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Geothermal Resources Council Transactions, Vol. 28, August 29 - September 1, 2004 Pico Alto, Terceira: A New Geothermal Field in the Azores Roger Henneberger 1, Rui Cabeças 2, Rita Martins 3, and Eduardo Granados 1 1 GeothermEx, Inc., Richmond, CA 2 SOGEO Sociedade Geotérmica dos Açores, Ponta Delgada, Açores (Portugal) 3 GeoTerceira Sociedade Geoeléctrica da Terceira, S.A., Angra do Heroísmo, Açores (Portugal) Keywords Pico Alto, Terceira, Azores, temperature gradient drilling, audiomagnetotelluric surveys, exploration ABSTRACT A new high-temperature geothermal resource has been identified on the island of Terceira in the Azores, through an exploration program undertaken by GeoTerceira with the assistance of GeothermEx. Previous exploration on the island did not provide enough information about the location and extent of the geothermal system to support deep drilling; the new program was designed specifically to reduce confirmation-drilling risk. A 624-station audiomagnetotelluric survey determined the resistivity structure of the area of interest with sufficient accuracy to develop a working model of the system, which was tested by drilling 4 temperature observation wells up to 600 m deep. Measured temperatures revealed a convective zone of at least 233 C, and a broad area in which temperatures greater than 200 C can be projected. A confirmation-drilling program is being planned in light of the positive results of the exploration work. Introduction The Azores archipelago is located in the North Atlantic Ocean between latitude 37-40 north and longitude 25-31 west. It consists of 9 inhabited islands that straddle the mid- Atlantic ridge off the coast of mainland Portugal, in a complex geotectonic setting associated with the triple junction point of the North American, Euro-Asian and African plates (Figure 1). All of the islands are volcanic in origin, and, on at least several of them, there is potentially exploitable geothermal energy. The Azores constitute an ultra-peripheral region of Europe, in which the strategy for the development and use of renewable energy favors indigenous sources. Geothermal energy has first-priority status among renewable sources due to its abundance and reliability. The development potential for other renewable energy resources (such as wind and hydropower) is limited by their volatility and the small scale of the systems they would serve. Geothermal power generation has been developed successfully on the largest and most populous island, São Miguel, since the early 1980s. Expansion of the generation capacity by Sociedade Geotérmica dos Açores (SOGEO) in the second half of the 1990s has led to the present situation, in which geothermal supplies about 25% of São Miguel s electrical energy needs as a base-load source of power. This success has encouraged interest in geothermal development on other islands. The most advanced exploration outside of São Miguel has taken place on Terceira, the second most populous island, where Sociedade Geoeléctrica da Terceira, S.A. (GeoTerceira), a sister-company of SOGEO, holds a concession and is actively pursuing geothermal development. GeoTerceira s objective is the installation of a geothermal power station, of about 12 MW capacity, to supply a significant Figure 1. Geographic and tectonic setting of Terceira island (from Nunes et al., 1990). AM = American plate; EU = European plate; AF = African plate; CMA = mid-atlantic ridge; RT = Terceira rift; TSJ = São Jorge transform; ZFNA = north Azores fracture zone; ZFOA = west Azores fracture zone; ZFEA = east Azores fracture zone; FG = Gloria fault. 345
fraction of Terceira s power demand, which includes the needs of a United States Air Force base as well as the island s more than 65,000 inhabitants. GeoTerceira, which is owned jointly by Electricidade dos Açores (EDA) and Electricidade de Portugal (EDP), is a private commercial enterprise, and therefore must follow a development approach that minimizes the time, cost and risk associated with bringing geothermal power on line. The remote location of the Azores, which leads to high costs and lead times for mobilizing heavy equipment, adds to the need for efficient planning. The following sections describe the successful critical-path program that GeoTerceira has used to identify a new high-temperature resource and prepare for confirmation well drilling. Previous Exploration Terceira island consists of 5 volcanic complexes: Cinco Picos, the caldera of Guilherme Moniz, the Pico Alto volcanic field, the shield volcano of Santa Bárbara, and the Central Rift Zone between Santa Bárbara and Pico Alto (Figure 2). The last three have erupted repeatedly within the last 20,000 years, and historic eruptions have occurred in the Central Rift Zone near the Pico Alto Field (in 1761), and off