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.     

Gas geochemistry of the Cordón Caulle geothermal system,Southern Chile

The Cordón Caulle geothermal system is located in a NW-trending volcano-tectonic depression of the Southern Andean Volcanic Zone of Chile. Outflows of low chloride water were previously interpreted as the surface expression of a shallow steam-heated aquifer, with subsurface temperatures of 150–170 °C. Gas data from fumaroles and hot springs have been used to assess the nature and temperature of the deeper, underlying geothermal reservoir. Fumaroles at the northeastern border of Cordón Caulle have ³He/⁴He ratios typical of subduction margins (6–7 R_A) and N₂/Ar ratios of about 40, indicating deep convection of air-saturated groundwater. Fumaroles at the southwestern border have N₂/Ar ratios >300, suggesting the presence of a deep volcanic component. Gas ratios of fumarole discharges yield equilibration temperatures >300 °C, whereas those of hot spring waters suggest temperatures of about 160 °C. Based on these data, and comparisons with well documented liquid and vapor-dominated geothermal systems, a model is proposed of a boiling liquid-dominated geothermal system overlain by a secondary steam-heated aquifer.     

Performance assessment of a geothermally heated building

This study deals with an exergetic performance evaluation of a geothermally heated building. This building used in the analysis has a volume of 1147.03 m³ and a net floor area of 95.59 m², while indoor and exterior air temperatures are 20 and 0 °C, respectively. The geothermal heating system used for the heat production was constructed in the Ozkilcik heating center, Izmir, Turkey. Thermal water has a pressure of 6.8 bar, a temperature of 122 °C and a mass flow rate of 54.73 kg/s, while it is reinjected at 3.2 bar and 72 °C. The system investigated feeds three regions. Among these, the Ozkanlar region has supply/return pressure and temperature values of 4.6/3 bar and 80/60 °C, respectively. Energy and exergy flows are studied to quantify and illustrate exergy destructions in the overall system. Total exergy input rate to the system is found to be 9.92 kW and the largest exergy destruction rate occurs in the primary energy transformation at 3.85 kW.     

Hydrogeochemistry and geothermometry of Changal thermal springs,Zagros region,Iran

This study addresses the hydrogeochemistry of thermal springs that emerge from the Asmari limestone in a gorge at Changal Anticline in the vicinity of the Salman-Farsi dam. The Changal thermal springs vary in temperature between 28 and 40 °C. Chemical and isotopic compositions of the thermal waters suggest two distinct hydrogeological systems: a deep, moderate-temperature (∼40 °C) geothermal system recharged by deeply circulating meteoric waters, and a shallow cold aquifer system related to local groundwater. The source geothermal fluid temperature was calculated using different geothermometers and mineral saturation indexes. Based on chemical and isotopic data, it is hypothesized that: (1) mixing occurs between the ascending geothermal water and shallow cold water; (2) the resulting thermal waters reaching surface are a mixture of 80% local, shallow meteoric water and 20% geothermal water; and (3) the circulation depth of the meteoric water is about 1500 m. The thermal reservoir temperature is estimated to be between 70 and 80 °C according to calculations using different geothermometers and computation of saturation indices for different solid phases.     

Integrated seismic and magnetotelluric exploration of the Skierniewice,Poland, geothermal test site

The purpose of the research activities at the Skierniewice geothermal test site is to develop and apply an exploration methodology for low-enthalpy systems in sedimentary formations. Work included seismic and magnetotelluric surveys carried out close to well Kompina-2 to create a detailed structural–geologic model and characterize the anisotropic fracture system around the borehole. The study included the reprocessing of archival data from selected boreholes and 2D seismic lines. The collected data were used to identify formations with high fracture permeability and the presumed flow path of geothermal (∼110 °C) brine in high productivity zones, and determining rock porosities and salinity distribution in the subsurface. The next stage of the investigations will focus on siting a second borehole and studying the possibility of installing a plant for electrical generation or direct geothermal heat applications.     

