Geophysical exploration of the Boku geothermal area, Central Ethiopian Rift

The Boku central volcano is located within the axial zone of the Central Ethiopian Rift near the town of Nazareth, Ethiopia. An integrated geophysical survey involving thermal, magnetic, electrical and gravimetric methods has been carried out over the Boku geothermal area in order to understand the circulation of fluids in the subsurface, and to localize the “hot spot” providing heat to the downward migrating groundwaters before they return to the surface. The aim of the investigations was to reconstruct the geometry of the aquifers and the fluid flow paths in the Boku geothermal system, the country's least studied. Geological studies show that it taps heat from the shallow acidic Quaternary volcanic rocks of the Rift floor. The aquifer system is hosted in Quaternary Rift floor ignimbrites that are intensively fractured and receive regional meteoric water recharge from the adjacent escarpment and locally from precipitation and the Awash River. Geophysical surveys have mapped Quaternary faults that are the major geologic structures that allow the ascent of the hotter fluids towards the surface, as well as the cold-water recharge of the geothermal system. The shallow aquifers are mapped, preferred borehole sites for the extraction of thermal fluids are delineated and the depths to deeper thermal aquifers are estimated.     

Chilean geothermal resources and their possible utilization

Most of the hot spring areas in Chile are located along the Andean Cordillera, associated with Quaternary volcanism. The volcanic—geothermal activity is mainly controlled by the subduction processes of the Nazca and Antarctic oceanic plates under the South America continental plate, and occurs at three well-defined zones of the Chilean Andes: the northern zone (17°30′–28°S), the central—south zone (33φ–46°S) and the southern-most or Austral zone (48°–56°S).Some tested high temperature geothermal fields, and geological and geochemical surveys of many other hot spring areas, evidence a great potential of geothermal resources in this country. Both electrical and non-electrical applications of this potential are considered in this paper.Taking into account the potentially available geothermal resources, the development of natural resources, the geographic and social—economic conditions existing in the different regions of Chile, it is concluded that power generation, desalination of geothermal waters, recovery of chemicals from evaporite deposits and brines and sulfur-refining are the main possible applications of geothermal energy in northern Chile; in central—south Chile geothermal energy is suitable for agribusiness such as greenhouses, aquaculture and animal husbandry.     

Fluid flow processes in the BEPPU geothermal system, Japan

The Beppu geothermal system is centred beneath the late Quaternary volcanoes of Tsurumi and Garandake at the northern end of the Ryukyu volcanic arc. The deep fluid has a temperature of at least 250–300°C, and an inferred chloride concentration of 1400–1600 mg/kg. Apart from fumarolic areas near the summits of the two volcanoes, most thermal activity occurs at low elevation along the two main outflow paths towards the coast. The hot spring waters of downtown Beppu have originated from outflow along the Asamigawa Fault, with their chemistry indicating predominantly dilution of the deep fluid by groundwater. The second outflow zone towards the hot spring area of downtown Kamegawa coincides with a ridge of lavas. Here boiling, steam loss, and subsequent mixing with steam-heated groundwaters have significantly modified both the deep fluid and host rocks. The area of the geothermal system above 200°C is at least 15 km² at sea level, and the total natural heat output is inferred to be at least 250 MW. Most of this heat output occurs as subsurface hot water outflows towards the coast due to the 1300 m of topographic relief across the system.     

Geochemistry in geothermal exploration

Techniques based on the variations in composition of water, gas and stable isotopes in the liquid and gas phases of the geothermal fluids have been applied for some time now in the major geothermal fields and are now also used regularly in geothermal exploration. There are numerous processes capable of modifying isotopic composition after infiltration of water from the surface, such as water-rock exchanges, formation of secondary minerals and exchange with the gaseous phase (CO₂ and H₂S). During ascent to the surface, the two main processes are steam separation and dilution and mixing with shallower waters. This paper also deals with the chemical characteristics of the waters, their classification and the water-rock interaction producing hydrothermal alteration. During exploration the chemical and isotopic geothermometers represent a unique method for investigating the deep system. The choice of geothermometer and interpretation of geothermometric data are two crucial steps in geothermal exploration. Finally, the paper discusses the geochemistry of gas mixtures, especially the origin of the gas species and the main chemical reactions that produce semi-empirical geothermometers and some recent non-empirical geothermometers based on models of a two-phase system in the reservoir. Gas-geothermometers can be developed to calculate the reservoir temperature for natural manifestations.     

