Hydrothermal metamorphism in the Larderello geothermal field

The various tectonic units underlying the Larderello — Travale geothermal region have undergone hydrothermal metamorphism. The hydrothermal mineral assemblages are generally consistent with the temperatures now measured in the wells, leading to the hypothesis that solid phases deposited from a liquid medium during a hot-water stage that preceded the vapour-dominated one.     

Interactions between the steam reservoir and surrounding aquifers in the Larderello geothermal field

Isotopic analyses of steam samples from Larderello geothermal field are studied in order to reveal the interference between the steam field and surrounding aquifers. The steam produced is made up of a mixture of waters which have undergone different circulation patterns. Surface distribution of values is strongly dependent on hydrogeological conditions. Variations with time reveal the possibility of utilizing isotopic analyses for studying the field's evolution and the effects on it of exploitation.     

Analysis of reservoir pressure and decline curves in Serrazzano zone, Larderello geothermal field

An estimate of reserves in the Serrazzano reservoir was obtained from mass balance studies and production decline curve analyses.The Serrazzano reservoir consists of a geometrically well-defined structural high of permeable formations separated from the other productive regions of the Larderello field.Deep drilling began in the 1930s and was limited to a small area exhibiting natural manifestations. After the second World War the area of drilling was extended to about 20 km². Currently the drilling area is about the same. Even though the reservoir has been producing steam since the 1930s, a systematic collection of production data did not begin until after 1953.Data on average reservoir pressures were not available for the material balance calculations made in the study reported here. Calculated bottom hole pressures of shut-in wells were taken therefore to represent local static reservoir pressures. These pressures were used to calculate an “average reservoir pressure” which was graphed as a function of cumulative production. The reservoir pressure history corresponding to the first half of current cumulative production is not known. Data for the second half indicate a linear relationship between “reservoir pressure” and cumulative production.The conventional straight-line p/z vs cumulative production material balance relationship is known to be correct, of course, for closed single-phase gas reservoirs. The validity of this linearity for stream-producing systems with boiling water has not been proved. Regardless of this, the following observations were made: a line connecting the available data points extrapolated back to zero production indicates an initial reservoir pressure approximating at least 40 atm. Extrapolating the same data to zero reservoir pressure indicates the total initial steam in place to be about 170 × 10⁶ tons.An empirical type-curve matching technique was applied to the production decline curves of wells in the reservoir. The curve for each well was extrapolated to infinite production time to obtain an estimate of total past and future production. Summing these values for all producing wells in the reservoir, an estimated total production (past and future) of 200 × 10⁶ tons was obtained.The agreement between the estimated total production applying material balance principles and decline curve analyses is remarkably good. Although these results may be useful, further field and theoretical work are necessary to prove their validity.     

Secondary changes in the chemical and isotopic composition of the geothermal fluids in Larderello field

Processes such as condensation of rising steam in colder horizons, mixing of deep steam with shallower waters and non-geothermal gas inflow are known to exist in the vapour-dominated geothermal system of Larderello. Deep steam drawn up by geothermal wells modifies its original chemico-physical characteristics in relation to the presence of one or more ‘secondary’ phenomena occurring in each point of the field. ¹³C/¹²C ratios in CO₂ and CH₄, chloride-ammonia-boric acid contents, H₂/H₂S and gas/steam ratios are used as useful parameters in delimiting the different zones in which this kind of phenomena predominate. Six main areas seem to be characterized geochemically, showing marked variations that are essentially due to differences in the geohydrological situations and thermal conditions.     

The oxygen isotope exchange between carbon dioxide and water in the Larderello geothermal field (Italy) during fluid reinjection

