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.     

A test of the transiel method on the travale geothermal field

An original electromagnetic method has been applied to geothermal prospecting on the Travale test site. The results show good correlations between observed polarization anomalies and productive zones. It is believed that these anomalies are related to reduction phenomena that occurred in the overburden (such as pyrite formation) caused by thermochemical exchanges between the reservoir and the overburden above those zones where the reservoir permeability is highest.     

P and S reflection seismic profiling and well logging in the travale geothermal field

Two high resolution seismic reflection profiles, obtained with vibrators as sources of P and of horizontally polarized SH waves, were recorded and elaborated. In the well CH-3, 2704 m deep and located close to the profiles, several geophysical logs were carried out: temperature, SP and induction, density, natural radioactivity, and sonic logs with P and S wave velocity measurements. These data enabled the calculation of the elasticity moduli and of the mechanical properties of the rocks affected. The sonic log results were subsequently calibrated by means of a vertical seismic profile which permitted the computation of synthetic seismograms and a VSP log for comparison with the seismic sections and correlation between S and P seismic reflectors. The structural maps obtained from data interpretation describe with sufficient detail the fracture trend of the Travale field.By comparing V_s and V_p velocities for the different geological units, as defined by the interpretation, the change in the α = V_p/V_s ratio and in the Poisson coefficient has been tentatively utilized to correlate the seismic data with material properties.The influence of lithology on the elastic constants is discussed, as well as the influence of the saturating fluids and of the degree of saturation. The different attenuation of the P and S waves, depending on the above-mentioned characteristics, demonstrates the possibilities offered by seismic methods in the evaluation of the potential of geothermal reservoirs.     

Differential magnetic soundings in the travale geothermal area

In 1980 and 1982 the Laboratory of Applied Geophysics of the C.N.R.S. attempted to establish, by magnetic differential sounding, a possible conductivity anomaly linked with the geothermal field of Travale, Tuscany. Some 25 sites were occupied along two profiles, one between Siena and Populonia, near Piombino, the other between Siena and Cecina. An important anomaly of the transient magnetic field (some 15% of the normal field) was discovered in 1980 between Gerfalco (in the SW) and Frosini (in the NE). It exactly covers the geothermal area of Travale. The direction of the telluric currents causing the anomaly is parallel to the magnetic meridian and their maximum depth is of some 2000 m. The 1982 campaign showed that in the north of Travale, anomalous currents move in a NW - SE direction or even completely EW (SW of Volterra) and meet in the sea near Livorno. One possible interpretation of these phenomena as a whole is to assume the presence of very conductive layers between Larderello and Travale. The currents which circulate parallel to the coast are channelled locally by this structure, which could be closely linked with the geothermal field.     

Natural electric field survey in three Southern Italy geothermal areas

The intensity of natural electric fields, as measured in three geothermal areas in southern Italy, varies from a few mV/m to more than 10 mV/m. Such extremely high values may be explained as due to self-potentials generated at the contact between the highly conductive water solutions inside the geothermal system and the waters outside the system and/or at the contact between altered and non-altered rocks.     

The geophysical exploration of geothermal reservoirs: the travale test site (Italy)

Geothermal reservoirs are usually tied to geological structures that are not easily explored by one geophysical method only. The combined application of several different techniques seems to be the most promising strategy. The development of the best combination of methods, comparison of the available instruments, and a number of processing and interpretation techniques have to be tested on a well-studied geothermal reservoir. The Travale geothermal field (Tuscany, Italy) is a high enthalpy hydrothermal system with single phase (vapour or liquid) or two-phase conditions, dominating in different parts of the reservoir. This field was chosen as a test site for geophysicists of the EEC member states. Since 1980 electrical, electromagnetic, magnetic and seismic exploration techniques have been tested in cooperation with ENEL, the Italian Electricity Agency, under the sponsorship of the Community.     

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.     

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.     

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.     

First results of the application of the dipole electrical sounding method in the geothermal area of travale - radicondoli (Tuscany)

A new geoelectric prospecting method has been tested in the Travale - Radicondoli geothermal area. This method is based on the dipolar technique that permits investigation at very great depths with much fewer problems than encountered when using the classical electric prospecting techniques.The following steps were taken in order to operate with relatively low power from a 2 kW generator:

1. (i) the ground was energized with a series of current square waves at a frequency of less than 0.05 Hz in order to avoid the effects of electromagnetic coupling and induced polarization;

2. (ii) the voltage was recorded digitally at the measuring dipole;

3. (iii) the voltage recordings were processed by the spectral analysis method of “maximum likelihood”.

The resulting apparent resistivity diagrams were transformed into Schlumberger diagrams and then interpreted quantitatively.The six soundings are too limited in number to represent a real prospecting but refer to different geological and structural situations typical of a geothermal area. Two electrosoundings were sited for this purpose so as to be directly calibrated by the wells in the local geothermal field. The quantitative analysis of the resistivity diagrams in particular revealed the low resistivity values of the carbonate formation forming the geothermal reservoir, where the hot fluid circulation is particularly strong (15 Ω.m).The dipolar method has proved capable of distinguishing, in the geological situation of Travale area, the various structural features of the geothermal field such as “cover”, “reservoir”. substratum, uplifted structures and tectonic depressions.     

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.     

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.     

P and S arrival time anomalies at a dense array: Marker of the travale field

Arrival times of seismic waves propagated to a dense array (20 three-component seismometers on a 3 × 3 km area) from natural sources beneath the Travale field, have been analysed for lateral variations which can be related to the extent of the productive reservoir. Significant teleseismic delays between sites only a few hundred meters apart correlate with productive well R9. Spatial variation of both P and S travel arrival times from local earthquakes at the edge of the array and 4 km deep beneath the reservoir consistently delineate its lateral extent to the south and east as documented by drillholes.     

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.     

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.     

Magnetotelluric soundings in the travale area, Tuscany

Long period MT soundings have been completed on ten sites already recorded on the Travale geothermal field in order to check the reproducibility of past data and provide new information on the geothermal structure and, eventually, on reservoir properties. A versatile equipment allowing for recording signals in the 100 – 0.01 Hz range was used with recording periods up to 6 hours. Although noise was a serious problem, especially in the 1 – 3 s window, it was possible to achieve acceptable signal to noise ratios and in most cases to produce reliable signal processing. Records also display a strong anisotropy depending on structural strikes. Interpretation did not call for sophisticated 2D or 3D modelling but instead focussed on 1D inversion along the less disturbed principal direction. It is concluded that because of a geoelectrical setting with a conductive cover overlying a resistive reservoir and basement, electrical conductance is a parameter representative neither of reservoir structure nor of soaking fluid properties. Essentially it reflects conductivity and/or thickness variations within cover formations. The best fit was found for a three layer structure associating a superficial resistive horizon, an intermediate, generally 1000 m thick, conductive complex and a resistive basement. It is not possible to discriminate within this resistive complex any conductive anomaly or other reservoir feature. In conclusion, the information yielded by the MT method is essentially of structural nature. The top of the carbonate reservoir could be mapped with fair confidence in good agreement with available drilling data.     

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.     

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.