Monitoring production using surface deformation: the Hijiori test site and the Okuaizu geothermal field,Japan

Production in geothermal reservoirs often leads to observable surface displacement. As shown in this paper, there is a direct relationship between such displacement and reservoir dynamics. This relationship is exploited in order to image fluid flow at two geothermal field sites. At the first locality, the Hijiori Hot Dry Rock (HDR) test site, 17 tilt meters record deformation associated with a 2.2 km deep injection experiment. Images of fluid migration along a ring fracture system of the collapsed Hijiori caldera are obtained. At the Okuaizu geothermal field, leveling and tilt meter data provide constraints on long- and short-term fluid movement within the reservoir. A set of 119 leveling data suggest that the north-to-northeast trending Takiyagawa fault acts as a barrier to flow. The northwesterly oriented Chinoikezawa and Sarukurazawa faults appear to channel fluid from the southeast. The tilt data from Okuaizu indicate that a fault paralleling the Takiyagawa fault zone acts as a conduit to transient flow, on a time scale of several weeks. The volume strain in a region adjacent to the injection wells reaches a maximum and then decreases with time. The transient propagation of fluid along the fault may be due to pressure build-up, resulting from the re-initiation of injection.     

Review of geothermal energy resources in Pakistan

Pakistan, despite the enormous potential of its energy resources, remains energy deficient and has to rely heavily on imports of hydrocarbon products to satisfy hardly its needs. Moreover, a very large part of the rural areas does not have the electrification facilities because they are either too remote and/or too expensive to connect to the national grid. Pakistan has wide spectrum of high potential renewable energy sources, conventional and as well non-conventional. Many of them have not been adequately explored, exploited and developed. Geothermal energy is one of them. Pakistan can be benefited by harnessing the geothermal option of energy generation as substitute energy in areas where sources exist. Most of the high enthalpy geothermal resources of the world are within the seismic belts associated with zones of crustal weakness like the seismo-tectonic belt that passes through Pakistan having inherited a long geological history of geotectonic events. The present study of the geotectonic framework suggests that Pakistan should not be lacking in commercially exploitable sources of geothermal energy. This view is further strengthened by (a) the fairly extensive development of alteration zones and fumeroles in many regions of Pakistan, (b) the presence of a fairly large number of hot springs in different parts of the country, and (c) the indications of Quaternary volcanism associated with the Chagai arc extending into Iran and Afghanistan border areas. These manifestations of geothermal energy are found within three geotectonic or geothermal environments, i.e., (i) geo-pressurized systems related to basin subsidence, (ii) seismo-tectonic or suture-related systems, and (iii) systems related to Neogene–Quaternary volcanism. A few localities, scattered sporadically all over the country, have been studied to evaluate only some of the basic characteristic parameters of the geothermal prospects. The present review study the geothermal activities of varying intensity and nature, associated with different geotectonic domains, and reveals the viable potential of the geothermal environments, which could be exploited for the generation of sustainable indigenous energy in Pakistan.     

Lumped-parameter models for low-temperature geothermal fields and their application

The behavior of low-temperature geothermal reservoirs under exploitation is simulated using analytical lumped-parameter models. These models consider the effects of fluid production and reinjection, as well as natural recharge, on the pressures (or water levels) of low-temperature, liquid-dominated geothermal systems. The computed responses for constant production/injection flow rates are given in the form of analytical expressions. Variable flow rate cases are modeled, based on the Duhamel's principle. Reservoir parameters are obtained by applying a weighted nonlinear least-squares estimation technique in which measured field data are history matched to the corresponding model response. By using history-matched models, the future performance of the reservoir can be predicted for different production/injection scenarios in order to optimize the management of a given geothermal system.We demonstrate the applicability of the models by simulating measured data from the Laugarnes geothermal field in Iceland, and the Balcova–Narlidere field in Turkey.     

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.     

Maximising the working fluid flow as a way of increasing power output of geothermal power plant

This paper presents a method of increasing the power output of a geothermal power plant based on organic working fluid. The power is raised by increasing the flow of geothermal water supplied to the evaporator by means of returning the stream of geothermal water from downstream of the evaporator for a repeated passage through that heat exchanger. Such arrangement increases the flow of the working fluid in the circuit. Analyses have been conducted for a power plant using several types of organic fluids. The results obtained show that there is one optimum evaporation temperature of the working fluid, which depends on the quantity of the recycled geothermal water, at which the capacity of the Clausius–Rankine is the highest.     