The oxygen isotope compositions of CO₂ and water vapor samples collected from Larderello geothermal wells after the start of the fluid reinjection program suggest that if the oxygen isotope exchange in the vapor phase does, in fact, exist, it is a very slow process when compared with the residence time of the fluids in the geothermal reservoir. This is because carbon dioxide and water vapor phases could not have equilibrated significantly in the vapor-dominated reservoir. This conclusion implies that the oxygen isotope composition of carbon dioxide may possibly be used as a tool in geothermal exploration for revealing the presence of liquid water in deep geothermal systems. Based on the interpretation of the oxygen isotope data of the CO₂, I propose that the origin of the low oxygen isotope ratios of carbon dioxide at Larderello is the high-temperature exchange with liquid water in the lower reservoir. In Larderello, the liquid water–rock interaction in the lower reservoir may have increased the ¹⁸O/¹⁶O ratio of the recharge meteoric component. By contrast, lack of high-temperature liquid water in the upper reservoir suggests that the large “δ¹⁸O shift” described for the upper-reservoir steam during the last decades reflects varying degrees of dilution of the lower-reservoir fluid by the low-¹⁸O vaporized liquid water of meteoric origin that recharges the field at shallow depth, with local contribution from still deeper high-¹⁸O water vapor of magmatic origin. The low oxygen isotope composition of the Mesozoic carbonaceous rocks that form the upper reservoir, consequently, likely represents a “fossil” record of the past hot-water geothermal stage.     

Mofete geothermal field

This report deals with the Mofete geothermal field, which was discovered in southern Italy by AGIP, the Italian state-owned company for exploration of oil, gas and other alternative energy resources, such as geothermal. AGIP is the main operator in the Mofete geothermal field, in joint venture with ENEL, the National Electricity Board. Seven wells have been drilled, and two producing aquifers found at depths between 500 and 2000 m. The preliminary interpretation of a long-term test (three months), together with the model of the field, suggest that a 10 MW power-plant will operate in the near future in the limited area considered favourable so far for geothermal production.     

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 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.     

Assessment of geothermal potential of central and southern Tuscany

Central and southern Tuscany contain all the presently exploited geothermal fields of Italy, and accordingly were chosen to test the geothermal resource assessment methodology described by Muffler and Cataldi (this issue). Using new compilations at 1:200,000 of all available drill-hole, geological, gravity, and thermal-gradient data, the region was divided into 31 zones, each of reasonably homogenous geology and thermal regime. The upper 3 km of each area was then divided by horizontal surfaces into three volumes consisting of (1) the impermeable cover (Neoautochthon, Ligurids, and the upper terrigenous part of the Tuscan Series), (2) the Jurassic and Triassic rocks that form the main reservoir complex (the lower carbonate part of the Tuscan Series) and (3) the underlying Triassic and Paleozoic quartzite and phyllite of low porosity, thus allowing calculation of geothermal energy in both rock and pore water. The aggregate of these values is the “accessible geothermal resource base” of central and southern Tuscany.     

Thermodynamic behaviour of the Bagnore geothermal field

In 1959 production began from the Bagnore reservoir near the Mt. Amiata Volcano. The reservoir gas was initially at a pressure of approximately 23 ata. Its noncondensable gas content was more than 80% by weight, most of which was CO₂. During the first few years of production the noncondensable gas content and the reservoir pressure dropped simultaneously to about 10% by weight and 7 ata respectively. They have been changing very slowly since that time.Detailed studies of hydrogeological data from the Bagnore field were made as a part of this work. The initial reservoir temperature was estimated to be between 170 and 180°C. The history of watering-out of individual wells on the periphery of the field was examined. The depth of fractures in these wells can be correlated with the gas-water interface in the reservoir which is assumed to rise in direct proportion to the drop in reservoir pressure.A mathematical model which accounts for thermodynamic and chemical equilibria between the vapor, liquid and solid carbonate phases in the reservoir was developed and applied to a study of the initial conditions in the reservoir. A lumped-parameter model of a 2-phase, 2-component system was then developed. This CO₂-H₂O, liquid-vapor model was used to calculate history of pressure and composition for the reservoir. These calculated histories compared favourably with those observed in the field.This research confirms the hypothesis that there was initially a large accumulation of noncondensable gas in the reservoir, and that it was drawn off during the first years of exploitation. Model calculations for the initial state of the reservoir indicate the CO₂ initially present could not be derived solely from local carbonate rocks. Calculations with the producing-state lumped-parameter model, furthermore, indicate that the long-term producing concentration of CO₂ cannot be accounted for by assuming reasonable amounts of CO₂-saturated liquid-water influx into the reservoir. These results point out that further investigation into the nature of CO₂ and water influx into the reservoir are required.     

