Hydrogeological and geochemical studies of the Efteni and Derdin geothermal areas,Turkey

The Efteni and Derdin geothermal areas are located in northwestern Turkey. Relatively low-temperature springs emerge from the Duzce Fault, a normal-component-dominated fault segment of the North Anatolian Fault System. The thermal waters of the Efteni and Derdin Springs show distinct geochemical and isotopic characteristics since they originate from different geothermal reservoirs and reflect the effects of different water–rock interaction processes. Geothermometry revealed higher reservoir temperatures for the Efteni system, however a strong δ¹⁸O shift, interpreted as being the result of isotopic exchange at high temperatures, was observed in the Derdin system. Hydrogeological and geochemical techniques are applied to identify recharge mechanisms, water–rock interaction processes and to construct conceptual models of these geothermal systems.     

Geothermal energy potential related to active volcanism in Indonesia

About 90 thermal areas in Indonesia are indicated, most of which could be grouped into hyperthermal areas located in active volcanic belts. The thermal manifestations are fumaroles, geysers, hot springs and hot mud-pools with surface temperatures generally at boiling point or more than 70°C. A tentative evaluation has been made of the potential of 54 thermal areas with a view to their further development for electrical power. The successful results of these studies in several thermal areas suggest that these volcanic geothermal systems have a high energy potential of about 13,000 – 14,000 MW.The Kawah Kamojang geothermal field in West Jawa is the first promising attempt at utilizing this geothermal energy for electrical power; a 30 MW geothermal power plant has already been installed, and a further 3 units totalling 165 MW are planned.     

Evidence for thermal mining in low temperature geothermal areas in Iceland

This study deals with thermal mining in several geothermal systems in Iceland. A number of 2500- to 3000-m deep drillholes have been drilled into low temperature geothermal areas in the country. The conductive gradient outside active geothermal areas has also been mapped, and shows a systematic variation from lower than 50°C/km in the outer parts of the Tertiary basalts to over 100°C/km on the borders of the volcanic zones (rift zones). The difference between formation temperatures inside geothermal systems and the surrounding conductive gradient can be computed as a function of depth. This difference is termed ΔT in this paper. The ΔT-curves show that the upper parts of the geothermal systems are heated and the lower parts are cooled compared to the undisturbed conductive gradient. In many cases the cooling of the lower part is greater than the heating in the upper part, so that a net thermal mining has occurred. This thermal mining is calculated for several geothermal systems, and the systems are compared. The net thermal mining in the top 3000 m appears to be much greater in formations of Pleistocene and Pliocene age. It gradually decreases to zero for formations older than 6 million years. However, the net thermal mining is critically dependent on the maximum depth of water convection in these systems, which is unknown.     

The Iceland Deep Drilling Project: a search for deep unconventional geothermal resources

The Iceland Deep Drilling Project (IDDP) is a long-term program to improve the economics of geothermal energy by producing supercritical hydrous fluids from drillable depths. Producing supercritical fluids will require the drilling of wells and the sampling of fluids and rocks to depths of 3.5–5 km, and at temperatures of 450–600 °C. The IDDP plans to drill and test a series of such deep boreholes in the Krafla, Nesjavellir and Reykjanes geothermal fields in Iceland. Beneath these three developed high-temperature systems frequent seismic activity continues below 5 km, indicating that, even at supercritical temperatures, the rocks are brittle and therefore likely to be permeable, even where the temperature is assumed to exceed 550–650 °C. Temperature gradients are greater and fluid salinities smaller at Nesjavellir and Krafla than at Reykjanes. However, an active drilling program is underway at Reykjanes to expand the existing generating capacity and the field operator has offered to make available one of a number of 2.5 km deep wells to be the first to be deepened to 5 km by the IDDP. In addition to its potential economic significance, drilling deep at this location, on the landward extension of the Mid-Atlantic Ridge, is of great interest to the international science community. This paper examines the prospect of producing geothermal fluids from deep wells drilled into a reservoir at supercritical temperatures and pressures. Since fluids drawn from a depth of 4000–5000 m may prove to be chemically hostile, the wellbore and casing must be protected while the fluid properties are being evaluated. This will be achieved by extracting the fluids through a narrow retrievable liner called the “pipe”. Modelling indicates that if the wellhead enthalpy is to exceed that of conventionally produced geothermal steam, the reservoir temperature must be higher than 450 °C. A deep well producing 0.67 m³/s steam (2400 m³/h) from a reservoir with a temperature significantly above 450 °C could, under favourable conditions, yield enough high-enthalpy steam to generate 40–50 MW of electric power. This exceeds by an order of magnitude the power typically obtained from a conventional geothermal well in Iceland. The aim of the IDDP is to determine whether utilization of heat from such an unconventional geothermal resource at supercritical conditions will lead to increased productivity of wells at a competitive cost. If the IDDP is an economic success, this same approach could be applied in other high-temperature volcanic geothermal systems elsewhere, an important step in enhancing the geothermal industry worldwide.     