Exploring for geothermal resources in Greece

In Greece the geothermal areas are located in regions of Quaternary or Miocene volcanism and in continental basins of high heat flow. The existence of high-temperature (>200 °C) resources has been proven by deep drilling on the islands of Milos and Nisyros and inferred on the island of Santorini by its active volcanism. Elsewhere, geological investigations, geochemical analyses of thermal springs and shallow drilling have identified many low-temperature (<100 °C) reservoirs, utilized for spas and greenhouse/soil heating. Ternary K–Na–Mg geothermometer data suggest deep, medium-temperature resources (100–200 °C) in Sousaki, the islands of Samothraki, Chios and Lesvos, in the basins of Nestos River Delta and Alexandroupolis and in the graben of Sperchios River. In the basins of northern Greece these resources are also inferred from deep oil exploration well data.     

Energetic performance test of an underground air tunnel system for greenhouse heating

The main objective of the present study is to investigate the performance characteristics of an underground air tunnel (UAT) for greenhouse heating with a 47 m horizontal, 56 cm nominal diameter U-bend buried galvanized ground heat exchanger. This system was installed in the Solar Energy Institute, Ege University, Izmir, Turkey. Based upon the measurements made in the heating mode, the average heat extraction rate to the soil is found to be 3.77 kW, or 80.21 W/m of tunnel length, while the required tunnel length in meters per kW of heating capacity is obtained as 12.46. The entering air temperature to the tunnel ranges from 14.3 to 21.5 °C, with an average value of 15.5 °C. When the system operates, the greenhouse air is at a minimum day temperature of 13.1 °C with a relative humidity of 32%. The maximum heating coefficient of performance of the UAT system is about 6.42, while its minimum value is about 0.98 at the end of a cloudy and cold day and fluctuates between these values at other times. The daily average maximum COP values for the system are also obtained to be 6.42. The total average COP in the heating season is found to be 5.16.     

Modelling of fluid flow and heat transfer to assess the geothermal potential of a flooded coal mine in Lorraine,France

The flooding of the Lorraine coal mines (France), representing a huge reservoir of about 154 × 10⁶ m³, began in June 2006. After attaining thermal equilibrium with the surrounding rocks, the water temperature in the deepest parts is expected to reach 55 °C, giving the opportunity for the extraction of low-enthalpy geothermal waters that may be suitable for district heating purposes. We present some numerical modelling results of the thermally driven convective flow in an open vertical shaft and in the entire mine reservoir. A dual permeability/porosity approach was used in the reservoir model, which includes open galleries and vertical shafts, coal panels backfilled with sand, and intact rock masses. Two scenarios of heat extraction with different flow regimes were investigated. A sensitivity analysis shows that the temperature decline in the production zone is highly dependent on the permeability of the surrounding porous rocks. Larger permeabilities result in higher water temperatures at the production shaft due to greater inflows of warm water from those rock masses.     

Shallow submarine and subaerial, low-enthalpy hydrothermal manifestations in Punta Banda, Baja California, Mexico: Geophysical and geochemical characterization

The low-enthalpy geothermal system at Punta Banda (NW Baja California Peninsula, Mexico) has been studied because it might provide heat to future desalination plants in the city of Ensenada. Utilization of subaerial, intertidal and submarine hot springs is evaluated based on geochemical and geophysical data. The results of the geochemical studies show that the geothermal fluids have a major meteoric water component because seawater is not present at the subaerial springs and hot wells. The highest estimated reservoir temperature (140 °C) calculated using a silica geothermometer corresponds to the Agua Caliente intertidal manifestation, a promising area also identified by geophysics. Geothermometric calculations applied to the computed composition of the thermal end member yield a reservoir temperature of 137 °C. Cl/B ratios indicate that the thermal fluids discharged by the intertidal vents and subaerial springs are similar, but they differ from those of submarine vents. Geoelectrical models depict an anomalous conductive trend from the La Jolla well to the Agua Caliente manifestation, suggesting the presence of a fault that allows upflow of hot water from depth. Lastly, integration of geochemical and geophysical data identified the best site for future exploration drilling at Punta Banda.     