Three-dimensional imaging of a geothermal system using temperature and geological models derived from a well-log dataset

The accurate imaging of geothermal systems from the ground surface down to great depths is an interdisciplinary problem common to geothermal resource exploration and development. Rocks can be characterized mainly in terms of their lithology, mineralogy, fracture distribution, permeability, thermal conductivity and porosity, and similarly the geothermal fluid (and its circulation) by its geochemistry, flow pattern, velocity, temperature and pressure. Some of these data are obtained by well logging and from laboratory tests conducted on drillhole cores. In general, the distribution of geothermal wells is not random, and well data are limited in terms of quantity and depth range. Accordingly, a sophisticated spatial modeling technique is indispensable in the three-dimensional imaging of geothermal systems. We describe a versatile 3-D modeling method that can be used to determine the temperature, flow velocity, and distribution of geological units within a geothermal field based on well log data. The model results for the Hohi geothermal area, Japan, provide plausible estimates of temperature, flow velocity, and geology to a depth of 3000 m. Superimposition of the three spatial models we obtained shows that, at Hohi, two geothermal reservoirs are localized near highly fractured fault zones that provide paths for the ascent of thermal fluids from depth.     

Numerical modelling of production from the Poihipi dry steam zone: Wairakei geothermal system,New Zealand

The Poihipi power station utilizes dry steam from a shallow zone near the margin of the Wairakei geothermal reservoir. The station has been in operation for several years, with daily variations in the fluid production rate following variations in time-of-day pricing of electricity. The corresponding varying pressure history provides a good database for testing models of the geothermal reservoir.     

Takigami geothermal system, northeastern Kyushu, Japan

The Takigami geothermal reservoir is bounded by a system of faults and fractures oriented along two main directions, north to south and east to west. The Noine fault has a large vertical displacement and trends north to south, dividing the subsurface characteristics of resistivity, permeability, temperature and reservoir depth. The Takigami geothermal fluid has a near neutral pH and is of the Na–Cl type, with a chloride content ranging from 400 to 600 ppm. The southwestern part of the area has the highest subsurface temperature, up to 250°C. The deep fluid originates from the southwest, and flow is mainly to the north and partly to the east along faults and fractures, decreasing in temperature with increasing lateral flow.     

Multiple integrated applications for low-to medium-temperature geothermal resources in Iceland

There is an increasing global demand for a faster, more expansive development in the energy sector, in order to improve the standard of living of the world's population by the creation of more jobs and better living conditions. The public is, however, well aware of the damage that has been done to the environment, in the form of deforestation, despoiling of lakes and rivers and, in particular, greenhouse effects, and it is unwilling to further sacrifice its natural environment. This decision puts pressure on scientists, engineers and developers to find ways and means of attaining “sustainable energy development”. In other words, the challenge now is to achieve the sustainable development of alternative renewable energy resources. Sustainability may be achieved in a number of ways, but the one most likely to result in a rapid increase in energy output without a deleterious impact on the environment is the revamping and integration of what we already have. This paper attempts to address sustainability as it applies to geothermal energy. We describe the concept of a multiple integrated use of geothermal energy, including the tenable benefits that can be obtained from applying this concept, such as a longer reservoir lifespan, a lower specific environmental impact, and greater marketing flexibility and profitability. The paper also emphasises the importance of achieving a maximum effective temperature drop across the application, commensurate with a minimum flow rate, optimal pumping characteristics and minimal fluid extraction from the geothermal reservoir. In geothermal house heating systems this means using large and effective radiators, dual-pipe heating systems, and thermostatic controls on each radiator. Where modifications to existing house heating systems are not feasible, e.g. by conversion from a single-pipe to a dual-pipe system or installation of larger radiators, an alternative solution is to adopt a cascaded flow of the geothermal fluid through a combination of heating systems operating at different temperature levels. For economic reasons it is always better to use the geothermal water directly if its chemical quality permits us to do so, otherwise heat exchangers made of resistant materials will be needed to isolate the geothermal fluid from the heating fluid in order to avoid corrosion or scaling in the pipes and radiators. The heat exchangers should be designed in such a way as to obtain a maximum temperature drop of the geothermal fluid. The paper also describes some heating system configurations, the characteristics of geothermal heating systems and their automatic control systems, as well as recommended geothermal field management and monitoring systems. The paper also includes a few examples of existing projects to demonstrate what has already been achieved and what could be done in the future; some suggestions are also made for new developments and innovations to make geothermal energy more generally attractive and useful worldwide.     