Impact of silica scaling on the efficiency of heat extraction from high-temperature geothermal fluids

In high-temperature geothermal fields, precipitation of amorphous silica from solution to form silica scales is the main obstacle to efficient heat extraction from the hot fluids. The silica deposits cause operational problems and may even clog pipelines and injection drillholes. The rate of silica-scale formation can be reduced by ageing amorphous silica super-saturated waters, thus allowing the aqueous silica in excess of saturation to polymerize. Polymeric silica shows less tendency to precipitate from solution than monomeric silica. Studies of separated water from the Nesjavellir geothermal power station, Iceland, indicate that silica-scale formation can be avoided during heat extraction by rapid cooling of the water in “capillary heat exchangers”, followed by ageing the water for 1–2 h and subsequently mixing it with condensed steam. It is thus possible to avoid scale formation during injection of the amorphous silica super-saturated water leaving the heat exchanger.     

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.     

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.     

Isotopic evidence for magmatic and meteoric water recharge and the processes affecting reservoir fluids in the palinpinon geothermal system, Philippines

Stable isotopic compositions of meteoric and geothermal waters indicate that the Palinpinon geothermal system of Southern Negros is fed by a parent water that originated from a mixture of local meteoric (80%) and magmatic (20%) waters. The meteoric water has an isotopic concentration of −8.5‰ and −54‰ in ¹⁸O and ²H, respectively, which corresponds to an average infiltration altitude of about 1000 m above sea level. With exploitation of the system and injection of wastewaters to the reservoir, the stable isotopic composition became heavier due to significant mixing of geothermal fluids with injection waters. Incursion of cooler meteoric waters, which is confirmed by the presence of tritium, also leads to the formation of acid-sulfate waters. Stable isotopes are effective as “natural tracers” to determine the origin and mixing of different fluids in the reservoir.     

Geochemistry of the surface and deep fluids of the Miravalles volcano geothermal system (Costa Rica)

The Miravalles high-temperature geothermal reservoir, located in the northwestern part of Costa Rica, is liquid-dominated. Reservoir temperatures generally range between 230 and 240 °C. The highest measured value is 255 °C. Bottom-hole measurements and solute geothermometry indicate that thermal conditions within the reservoir are very stable over time. The waters discharged from the wells have a neutral or slightly alkaline pH and are of the sodium-chloride type. Based on isotope data, the main recharge zone appears to be located on the northeastern side of the Guanacaste Cordillera. Several mixing trends have been identified between reservoir fluids and regional groundwaters. Gas discharges are dominated by CO₂, with minor amounts of H₂S and N₂. Relative N₂, Ar and He contents reveal a typical arc-type signature and significant inflow of meteoric-derived gases. Cl–SiO₂-enthalpy and δ¹⁸O–δ²H–Cl relationships suggest the existence of a maturation trend that is the result of both natural (i.e. direct drainage of deeper fluids) and anthropogenic causes (reinjection of Cl-rich waste waters). Acid fluids with SO₄-acidity (pH ranging between 2.4 and 3.7) have been encountered in three wells at the eastern border of the well field. Preliminary data assessment indicates two possible sources, either superficial H₂S oxidation or inflow of “immature” volcanic waters.     