Electrical conductivity and P-wave velocity in rock samples from high-temperature Icelandic geothermal fields

Measurements of electrical conductivity and P-wave velocity of seven rock samples were made in the laboratory under inferred in situ conditions. The samples were collected from smectite and chlorite alteration zones in boreholes from the Krafla and Hengill, Iceland, geothermal areas. The measurements were done in the 25–250 °C range, with pore pressure and confining pressure equal to inferred in situ hydrostatic and lithostatic pressures, respectively. Conductivity increases linearly with temperature over the 30–170 °C range; that rise is considerably smaller above 170 °C. Time-dependent effects on conductivity occur above approximately 100 °C. These effects may be related to ion exchange between the clay minerals or the Stern layer, and the pore fluid. The temperature coefficient of conductivity is found to be considerably higher than attributed to pore fluid conduction alone, indicating interface conduction in an electrical double layer on the mineral-water interface in the pores. The results also show that there is no distinction in electrical conduction mechanism in the smectite and chlorite alteration zones; both are dominated by interface conductivity under in situ conditions. The sharp decrease in conductivity at the top of the chlorite alteration zone, commonly observed in resistivity surveys in high-temperature geothermal systems, is most likely due to the lower cation exchange capacity of chlorite compared to that of smectite.     

Optimal design of binary cycle power plants for water-dominated,medium-temperature geothermal fields

Exploitation of lower temperature, water-dominated geothermal fields is analyzed, and a methodology for optimizing geothermal binary plants is discussed. The geothermal fluid inlet temperatures considered are in the 110–160 °C range, while the return temperature of the brine is assumed to be between 70 and 100 °C. The analysis shows that the brine specific consumption, ranging from 20 to 120 kg s⁻¹ for each net MW produced, and the efficiency of the plants, ranging from 20% to 45% in terms of Second Law efficiency, are dictated mainly by the combination of the brine inlet temperature, the brine rejection temperature and the energy conversion cycle being used. For given operating conditions and with correct matching between working fluid and energy conversion cycle, it is possible to obtain very similar performances in a number of different cases. It is shown that optimization of the plant can yield improvements of up to 30–40% in terms of reduction of brine specific consumption compared to conventional design.     

Hydrology,hydrochemistry and geothermal potential of El Chichón volcano-hydrothermal system,Mexico

Fluid and heat discharge rates of thermal springs of El Chichón volcano were measured using the chloride inventory method. Four of the five known groups of hot springs discharge near-neutral Na–Ca–Cl–SO₄ waters with a similar composition (Cl ∼ 1500–2000 mg kg⁻¹ and Cl/SO₄ ∼ 3) and temperatures in the 50–74 °C range. The other group discharges acidic (pH 2.2–2.7) Na–Cl water of high salinity (>15 g/L). All five groups are located on the volcano slopes, 2–3 km in a straight line from the bottom of the volcano crater. They are in the upper parts of canyons where thermal waters mix with surface meteoric waters and form thermal streams. All these streams flow into the Río Magdalena, which is the only drainage of all thermal waters coming from the volcano. The total Cl and SO₄ discharges measured in the Río Magdalena downstream from its junction with all the thermal streams are very close to the sum of the transported Cl and SO₄ by each of these streams, indicating that the infiltration through the river bed is low. The net discharge rate of hydrothermal Cl measured for all thermal springs is about 468 g s⁻¹, which corresponds to 234 kg s⁻¹ of hot water with Cl = 2000 mg kg⁻¹. Together with earlier calculations of the hydrothermal steam output from the volcano crater, the total natural heat output from El Chichón is estimated to be about 160 MWt. Such a high and concentrated discharge of thermal waters from a hydrothermal system is not common and may indicate the high geothermal potential of the system. For the deep water temperatures in the 200–250 °C range (based on geothermometry), and a mass flow rate of 234 kg s⁻¹, the total heat being discharged by the upflowing hot waters may be 175–210 MWt.     

