Hydrothermal alteration in the Aluto-Langano geothermal field, Ethiopia

The hydrothermal mineral assemblages found in eight wells (with a depth range of 1320–2500 m) of the active geothermal field of Aluto-Langano (Ethiopia) indicate a complex evolution of water-rock interaction processes. The zone of upflow is characterized by high temperatures (up to 335°C) and the presence of a propylitic alteration (epidote, calcite, quartz and chlorite, as major phases) coexisting with calcite and clay minerals. The zone of lateral outflow is characterized by mixing of deep and shallow waters and the occurrence of a calcite-clay alteration that overprints a previous propylitic assemblage. Clay minerals have a mushroom-shaped zonal distribution consistent with the present thermal structure of the field. Microprobe analyses have been carried out on chlorite and illite in order to apply several geothermometers. Most of the chlorite is iron-rich chlorite. It is found that the temperatures calculated from the chlorite geothermometer (159–292°C) after Cathelineau and Nieva [Contrib. Mineral. Petrol. 91, 235–244 (1985)] are in good agreement with in-hole measured temperatures (155–300°C). In the upflow zone, temperatures calculated from this geothermometer (217–292°C), together with fluid inclusion data of Valori et al. [Eur. J. Mineral. 4, 907–919 (1992)], and computed saturation indices of alteration minerals, indicate thermal stability or slight heating. On the other hand, evidence of a significant cooling process (up to 171°C) in the outflow zone is provided by the comparison between fluid inclusion homogenization temperature (240–326°C) and in-hole temperature (155–250°C). The apparent salinities (0.8–2.3 wt% NaCl eq.) of the fluid inclusions are generally higher than the salinity of the present reservoir fluid (0.29–0.36 wt% NaCl eq.). Clay minerals (illite, smectite, Ill/S mixed layers, vermiculite and chloritic intergrades) generally occur at temperatures consistent with their stability fields.     

Reservoir engineering assessment of low-temperature geothermal resources in the Skierniewice municipality (Poland)

Low temperature geothermal resources and their production potential in the Skierniewice area of Poland were evaluated assuming conventional well doublet arrays. The reservoir engineering assessment was carried out, within the framework of a World Bank project, to study the feasibility of providing heat to the local district heating system, using data from two existing wells and from geophysical surveys, and by evaluating results of production, injection and interference well tests. Two options were simulated mathematically, using both semianalytical and numerical codes, considering a simplified reservoir model: one based on the two existing wells, the other considering four wells, two to be drilled. The injection of spent brines into a different reservoir was also studied.     

Reservoir analysis of the Palinpinon geothermal field, Negros Oriental, Philippines

The Philippine National Oil Company and Lawrence Berkeley Laboratory have conducted an informal cooperative project on the reservoir evaluation of the Palinpinon geothermal field in the Philippines. The work involved the development of various numerical models of the field in order to understand the observed data. A three-dimensional porous medium model of the reservoir has been developed that matches well the observed pressure declines and enthalpy transients of the wells. Submodels representing the reservoir as a fractured porous medium were developed for the analysis of chemical transport of chlorides within the reservoir and the movement of the cold water front away from injection wells. These models indicate that the effective porosity of the reservoir varies between 1 and 7% and the effective permeability between 1 and 45 millidarcies. The numerical models were used to predict the future performance of the Palinpinon reservoir using various possible exploitation scenarios. A limited number of make-up wells were allocated to each sector of the field. When all the make-up wells had been put on line, power production gradually began to decline. The model indicates that under the assumed conditions it will not be possible to maintain the planned power production of 112.5 MWe at Palinpinon 1 and 80 MWe at Palinpinon II for the next 30 years, but the decline in power output will be within acceptable normal operating capacities of the plants.     

Reservoir characteristics and development plan of the Oguni geothermal field, Kyushu, Japan

Geothermal power development at the Oguni field, central Kyushu, is planned to begin in the year 2001 with a double flash system generating 20 MW_e. The Oguni reservoir has been delineated by systematic geothermal surveys, well tests and reservoir engineering studies. The fractured reservoir is horizontally layered and divided into northern and southern portions. Both of them have NaCl dominant fluids ranging from 200 to 240°C. The northern reservoir covers a large area including the Takenoyu Fault Zone and has a relatively high permeability (kh≈80–230 darcy-m). By contrast, the southern reservoir covers a relatively small area and has limited transmissivity. The southern reservoir has a higher pressure (1 MPa) than the northern reservoir, indicating little connectivity between them. Based on numerical calculations, as well as the surface topography and environmental aspects, the production and reinjection zones have been separated, and a large part of the necessary fluid will be produced from the northern reservoir along the Takenoyu Fault and a small part will be taken from the southern reservoir. The separated water will be reinjected into the northernmost part of the northern reservoir, in order to prevent a cold sweep problem for production.     