Low-temperature geothermal utilization in Iceland – Decades of experience

Geothermal energy plays a key role in the economy of Iceland and it supplies about 89% of the space heating requirements. A large fraction of the country's district heating services (hitaveitas) use energy from low-temperature geothermal systems, which are mostly located outside the volcanic zone. Many of the geothermal district heating services have been in operation for several decades and much can be learned from their operation, in particular regarding long-term management of low-temperature geothermal resources. In most cases down-hole pumps are used, but there are examples of large-scale artesian flow still being maintained. The Reykjavík geothermal district heating service is the world's largest such service. It started operation on a small scale in 1930, and today it serves Reykjavík and surrounding communities, about 58% of the total population of Iceland. The Reykjavík district heating service utilizes three low-temperature systems. The production and response (pressure, chemistry, and temperature) histories of these systems and six other low-temperature geothermal systems are discussed. Four of the systems are very productive and reach equilibrium at constant production. Two are much less productive and do not attain equilibrium, while three are of intermediate productivity. Groundwater inflow has caused temperature decline and chemical changes in two of the systems. Several problems have faced the Icelandic low-temperature operations, such as excessive pressure drawdown caused by overexploitation, colder water inflow, and sea water incursion. None of the district heating systems has ceased operation and solutions have been found to these problems. The solutions include improving the energy efficiency of the associated heating systems, deeper and more focussed drilling (e.g., directional drilling), finding new drilling targets (even new drilling areas), and injection, as well as technical solutions on the surface. The long utilization case histories provide important information pertaining to sustainable management of geothermal resources.     

Low-temperature geothermal energy use in Iceland

The results are given of a recent survey of the utilization of geothermal energy produced in low-temperature areas in Iceland. About 70% of Icelanders enjoyed geothermal district heating in 1979 and in the next 3–5 years this percentage should increase to about 80%. Most of the district heating systems receive hot water from low-temperature (reservoir temperature less than 150°C) geothermal areas. In late 1980 the thermal power above 15°C used for district heating amounted to 850 MW while the total low-temperature use was about 950 MW-thermal.     

Geochemistry of thermal waters in the Tengchong volcanic geothermal area, West Yunnan Province, China

The Tengchong volcanic geothermal area is one of the areas in China which has powerful geothermal energy potential. The chemical compositions of the thermal waters discharged in this area were studied to obtain information on boiling and mixing relationships and average reservoir temperatures. Then a conceptual model of the Tengchong volcanic geothermal area was formulated. Hydrothermal areas have reservoir temperatures ranging from 90 to 150°C; such temperatures can be found in up to 60% of the 58 hydrothermal areas. Five hydrothermal areas have high temperatures, with an average reservoir temperature of more than 150°C, and occupy less than 10% of the total. The Hot Sea geothermal field is one of the five high temperature hydrothermal areas where a more detailed investigation was made.     

Thermal regime, groundwater flow and petroleum occurrences in the Cap Bon region, northeastern Tunisia

The Cap Bon region of northeastern Tunisia is part of a young continental margin that presents a thick column of sediments deposited mainly during Cretaceous and Miocene extended tectonic episodes. This sedimentary package is characterised by broad synclines alternating with NE–SW trending anticlines, and is affected by numerous NE–SW, NW–SE and E–W striking faults. Oligo-Miocene sandstones constitute the most important potential reservoir rocks in the region.The distribution of subsurface temperatures in the Cap Bon basin reflects local groundwater circulation patterns and correlates with the location of known oil and gas fields. The results of geothermal studies could therefore prove useful in the search for new hydrocarbon resources in the region. Subsurface temperatures were measured in deep oil exploration and shallow water wells. Local geothermal gradients range from 25 to 35 °C/km, showing higher values in the Korbous and Zennia areas, which correspond to zones of groundwater discharge and convergence in the Oligo-Miocene aquifer system, respectively.Analysis of thermo-hydraulic and geochemical data relative to the thermal springs in the Korbous region along the Mediterranean coast has made a useful contribution to geothermal prospecting for potential deep reservoirs. Positive geothermal gradient anomalies correspond to areas of ascending thermal waters (i.e. discharge areas), whereas negative anomalies indicate areas of infiltrating colder meteoric waters (i.e. recharge areas). The zones of convergence of upward-moving water and groundwater may be associated with petroleum occurrences.     