Gravity changes in the Tauhara sector of the Wairakei–Tauhara geothermal field,New Zealand

Five microgravity surveys, done between 1972 and 2006, show that in the northern part of the Tauhara geothermal field there were large gravity decreases prior to 1985 associated with the expansion of steam zones resulting from pressure drawdown caused by fluid extraction at Wairakei. Since 1985 there have been gravity increases of up to 240 μgal in the northern part of Tauhara, corresponding to a mass increase of about 20 Mt. The gravity increases are centred near the unused deep well TH4, and are inferred to result mainly from resaturation of a deep steam zone due to a downflow of water in the well. We suggest that the water entered the well from a confined groundwater aquifer at a known casing break at 393 m depth and exited in the region of slotted casing at about 900–1000 m depth causing displacement of single-phase liquid upwards into the overlying steam zone. The average downflow rate is estimated to be about 110 t/h (30 kg/s); however, no downhole measurements in the well have been possible due to casing breaks. Simple modelling of the gravity data suggests the region of resaturation had the form of a cone of impression 150–250 m high and extending laterally for 1–2 km. Since 1985, gravity changes in the central and southern parts of the Tauhara field have been less than 50 μgal, indicating little net mass loss (<2 Mt), and hence little effect in this area from the continuing production at Wairakei. The subsidence centred near Crown Road has been attributed to compaction of a thin, elliptical lens of porous, thermally-altered volcanic deposits at shallow depth as a result of a water level decline in the near-surface, steam-heated groundwater aquifer. Gravity data (1994 onwards) at a point near the centre of the subsidence bowl show that, despite ground subsidence of about 0.55 m, there have been no significant gravity changes. One explanation for the absence of gravity changes is that the water lost from the near-surface aquifer has drained downwards increasing the saturation in part of a deeper, partly saturated layer.     

Recent ground subsidence at Crown Road,Tauhara and its probable causes

A localised ground subsidence anomaly at Crown Road, Taupo, within the Tauhara field of the Wairakei–Tauhara geothermal system, has been subjected to intense scrutiny because of its relatively recent onset and proximity to urban areas. Over a period of 20 years a maximum of 0.63 m of subsidence has accumulated. Uncertainties regarding its cause remain, but the evidence now strongly favours a relatively shallow (about 50 m depth) origin, compared with other geothermal subsidence bowls at Wairakei, Tauhara and Ohaaki. Declining water levels in a shallow boiling aquifer are considered to be the principal driving mechanism at Crown Road. The source of the subsidence is an anomalously compressible formation of intensely altered ignimbrite found at the base of a buried hydrothermal eruption deposit. This formation is dominated by soft kaolinite and smectite-illite clays of high plasticity and water content, resulting from alteration of highly vesiculated pumice, and is capped by a thin hardpan of silicified pyroclastic material, characterised by vuggy macro-porosity, at about 33 m depth. During initiation of the subsidence event, this hardpan may have failed in shear mode around the edges of a buried eruption crater, allowing the overburden to fully load the underlying compressible clays.     

Investigations of deep resistivity structures at the Wairakei geothermal field

The Wairakei geothermal field was the proving ground for the use of electrical resistivity methods for geothermal exploration. At this site it was first demonstrated that a large contrast in resistivity existed between geothermal ground and the cold surroundings. Within the top 500 m of the geothermal field, low-resistivity (5–10 Ωm) reflects the effects of both the hot saline water in the pore spaces and the conductive rock-matrix. The first surveys at Wairakei used a Wenner array (a ∼550 m) to measure the resistivity values along tracks throughout the field; contour maps of the resistivities were used to estimate the lateral extent of the geothermal waters at a few hundred metres depth. In the late 1960s the Wenner array was superseded by the Schlumberger array (AB/2 = 500 m and 1000 m), which enabled deeper penetration and better definition of the extent of the geothermal waters. These early surveys showed that the bounds of the geothermal waters were often sharp, leading to the concept that a ‘resistivity boundary’ could be defined for New Zealand's liquid-dominated geothermal fields. As new methods of measuring electrical structure with greater precision became available, Wairakei was often chosen as the testing ground.     