Chemical and isotopic techniques in geothermal investigations

A review is given of current thinking on the origin of chemical constituents in geothermal fluids, focussing attention on the more “soluble” elements such as Cl, Br, I, Li, Cs, As and B, and the dissolved gases. Mineral and solution equilibria are evaluated in order to derive underground temperatures, water pH and P_CO2 from the analysis of steam and water flows, natural or from geothermal wells. Isotopic exchange equilibria involving H, C, O, or S, provide additional geothermometers which may often be applied when chemical methods are complicated by steam separation or mixing processes.The isotopic make-up of water (H, D, T, ¹⁶O, ¹⁸O) serves to identify water sources, water ages, and mixing processes in geothermal systems, and to delineate trends with time during the operation of a geothermal field. Chemical and isotope analyses also indicate the occurrence of changes in underground temperature, degree of rock/water interaction, and the tendency for mineral deposition.     

Intersecting faults and sandstone stratigraphy at the cerro prieto geothermal field

The NW - SE trending Cerro Prieto fault zone is part of a major regional lineament that extends into Sonora, Mexico, and has characteristics of both a wrench fault and an oceanic transform fault. The zone includes a number of separate identified faults and apparently penetrates deep into the basement and crustal rocks in the area. The zone serves as a conduit for both large and rapid heat flow. Near well M-103, where the Michoacán fault obliquely intersects a shorter NE - SW trending fault (i.e., the Pátzcuaro fault), large circulation losses during drilling indicate greater permeability and hence increased natural convective fluid flow. Temperature contour maps for the southern portion of the field suggest that a shear fault zone also exists in the vicinity of wells M-48, M-91 and M-101. This shear zone aids in rapidly distributing geothermal fluid away from the Cerro Prieto fault zone, thus enhancing recharge to the western part of the reservoir.We have studied the distribution of lithologies and temperature within the field by comparing data from well cuttings, cores, well logs and geochemical analyses. Across the earliest developed portion of the field, in particular along a 1.25 km NE - SW section from well M-9 to M-10, interesting correlations emerge that indicate a relationship among lithology, microfracturing and temperature distribution. In the upper portion of the reservoir of this section, between 1200 and 1400 m, the percentage of sandstones ranges from 20 to 55. Well logs, calcite isotope maxima, and the Na - K - Ca geothermometer indicate temperatures of 225–275°C. The isothermal high in this vicinity corresponds to the lowest total percentage of sandstones. Scanning electron microphotographs of well cores and cuttings from sandstone and shale units reveal open microfractures, mineral dissolution and mineral precipitation along microfractures and in pores between sand grains. Our working hypothesis is that these sandy shale and siltstone facies are most amenable to increased microfracturing and, in turn, such microfracturing allows for higher temperature fluid to rise to shallower depths in this part of the reservoir.Our ongoing research is aimed at achieving a coherent geological model that provides a basis for estimating reservoir capacity, and that illustrates our understanding of fluid flow along major faults, laterally through fault shear zones, and within predominantly silty and shaley deltaic clastics that have been microfractured.     

History of geothermal exploration in Indonesia from 1970 to 2000

Reconnaissance surveys undertaken since the 1960s show that more than 200 geothermal prospects with significant active surface manifestations occur throughout Indonesia. Some 70 of these were identified by the mid-1980s as potential high-temperature systems using geochemical criteria of discharged thermal fluids. Between 1970 and 1995, about 40 of these were explored using geological mapping, geochemical and detailed geophysical surveys. Almost half of the surveyed prospects were tested by deep (0.5–3 km) exploratory drilling, which led to the discovery of 15 productive high-temperature reservoirs. Several types of reservoirs were encountered: liquid-dominated, vapour-dominated, and a vapour layer/liquid-saturated substratum type. All three may be modified by upflows (plumes) containing magmatic fluid components (volcanic geothermal systems). Large, concealed outflows are a common feature of liquid-dominated systems in mountainous terrain. All explored prospects are hosted by Quaternary volcanic rocks, associated with arc volcanism, and half occur beneath the slopes of active or dormant stratovolcanoes. By 1995, five fields had been developed by drilling of production wells; three of them supplied steam to plants with a total installed capacity of 305 MW_e. By 2000, with input from foreign investors, the installed capacity had reached 800 MWe in six fields, but geothermal developments had stalled because of the 1997–1998 financial crisis.     

