot pressurized water at 200°C containing about 2% CO₂ dissolved in the fluid by weight. During production, as the pressure decreases this gas is released into the vapor phase. When the reservoir fluid loses its dissolved CO₂ the equilibrium of the dissolved solids is destroyed and deposition occurs, either in the reservoir or in the well. Scaling in the well can be cleaned mechanically but it is more or less impossible to clean the reservoir. Thus the gas must be kept dissolved in the water until it reaches the well. At Kizildere the wells are cleaned every six months of production. After cleaning it has been observed that some of the wells regain their original production values and others do not. Well KD-14 was deepened and incrustation noted on the cuttings coming from the reservoir. On closing the wells some of them retain their well-head pressure at a constant value, whereas in others a pressure build-up continues at the well-head. Normally the static WHP are around 5 – 6 kg/cm². However, after closure of the master-valve in wells KD14 and KD15 the WHP continued to increase until all the water column in the well was changed by the gas. This phenomenon occurs because of the presence of free gas in the reservoir.     

Chemical study of geothermal waters of Central Tibet (China)

Thirteen of about 300 areas of thermal springs in Tibet have been studied in detail. The springs fall into four groups, the most important and interesting of which produces silica, boron, lithium and cesium-rich waters. Chemical geothermometers agree within a few degrees in the range 200–220°C. A second group includes waters of the same type, but secondary reactions decrease the silica concentrations and increase the Ca and Mg contents. The third and fourth groups are similar to Pyrenean alkaline waters and to CO₂-rich waters from Massif Central respectively; deep temperatures are low to moderate.     

Galerkin finite-element simulation of a geothermal reservoir

The equations describing fluid flow and energy transport in a porous medium can be used to formulate a mathematical model capable of simulating the transient response of a hot-water geothermal reservoir. The resulting equations can be solved accurately and efficiently using a numerical scheme which combines the finite element approach with the Galerkin method of approximation. Application of this numerical model to the Wairakei geothermal field demonstrates that hot-water geothermal fields can be simulated using numerical techniques currently available and under development.     

Coupled reservoir-wellbore simulation of geothermal reservoir behavior

The reservoir simulator TOUGH and the wellbore simulator WFSA have been coupled to model flow of geothermal brine in the reservoir as well as in the wellbore. An outline of the structure of the two computer codes is given, together with the relevant equations. A new module, COUPLE, has been written to serve as an interface between TOUGH and WFSA. Two sample problems are given to illustrate the use of the coupled codes. One of these problems compares the results of the new simulation method to those obtained by using the deliverability option in TOUGH. The coupled computing procedure is shown to simulate more accurately the behavior of a geothermal reservoir under exploitation.     

Controls on the Karaha–Telaga Bodas geothermal reservoir,Indonesia

Karaha–Telaga Bodas is a partially vapor-dominated, fracture-controlled geothermal system located adjacent to Galunggung Volcano in western Java, Indonesia. The geothermal system consists of: (1) a caprock, ranging from several hundred to 1600 m in thickness, and characterized by a steep, conductive temperature gradient and low permeability; (2) an underlying vapor-dominated zone that extends below sea level; and (3) a deep liquid-dominated zone with measured temperatures up to 353 °C. Heat is provided by a tabular granodiorite stock encountered at about 3 km depth. A structural analysis of the geothermal system shows that the effective base of the reservoir is controlled either by the boundary between brittle and ductile deformational regimes or by the closure and collapse of fractures within volcanic rocks located above the brittle/ductile transition. The base of the caprock is determined by the distribution of initially low-permeability lithologies above the reservoir; the extent of pervasive clay alteration that has significantly reduced primary rock permeabilities; the distribution of secondary minerals deposited by descending waters; and, locally, by a downward change from a strike-slip to an extensional stress regime. Fluid-producing zones are controlled by both matrix and fracture permeabilities. High matrix permeabilities are associated with lacustrine, pyroclastic, and epiclastic deposits. Productive fractures are those showing the greatest tendency to slip and dilate under the present-day stress conditions. Although the reservoir appears to be in pressure communication across its length, fluid, and gas chemistries vary laterally, suggesting the presence of isolated convection cells.     

Characteristics and management of the Sumikawa geothermal reservoir, northeastern Japan

The subsurface temperature gradually increases southward in the Sumikawa geothermal field and decreases sharply toward the north. The geothermal reservoir contains a two-phase zone between the cap rock and hot water zone. The target for production was designated in the deep zone, in the high temperature southern area. The production and reinjection areas have been separated to recover thermal energy efficiently during the recycling of reinjection fluid; the wells have been spaced as far apart as possible to reduce well interference. To improve productivity and injectivity, cold-water well stimulation was applied, and this experiment reduced the number of wells required for 50 MW_e power generation.     

The Cerro Prieto IV (Mexico) geothermal reservoir: Pre-exploitation thermodynamic conditions and main processes related to exploitation (2000–2005)

The Cerro Prieto IV (CP IV) reservoir, located in the northeastern part of the Cerro Prieto (Mexico) geothermal field, was studied in order to define its pre-exploitation conditions and initial (2000–2005) response to exploitation. Bottomhole thermodynamic conditions were estimated by modeling heat and fluid flows using the WELLSIM program and well production data. Produced fluid chemical and isotopic data were also analyzed to investigate characteristic patterns of behavior over time, which were then compared against simulation results to obtain a conceptual model of the CP IV reservoir. According to the proposed model, two zones in the reservoir – separated by Fault H and producing fluids of different characteristics – were identified under pre-exploitation conditions. Wells in the area to the east-southeast (south block) produce very high-enthalpy fluids (≥2000 kJ/kg), with very low chloride (≤7000 mg/kg) and high CO₂ (>6‰ molar) and δD (<−94‰). In contrast, wells toward the west-northwest (north block) show moderate-enthalpy fluids (1400–1800 kJ/kg), with high chloride (∼12,000 mg/kg) and relatively low CO₂ (<6‰ molar) and δD (<−94‰). Dilution caused by cooler water entry, boiling due to steam gain, both occurring in the north block, and steam condensation in the south block were identified as the three main reservoir processes associated with exploitation. Also, it was found that the dynamics of the CP IV reservoir is controlled by the Fault H system.     

Geothermal reservoir monitoring by continuous self-potential measurements,Mori geothermal field,Japan

A self-potential (SP) monitoring study was conducted at the Mori geothermal field, Japan, to detect fluid flow changes in the reservoir caused by well operations. In 2000, eight observation points were deployed in and around the reservoir region for about 2.5 months, during which the production and reinjection wells were shut in and then re-opened. Since no reliable remote reference was available, the SP results were plotted in two ways: adopting an observation point in the target area as reference, and using the average value of all the observation points as a reference (called the “relative SP” plot). Although changes in SP corresponding to well operations were detected in both plots, more reliable results were obtained with the relative SP plot. Based on numerical simulation of SP using a simplified model of the Mori reservoir, the observed changes are interpreted as being caused by reservoir pressure changes through electrokinetic coupling.