Sustainable development of the Kamojang geothermal field

Geothermal electricity production in Indonesia began with the operation of a 0.25 MW_e pilot project in Kamojang geothermal field, in 1978. Commercial operation started in 1983, with the commissioning of the 30 MW_e Unit-1 power plant. In 1987, an additional capacity of 110 MW_e was provided by the Unit-2 and Unit-3 power plants. The addition of the 60 MWe Unit-4 power plant in 2008 increased the total generating capacity to 200 MW_e. The 27 years of commercial operation have led to a slight decline in reservoir pressure and temperature within the active production sector. The most recent significant change in the field conditions and performance occurred following the 2008 increase in generating capacity from 140 to 200 MWe. The production decline of individual wells has been relatively low, at an average of 3%/yr. However, the increased rate of steam withdrawal might negatively affect long-term sustainability of energy production at Kamojang unless suitable field management strategies are implemented. In order to stabilize the steam flow, it has been necessary to drill about three make-up wells every 2–3 years. The unbalanced mass extraction, where less than 30% of the produced steam mass can be injected, is a serious concern for long-term reservoir management in Kamojang. The field operator (Pertamina) plans to increase the Kamojang generating capacity from 200 to 230 MWe (Unit 5) and optimize the long-term performance of the Kamojang geothermal resource. The response of the reservoir during the previous three decades is being used to guide reservoir development for the planned increase in production capacity.     

Geology of the Cerro Prieto geothermal field

The Cerro Prieto geothermal field is located in the Mexicali Valley which is characterized morphologically by the presence of the elongated Cucapah range striking predominantly NW-SE. This range consists of Upper Cretaceous granite which has intruded and metamorphized the Cretaceous and/or Paleozoic sediments. Near the field is the rhyodacitic Cerro Prieto volcano which pierces through the Cenozoic deltaic sediments.We divided the deltaic sediments into two lithostratigraphic units: Unit A: Unconsolidated Quaternary deltaic sediments composed of clays, sands and gravels. Unit B: Consolidated Tertiary deltaic sediments composed of siltstone, shales and sandstones.Since the geothermal aquifer is intimately related to the structural behavior of the geological formations which are overlaid by unconsolidated deltaic sediments, it was necessary to apply different geophysical methods to understand the behavior of the consolidated sediments and the basement.Based upon geophysical surveys and the wells completed to date, a regional geologic model and several cross-sections of the Cerro Prieto field have been developed.Two fault systems have been defined. The principal one, which we called the Cerro Prieto system, has a predominantly NW-SE strike, parallel to important faults such as the San Andreas, San Jacinto, Cucapah, Imperial etc.Normal to the NW-SE system are faults with predominantly SW-NE strikes, which we designated the Volcano system. At depth, these two systems have apparently created a step-faulted horst and graben structure striking NW-SE with its eastern and western sides out.A number of cross-sections were made based on petrographic analyses of cuttings and core samples. These sections confirm the existence of two fault systems passing through the geothermal field, which complicates the structural interpretation of the field.The geologic sections based on the petrographic analysis of rock samples obtained from the drillings render very complicated a structural interpretation of the two fault systems which affect the geothermal field and which occur locally in great frequency.     

Isotope geothermometry in the larderello geothermal field

The isotope geothermometers based on the ¹³C/¹²C fractionation between carbon dioxide and methane and on the ¹⁸O/¹⁶O fractionation between carbon dioxide and water vapour have been applied in Larderello geothermal field. The CO₂ - CH₄ thermometer gives temperatures which are 50–200°C higher than those measured at the well head. The distribution of the isotopic temperatures within the field follows more or less similar patterns to those given by the well-head temperatures. They are believed to reflect the temperatures of formation of CO₂ and CH₄.The CO₂ - H₂O thermometer gives the temperature of the geothermal reservoirs tapped, and the difference between the isotopic temperature and the temperature measured at the well head is a measure of the cooling undergone by geothermal fluid on its way up to the surface.     

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.     

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.     

High salinity fluid handling in Milos geothermal field

On Milos island, where a high enthalpy gecthermal field capable:of producing high salinity fluids has been discovered a 2MW Pilot plant was installed. Systematic production tests have been carried out at the production well, with emphasis on geothermal fluid chemistry and behaviour. During successive long term operation trials of the Unit (turbogenerator, steam gathering and brine transmission system), problems arose in both steam and brine cycles due to severe scaling phenomena encountered. The sequence of events in identifying the scaling problems and the technical approaches applied to remedy them, are reported in this paper together with data concerning the plant's operational problems and with the direct hot reinjection used as brine disposal method.