Geothermal energy in Turkey: 2008 update

Geological studies indicate that the most important geothermal systems of western Turkey are located in the major grabens of the Menderes Metamorphic Massif, while those that are associated with local volcanism are more common in the central and eastern parts of the country. The present (2008) installed geothermal power generation capacity in Turkey is about 32.65 MWe, while that of direct use projects is around 795 MWt. Eleven major, high-to-medium enthalpy fields in western part of the country have 570 MWe of proven, 905 MWe of probable and 1389 MWe of possible geothermal reserves for power generation. In spite of the complex legal issues related to the development of Turkey's geothermal resources, their use is expected to increase in the future, particularly for electricity generation and for greenhouse heating.     

Dual-fluid-hybrid power plant co-powered by low-temperature geothermal water

Three variants of power plants fuelled or co-fuelled by geothermal water have been assessed, with the aim of making the best use of the energy contained in a stream of 80–120 °C geothermal water. Heat-flow calculations for three power plant types, namely an Organic Rankine Cycle (ORC) power plant, a dual-fluid-hybrid power plant and a single-fluid hybrid-fuelled power plant, are presented. The analysis shows the thermodynamic benefits, in terms of the extent of using the thermal energy of low-temperature geothermal water, that arise from utilizing hybrid and dual-fluid-hybrid power plants rather than ORC power plants. The dual-fluid plant optimizes the use of the geothermal water, but the hybrid plant makes the best overall utilization of the energy compared to separate ORC and fuel-fired plants.     

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.     

Low enthalpy geothermal energy utilisation schemes for greenhouse and district heating at Traianoupolis Evros, Greece

A socio-economic study has been made of the possible use of low enthalpy geothermal resources for district and greenhouse heating in the Traianoupolis Evros region. The thermal energy potential of the Aristino-Traianoupolis geothermal field has been estimated at 10.8 MW_th (discharge temperature of 25 °C). Geothermal wellhead water temperatures range from 53 to 92 °C, from 300 m deep wells yielding over 250 m³/h. Our conclusions show, amongst the different scenarios examined and on the basis of a market study, that utilisation of this geothermal energy capacity for district heating of nearby villages, and/or greenhouse heating directed at serving local vegetable markets, would be an attractive investment.     

Evolution of the Wairakei geothermal reservoir during 50 years of production

The Wairakei geothermal field has been under production for more than 50 years. Exploration wells show that the high-temperature and very permeable, productive resource extends over about 12 km² within a greater area of about 25 km² that shows various effects of thermal activity. Up to 2006, 3 km³ of fluid and 2750 PJ of energy had been extracted at an average rate of 5250 t/h and enthalpy of 1130 kJ/kg. Significant production started in 1955 and up to 1978 there was no injection of cooled geothermal fluids. During the first decade of operation a pressure drawdown of up to 20 bars (2 MPa) developed and spread evenly across the reservoir, even though fluid extraction was focused within an area of 1 km² close to the northeastern field boundary. This pressure reduction resulted in widespread boiling and formation of segregated steam zones at the top of the reservoir together with inflow of cooler fluids into its northeastern part via the original natural outflow channels. From 1975 to 1997 pressures in the deep liquid reservoir stabilized at 23–25 bars (2.3–2.5 MPa) below the original pressure, with little change up to the time injection commenced in 1998. This natural pressure support indicates that prior to injection there was substantial recharge, 80% of which is assessed as high-temperature deep inflow. Since 1998 about 30% of the extracted fluids have been injected and reservoir pressures have increased by 3–4 bars (0.3–0.4 MPa). To date, significant returns of injected fluids have not been detected in the production areas. Over the 50 years of operation, temperatures in the main production areas have declined from 250 to 220 °C while deeper production zones toward the western boundary of the reservoir have remained at about 250 °C. A series of deeper makeup wells to maintain future production have been drilled in the high-temperature recharge area. An increasing fraction of injection, both in-field and out-field is planned over the next few years.