Temporal and spatial distribution of local seismicity in the Chipilapa-Ahuachapán geothermal area, El Salvador

Microseismic monitoring of the Chipilapa-Ahuachapán area was carried out during August-November 1988 and October 1991–April 1992. The objective was to use the study of microearthquakes as an exploration tool to invvestigate the geothermal potential of the Chipilapa area and to evaluate the main characteristics of the seismic activity, prior to and during the exploitation tests. Since 1989, seven wells have been drilled in the area, two of which have encountered three geothermal aquifers that could be exploited for electricity generation by means of binary-cycle technology. The 1988 survey detected important, shallow and low magnitude seismic activity, located mainly south and southwest of the explored area. This activity is possibly related to the recharge zone of the Chipilapa-Ahuachapán geothermal system, located further south, beneath the Pleistocene Pacific Volcanic Chain. The 1991–1992 survey confirmed the existence of seismicity beneath the southern volcanic axis, but other important clusters of activity were recorded northward, related to the deeper structures of the Central Graben, and southwest of the Ahuachapán geothermal field, close to the 1990 hydrothermal eruption of Agua Shuca. Shallow microseismic activity also appeared along the faults limiting the Chipilapa geothermal field to the east. Although it is probable that this seismicity is due to fluid circulation in fractures, no geothermal reservoirs were intercepted by wells CHA and CH8. Moreover, no significant induced seismicity was recorded during production and injection tests.     

The source and heating mechanism for the Ahram, Mirahmad and Garu thermal springs, Zagros Mountains, Iran

The Ahram, Mirahmad and Garu low-temperature geothermal springs in the Zagros Mountains, Boshehr Province, Iran, emerge along the Ghatar-Kazeroon fault. The average temperature of the springs is about 40 °C and the waters have appreciable amounts of dissolved solids and hydrogen sulfide. Based on chemical analyses, including stable isotopes of the thermal waters and data interpretations, and on a comparison with fresh water springs and wells in the study area, we conclude that the hot waters are of meteoric origin. Because of the prevailing geothermal gradient, the waters are heated as they circulate deep in the system through joints, fractures and the Ghatar-Kazeroon fault. During their deep circulation, the waters come into contact with Hormoz Series evaporites and the oilfield brines, resulting in an increase in dissolved ion concentrations.     

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.     

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.     

Chemical and isotopic measurements of geothermal discharges in the Acqui terme district, Piedmont, Italy

Chemical and isotopic studies have been carried out on samples from the Acqui geothermal district (Piedmont, Italy). The results indicate that the waters represent mixtures of meteoric waters and a fossil brine; the contribution of meteoric waters ranges between 93 and 98%. The recharge zone of meteoric waters is most likely in the Voltri-Savona massif at an isotopic recharge altitude of 590 m. On the basis of chemical and isotopic data the reservoir temperature has been estimated at about 200°C. This high value renders the Acqui district possibly the most promising geothermal system in northern Italy.     

Geochemical study of fluids on Lesbos island, Greece

Sixty-five water samples and seven associated gas samples have been collected on Lesbos island. The lithology and structural setting have resulted in two main types of hydrological circulation: a shallow circulation hosting low-salinity cold waters and a deeper one, hosting high-salinity hot waters that often emerge in thermal springs near the coast. The cold waters are characterized by Ca(Mg)-HCO₃(SO₄) composition, while the thermal waters generally have an Na-Cl composition. The chemical features of the former can be explained by their circulation in the ophiolite-bearing phyllitic basement and volcanic rocks. Waters circulating in the ultramafic layers of the basement are richer in Mg than the waters whose circulation is mainly within marble levels or volcanic rocks. The Na-Cl thermal waters are characterized by salinities ranging from 1910 to 35,700 mg/kg. As indicated by previous hydrogeochemical and isotopic studies, the Na-Cl composition of the thermal waters on Lesbos is the result of mixing between shallow meteoric waters and marine waters. While interacting with the minerals of the geothermal reservoir, the saline waters retain the Na/Cl sea water ratio but become enriched in Ca²⁺ and depleted in Mg²⁺ with respect to sea water.Processes of hydrothermal alteration at depth are activated by a gas phase enriched in CO₂, which reaches the geothermal reservoir by rising along the deep fractures of the basement. Thermodynamic calculations based on hydrothermal alteration processes occurring at the estimated temperatures of the geothermal reservoir (about 120 °C) indicate that the thermal waters of Lesbos are in equilibrium with talc and dolomite.