The Bulalo geothermal field, Philippines: Reservoir characteristics and response to production

The Bulalo geothermal field has been operating since 1979, and currently has 330 MWe of installed capacity. The field is associated with a 0.5 Ma dacite dome on the southeastern flank of the Late Pliocene to Quaternary Mt. Makiling stratovolcano. The reservoir occurs within pre-Makiling andesite flows and pyroclastic rocks capped by the volcanic products of Mt. Makiling. Initially, the reservoir was liquid-dominated with a two-phase zone overlying the neutral-pH liquid. Exploitation has resulted in an enlargement of the two-phase zone, return to the reservoir of separated waste liquid that has been injected, scaling in the wellbores and rock formation, and influx of cooler groundwaters. Return of injected waters to the reservoir and scaling have been the major reservoir management concerns. These have been mitigated effectively by relocating injection wells farther away from the production area and by dissolving scale from wells with an acid treatment.     

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.     

Hydrothermal alteration minerals in Aluto Langano geothermal wells, Ethiopia

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

Influence of late cenozoic glaciations on the geothermal field in Northern and Central Europe

On the basis of the numerical solution of Stefan-like boundary-value problems the simplest palaeoclimatic and geothermal models are analyzed. Typical histories of freezing isotherms, temperature and heat flow changes are calculated taking into account the effects of accumulation and denudation. The results show that the geothermal field demonstrates a better ability to recollect palaeocryologic environment when the phase transitions of subsurface water are involved. However, the geothermal traces of past cryogenic events in Northern and Central Europe before the middle of the Pleistocene could hardly remain up to the present day.     

Gas-water interface rise during early exploitation tests in alfina geothermal field (Northern Latium, Italy)

Alfina geothermal field is made up of a gas-cap (CO₂) overlying an almost closed aquifer. The wells drilled in the highest part of the reservoir produce gas while the others either produce hot water or are sterile.During the first production tests the analysis of pressure and water-levels showed that the gas-water interface had risen. This was later confirmed when a gas-producing well began carrying water.This paper attempts to explain field behaviour by means of simple calculations and assuming an idealized geological structure.This study gives an estimation of the average porosity and initial gas in place.     

Sustainability of the Hatchobaru geothermal field, Japan

The Hatchobaru power plant Unit No. 1 (55 MW) has been operating since 1977 and Unit No. 2 (55 MW) since 1990. The mean capacity factor of the power plant has reached about 90%. Considering that the long-term operation of the plant, over 30 years for Unit No. 1 and nearly 20 years for Unit No. 2, has been maintained with such a high capacity factor, sustainable development in terms of economic production has been achieved. To maintain a stable operation, systematic reservoir monitoring and reservoir simulation studies have been conducted. The monitoring of changes in reservoir pressure, temperature and gravity indicates that the reservoir is currently approaching a stable state. Results of a simulation study suggest that the sustainable power output of the Hatchobaru reservoir is approximately 120 MW, and each productive fault has the capacity to produce enough steam to generate from 11 to 55 MW. Therefore, it would be possible to maintain the rated power output of 110 MW by optimizing well alignments so that the mass production can be kept within the sustainable productivity of each fault, and the injected water does not cool the production zones.     

Preliminary geothermal model of the travale field, Italy

Based on geological and geothermal knowledge of the Travale geothermal field, a mathematical model is proposed for the heat transfer from the top of the reservoir to the soil surface.Consistency between observed and calculated temperatures demonstrates that the heat is transferred by conduction and that the established temperature field depends essentially on the temperature at the top of the reservoir and its slope.     

Development history of the Tiwi geothermal field, Philippines

Commercial production of electricity from the Tiwi geothermal system began in 1979. In 1982, Tiwi became the world's first water-dominated system to produce more than 160 MWe. Today the field supplies about 11% of Luzon's electricity. Initially, the reservoir was single-phase liquid with a small, shallow steam zone on the east side. Temperature reversals in the first wells showed the east to be an outflow zone. As production began, reservoir pressure declined, two-phase conditions developed, and groundwater entered the reservoir from the east. As many production wells cooled, brine production increased and generation decreased from about 280 MWe in 1983 to about 190 MWe in 1986. Improvements to surface facilities and new wells drilled farther west raised generation to about 280 MWe by mid-1993. Separated brine was first injected into the reservoir, but this lowered steam production; injection is now outside the field.     

