Lithological correlations of the cerro prieto wells, based on well - log interpretation

This paper presents the results of correlations of the geophysical well logs obtained from wells drilled at the Cerro Prieto geothermal field. From this correlation, a structural interpretation of the ‘receptacle’ holding the geothermal fluids has been obtained.Based on the temperature and electrical resistivity logs, the presence of two main geothermal reservoirs has been established. It was also found that most of the wells were completed such that production is obtained simultaneously from several sandy layers. In all probability this has created some mechanical problems in the wells in addition to a certain confusion in the interpretations based on fluid production data.An hypothesis is postulated about the deposition environment and the genesis of the reservoir, on the basis of which it is concluded that the reservoir is a paleochannel. It is also postulated that thinning of the basement rocks is the fundamental cause of the heat transmission to the reservoirs.Tables listing the correlating marker horizons for each well and maps showing the top of the two main reservoirs and the isopacks for the shallow one are presented. From these data have been inferred the areas showing the best possibilities for immediate geothermal development.     

Calcite deposition at Miravalles geothermal field Costa Rica

The calcite deposition problem at Miravalles has been studied since it was observed in the first three wells drilled on the slopes of the Miravalles Volcano. Long-term tests have been carried out to study reservoir characteristics. The change in the production behavior of the wells with the restriction imposed by the deposited calcite has been studied trying to evaluate and quantify the scaling problem. Work is being done on predictions of the deposition rate, location and distribution of the deposited mineral inside the wells. This work was compared with real data obtained from caliper logs of the wells before and after production. The feasibility of the first 55 MW power plant has been demonstrated. It was considered that the solution for the calcite problem is the “reaming during discharge of the wells” trying at the same time to minimize the cleaning interventions with a new well design. A long-term inhibitor test has been scheduled by the beginning of 1989. The economic evaluation of this test may affect the decision of reaming the wells as a solution for the feasibility of the first and subsequent power units. It is also believed, due to the thermodynamics and chemical characteristics of the extracted fluids, that it is possible to find a “non-deposition zone” which will permit the drilling of wells without a scaling problem.     

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.     

Industrial use of the high-temperature geothermal field at Theistareykir, Iceland

The Theistareykir high-temperature geothermal field is located in the region Thingeyjarsýsla about 32 km from the coast and its principal town, Húsavík in northern Iceland. The paper shows that geothermal steam from the Theistareykir field can be piped to an industrial area close to Húsavík having a temperature of 170°C – 180°C at a cost of approximately US$ 4 per tonne while a steam cost of US$ 10 a tonne is quite common in steam consuming industries.The production of alumina is used as an illustration of the possible industrial use of geothermal steam. It is probable that the requirements for aluminium smelter capacity in Iceland may exceed 500,000 tonnes per annum within a decade. One million tonnes of alumina would be needed to achieve this annual aluminium production rate.     

Waste water reinjection at tongonan geothermal field: Results and implications

Reinjection of waste water in geothermal reservoirs is primarily intended for the disposal of the separated brine containing toxic minerals. It also increases the recovery of internal energy in the reservoir, aside from sustaining pressure and fluid mass in-place. However, there are also associated detrimental effects in the system once premature breakthrough of reinjected fluid occurs. There will be losses in well deliverability, due to decline in enthalpy and steam fraction, and reduction in permeability if deposition takes place. Induced seismicity also increases in the area, especially along faulted regions. Increases in iniectivity during the early span of reinjection in Tongonan have been experienced, followed by levelling and reduction after a certain period. A drop in well output of the well followed a premature fluid breakthrough. In the production sector, pressure drawdown is minimized due to reinjection returns.     

Reinjection and gravity changes at Rotokawa geothermal field,New Zealand

Since commissioning in late 1997, heat-depleted wastewaters from Rotokawa power station have been reinjected into a shallow two-phase aquifer, which is separated by a low-permeability zone from the deeper exploited reservoir. Gravity changes for 1997–2003 show a large positive (341 μgal), near-circular anomaly, centred near the reinjection wells. Modelling shows that the anomaly is associated mainly with the introduction of the cooler reinjected waters into the reinjection aquifer, resaturating the pores by displacing steam. The region of liquid resaturation had the shape of a cone of impression, which reached from the bottom of the reinjection aquifer up to shallow (100–200 m) depths. The gravity changes represent a net mass gain of 8.6 million tonnes and the near-circular shape indicates that the horizontal permeability of the resaturated region was isotropic. Gravity changes for 2003–2004 occurred at most of the points that previously had large gravity increases, however, the region of the gravity increases is not radially symmetrical, suggesting that resaturation had become anisotropic in the horizontal plane. Modelling also suggests that the cone of impression had not increased in height during 2003–2004 but had expanded laterally by up to 30 m, except in a southerly direction, where it may have extended up to 2 km.     

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.     

Interpretation of a well interference test at the Chingshui geothermal field,Taiwan

Production in the liquid-dominated Chingshui geothermal field is largely from a fractured zone in the Jentse Member of the Miocene Lushan Formation. The geological data strongly indicate a possibility of linear-flow geometry on a field-wide scale. This was confirmed by re-analyzing the results of a multiple-well interference test performed in 1979. Radial and linear-flow models were used in this process. An evaluation of computed reservoir transmissivities and well capacities indicated that a linear model fitted the interference test data significantly better than a radial model. The linear-flow model that was developed for the Chingshui reservoir was also instrumental in obtaining an improved estimation of the geothermal fluid reserves (i.e., fluid-in-place).     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Hydrothermal alteration mineralogy as an indicator of hydrology at the Ngawha geothermal field,New Zealand

Interaction between geothermal fluids and the rocks through which they migrate alters many earlier formed minerals and produces others. The minerals thus formed preserved evidence of hydrological conditions prevailing within an active geothermal system; in particular, they can reflect the range of temperatures under which they formed. This feature was tested at the Ngawha geothermal system, which is different from others in New Zealand in that its reservoir comprises fractured basement rocks covered by a 500–600m thick sequence of sedimentary rocks. Petrographic examination of cores and cuttings recovered from drillholes at Ngawha shows that the secondary minerals present within the rock matrices and veins are of different ages. The thermally sensitive minerals include epidote, titanite, biotite and clays, including some that are interlayered. Comparison of the measured downwell temperatures with those deduced from the secondary mineralogy and by homogenizing fluid inclusions, shows that the central part of the field has remained thermally stable since the youngest secondary minerals deposited there but its southern margin has cooled by 20–40°C or perhaps more. A likely cause of this is an inflow of cooler water from the east, which also causes the temperature inversion clearly evident in hole Ng8. By contrast, some fluid inclusion geothermometry results suggest that the northern part of the drilled field has heated since their host hydrothermal quartz crystals formed.