Numerical modeling of Basin and Range geothermal systems

Basic qualitative relationships for extensional geothermal systems that include structure, heat input, and permeability distribution have been established using numerical models. Extensional geothermal systems, as described in this paper, rely on deep circulation of groundwater rather than on cooling igneous bodies for heat, and rely on extensional fracture systems to provide permeable upflow paths. A series of steady-state, two-dimensional simulation models is used to evaluate the effect of permeability and structural variations on an idealized, generic Basin and Range geothermal system of the western U.S.Extensional geothermal systems can only exist in a relatively narrow range of basement (bulk) permeability (10⁻¹⁵ m² to 10⁻¹⁶ m²). Outside of this window, shallow subsurface fault zone temperatures decrease rapidly. Mineral self-sealing does not significantly affect the flow system until the flow path is almost completely sealed off. While topography gives an extra “kick” to convective circulation, it is not a requirement for geothermal system development. Flow from the ranges to the fault dominates the circulation, while secondary flow systems exist on the range front slopes. A permeable fault in one valley can also induce cross-range flow if there are no equally good upflow paths in the adjacent valleys. When bulk permeability is high enough, additional deep circulation cells develop in adjacent valleys, diverting heat and fluid from the fault and consequently reducing temperatures in the fault itself. Qualitative comparison between temperature–depth logs from actual geothermal systems and from the generic models is a significant aid to understanding real-world geothermal fluid flow, and suggests new or better interpretations of existing systems.     

Efficiency of Vertical Geothermal Heat Exchangers in the Ground Source Heat Pump System

Taking the fluid temperature distribution along the borehole depth into account, a new quasi-three-dimensional model for vertical ground heat exchangers has been established, which provides a better understanding of the heat transfer processes in the geothermal heat exchangers. On this basis the efficiency of the borehole has been defined and its analytical expression derived. Comparison with the previous two-dimensional model shows that the quasi-three-dimensional model is more rational and more accurate to depict the practical feature of the conduction of geothermal heat exchanger, and the efficiency notion can be easily used to determine the inlet and outlet temperature of the circulating fluid inside the heat exchanger.     

The use of soil Hg to delineate zones of upwelling in low-to-moderate temperature geothermal systems

A soil mercury survey was conducted near the town of Calistoga, California to identify and delineate a buried fault system that is thought to control the upwelling of low-to-moderate temperature geothermal fluids in the upper Napa Valley. Soil samples were collected at 100 m intervals along traverses that crossed hot springs and existing geothermal well sites. Strong mercury anomalies occur along a broadly-defined zone and in close proximity to surface thermal activity including active hot springs and silica sinter formations. In contrast, background mercury concentrations are present in locations with little or no indication of subsurface thermal activity, such as along the margins of the valley or near groundwater wells producing non-thermal water. Analysis of smoothed Hg values reveals a N65W-trending lineament of high Hg concentrations. These results suggest that soil mercury surveys can be a useful and cost-effective method for the identification and mapping of structures controlling subsurface fluid flow in low-to-moderate temperature geothermal systems.     

Thermal and geochemical structure of the Uenotai geothermal system, Japan

The Uenotai geothermal area is located in southern Akita prefecture of northern Honshu Island. The Uenotai geothermal system is a liquid-dominated system with a central zone of aquifer boiling. The two-phase reservoir has evolved from liquid in the natural state due to exploitation. Gas composition of the vapor phase in the reservoir is nearly in equilibrium and correlates with the vapor fraction in the reservoir and with discharging steam quality. The marginal part of the Uenotai system has cooled with the drop in ground-water level. The chemical characteristics of the geothermal water indicate mixing of the immature high Cl source water with conductively heated or steam-heated shallow water or surface water, as well as boiling and steam gain.