Appraisal of the Tokaanu–Waihi geothermal field and its relationship with the Tongariro geothermal field, New Zealand

Tokaanu–Waihi geothermal field is situated near the southern end of the Taupo Volcanic Zone, New Zealand. Neutral chloride thermal waters discharge at Tokaanu and Waihi in the north of the field on flat land between the andesite volcanoes Tihia and Kakaramea and the shore of Lake Taupo, while steam-heated thermal features occur at Hipaua on the northern flanks of Kakaramea. Electrical resistivity surveys have been made over the field using several different measurement techniques. In the north of the field where roads and tracks allow vehicle access, resistivity profiling using Schlumberger arrays with electrode spacings (AB/2) of 500 m and 1000 m show that Tokaanu, Waihi and Hipaua all lie within a continuous region of low apparent resistivity (5–20 Ωm) and are thus part of the same geothermal system. Along the eastern edge of the system there is a sharp transition to apparent resistivities greater than 100 Ωm in the cold surrounding region. Surveys on Lake Taupo using an equatorial bipole-bipole electrode array towed behind boats (spacing equivalent to AB/2=500 m) found that the low resistivity zone extends offshore by about 1 km. The steep, bush-clad, southern part of the field was surveyed with magnetotelluric (MT) resistivity measurements using both naturally occurring signals and the 50 Hz radiation from the power wires as sources. These measurements found low resistivities over the north-eastern slopes and around the summits of Tihia and Kakaramea, indicating thermal activity. However, the measurements were too widely spaced to allow the field boundary to be clearly delineated. Interpretation of the resistivity and other data suggests that the Tokaanu–Waihi thermal waters rise nearly vertically from a source deep beneath the elevated southwestern part of the field to the water table. These waters then flow north to discharge at the surface near Lake Taupo. Neighbouring geothermal systems, which occur at Tongariro about 18 km south of Tokaanu–Waihi, and at Motuoapa about 10 km to the northeast, are separated from the Tokaanu–Waihi field by high resistivity ground. This suggests that the thermal fluids discharging at the three fields do not have a common source, as has been suggested previously.     

Production testing in miravalles geothermal field, Costa Rica

Wellbore deposition of calcium carbonate has occurred in the first wells drilled in the Miravalles geothermal field, Costa Rica. Well PGM-1 was flowtested for periods of 1, 5 and 6 months in 1980, 1981 and 1982, respectively. The well is 1300 m deep and produces from a liquid-dominated reservoir at about 240°C. The flowtests illustrate the time behavior of wells suffering from wellbore deposition. At early time the decrease in output flow rate and wellhead pressure is slow but at late time it becomes rapid; the curves are concave down. The ratio (flow rate/wellhead pressure) remains nearly constant with time for wells suffering calcium carbonate wellbore deposition, at least in the Miravalles geothermal field.     

Characterization of geothermal reservoirs with electrical surveys: Beowawe geothermal field

The work reported here was undertaken to test the utility of electrical surveys for geothermal reservoir characterization using existing exploration and well data sets from the operating Beowawe geothermal field located in the Basin and Range Province of western USA. The STAR geothermal reservoir simulator was used to model the natural state of the system, and to compute the subsurface distributions of temperature and salinity, which were in turn utilized to calculate pore-fluid resistivity. Archie's law, which relates formation resistivity to porosity and pore-fluid resistivity, was adopted to infer the formation resistivity distribution. Subsequently, direct current (DC) resistivity, magnetotelluric (MT) and self-potential (SP) postprocessors were used to compute the expected response corresponding to available survey data. The measured apparent resistivity distribution from a dipole–dipole DC resistivity survey is in good agreement with the computed values. The calculated self-potential distribution agrees with the main features of an available SP survey. Although the computed MT apparent resistivity sounding curves reproduce the shapes of the measured MT sounding curves, an overall scale factor exists between the measured and calculated MT responses, and similarly with the computed dipole–dipole resistivity model. Possible reasons are static shifts in the coarsely sampled MT stations, and resistivity anisotropy due to the stratigraphy. Taken as a whole, the results of this study support the view that a suite of carefully designed electrical surveys (DC, MT, and SP) may be employed to infer favorable subsurface geothermal reservoir characteristics.     

Geology, mineralogy and stable isotope geochemistry of the cesano geothermal field (Sabatini Mts. volcanic system, Northern Latium, Italy)

Some deep wells (1400–3000 m) have been drilled into the volcanic system of the Sabatini Mts., north of Rome, in search of high enthalpy geothermal fluids.Baccano caldera has formed over a carbonate structural high with a Quaternary volcanic cover, on the inside of a large graben that developed from the Upper Miocene and is characterized by a strong positive thermal anomaly. A spatially irregular Neogenic neoautochthon cycle, an allochthonous flysch complex (‘Sicilidi’ s.l.) and a Meso-Cenozoic ‘basal’ carbonate sequence were recognized in the sediments crossed by these wells below the volcanic cover. The observed structural setting can be correlated to that of analogous outcropping structures (Mt. Soratte) and shows a tensional tectonic phase overlying an older, mainly compressional phase.The stratigraphic and mineralogic analyses revealed important differences in the Trias, which is mainly calcareous in the western sector of the area and anhydritic in the eastern: the two facies at present seem to be tectonically brought closer to one another.The isotopic analyses all show values comparable to those of the sedimentary carbonate host rocks, suggesting limited circulation of the hydrothermal fluids. ¹⁸O/¹⁶O analyses on specimens in the most restrictive locations next to interfaces show a relatively larger extent of isotopic exchange with aqueous fluids.Fluid circulation, which is mainly extinct, seems to belong to a closed system. Two types of circulation have been recognized: the first of these is connected to the stratigraphic discontinuities (especially at the contact between the allochthonous flysch complex and the basal carbonate sequence) and is characterized by mainly K-feldspar and Sulphide-Sulphate mineralizations; the second circulation is tied to the fracture systems and is particularly widespread in the pyroclastic cover where it appears in the form of sulphate- and carbonate-rich dykes. This system seems to have undergone a mixing with surface waters.     

Geology of the Olkaria geothermal field

Up to now development of the resource in Olkaria geothermal field, Kenya, has been based on fragmental information that is inconclusive in most respects. Development has been concentrated in an area of 4 km² at most, with well to well spacing of less than 300 m. The move now is to understand the greater Olkaria field by siting exploratory wells in different parts of the area considered of reasonable potential. To correlate the data available from the different parts of the field, the geology of the area, as a base for the composite field model, is discussed and shown to have major controls over fluid movements in the area and other features.     

Exploration of Lahendong geothermal field in North Sulawesi, Indonesia

This paper describes the progress made in developing the geothermal resources at Lahendong, North Sulawesi, Indonesia for utilization in power generation. Exploration of the whole region included a geophysical survey undertaken exclusively by the Volcanological Survey of Indonesia (VSI). A temperature survey at various depths was conducted through gradient boreholes. The results show that the area of anomalous temperature corresponds to the area of low resistivity revealed by the seismic survey. Two shallow exploratory boreholes (300–400 m) drilled by VSI confirmed the existence of the resources. The deep reservoir in Lahendong field extends over an area of 10 km²; the upper parts of the reservoir are presumed to be water dominated (temperatures in excess of 200°C) and to overlie a zone of hot chloride water at an undetermined depth. The potential of Lahendong field is estimated to about 90 MW.In Pelita IV (1984–1989), the fourth 5-year plan, the State Electricity Public Corporation plans to construct a 30 MW geothermal power-plant in the Lahendong field.     

First decade of geothermal development in Yangbajain field, China

The electric use of geothermal energy in China on a trial basis began in the early 1970s. Since then a total of nine small-scale pilot units, with the exception of Yangbajain, using underground thermal water for electricity generation, have been constructed in different parts of China. All of these localities are already covered by regional electric grids; these units cannot be run safely, steadily and continuously, so it is not possible to attach great importance to these miniplants. Based on their operating experience and thermodynamic and economic analyses in the present technological conditions, it appears that a geothermal water with temperature well below 100°C is favorable only to non-electric uses, except in remote areas with an extreme shortage of conventional energy sources (Liu, 1985). At present the State and local governments take only the Yangbajain geothermal field seriously. The significant achievements in Yangbajain have certainly been made possible by a number of conditions (Wu et al., 1985): a serious lack of oil and coal, and a great difficulty in developing the Xizang hydropower resources, for which an enormous capital investment is evidently needed; hydrothermal resources are present all over the Xizang plateau, usually at relatively shallow depths and consequently more convenient for exploration and exploitation; a willingness on the part of geothermal experts from the hinterlands of China to explore this new energy resource. In addition, the scientific significance of the geothermal phenomena in Xizang Plateau is of considerable interest to many geoscientists and engineers.