the west coast of the island (as recently as 1999). There are few surface manifestations of hydrothermal activity on the island, but an area of fumaroles is present at Furnas do Enxofre, within the Pico Alto volcanic field. Figure 2. Map of Terceira island, showing areas of interest determined from pre-1999 exploration. Apart from basic studies of the thermal features of the island (e.g., Zbyzewski et al., 1971; Self, 1973), there was very little investigation of the geothermal potential of Terceira before the late 1970s. During the late 1970s, the Laboratório de Geociências e Tecnologia (LGT) of the Regional Government of the Azores carried out geoelectrical surveys of limited extent (consisting of several dipole-dipole surveys and at least one vertical electric sounding) and drilled 9 shallow temperature-observation wells ranging in depth from 85 to 222 meters. This work was complemented by studies of surface geology, volcanology and tectonics, as well as a limited seismic reflection survey performed by the Instituto de Geociências dos Açores. In 1980 the United States Geological Survey included Terceira in its inventory of the geothermal resources of Portugal, making a resource estimate based mainly on the characteristics of the volcanoes and surface thermal manifestations (Johnston, 1980). At about the same time, LGT contracted exploration and development services to be performed on the islands of São Miguel, Terceira and Faial. On Terceira, the contracted work included a two-phase program of basic geothermal exploration, which was carried out by Geothermal Energy New Zealand Ltd. in association with Mitsubishi Corporation (1981, 1982). The first phase consisted of geologic and geochemical studies; the second was a geophysical program that included gravimetry, DC resistivity and a 100-station MT survey. Little new exploration occurred on Terceira during 1983-1992, while geothermal development work in the Azores was focused on São Miguel. Around 1993, the Commission of the European Community authorized a new program of exploratory studies. This work included additional tectonic mapping (based mainly on remote sensing imagery), an 18-station MT survey, and new geochemical studies that included surveys of soil mercury, radon and helium (IDRO- GEO and 3R Research, 1994). Another hiatus occurred until 1999-2000, when EDA, through SOGEO, renewed exploration activities on the island. In September 2000, the partners that eventually formed GeoTerceira sought a geothermal concession with the aim of commercially developing the resource. Design of Exploration Program A review of the exploration results through 1999, carried out by SOGEO and GeothermEx, Inc., showed that further exploration work was needed in order proceed to confirm the resource by deep drilling without excessive risk. The available data were sufficient to define an area of interest, in the vicinity of Furnas do Enxofre and the Pico Alto volcanic field (Figure 2), but, apart from the fumaroles, there was little evidence from which to assess the size, shape and location of the inferred geothermal system. Geoelectrical surveys had confirmed the presence of a resistivity low, but did not present a consistent enough picture of the resistivity structure to infer much about subsurface conditions. Five of the 9 temperature-observation wells were clustered around Furnas do Enxofre, revealing mainly the effects of the fumaroles. The others were drilled at scattered locations in the central part of the island, and showed low temperatures with profiles indicating that they had not penetrated below the zone of shallow, cold groundwater. Soil mercury and radon surveys showed scattered anomalies that clouded more than clarified the geothermal picture (Figure 3). A program of additional exploration was designed to meet the specific objective of being able to site deep confirmation wells with an acceptable level of risk. Reducing risk meant obtaining better information about the extent and location 346
Figure 3. Ground radon and soil mercury anomalies (data from IDROGEO and 3R Research, 1994). of the geothermal system that feeds the fumaroles at Furnas do Enxofre. Taking into account the geologic setting, project objectives, and logistical situation, a two-stage program was established. The program included: 1) A detailed geoelectrical survey, using upto-date methods, in and around the area of interest, to define the subsurface resistivity structure as accurately as possible. 2) A series of intermediate-depth temperature observation wells, to observe directly the temperature gradients produced by the geothermal system. Various electric survey methods were considered, and it was determined that the audiomagnetotelluric (AMT) method would be the most appropriate for the geologic terrane and logistical conditions. A depth range of 500 to 600 meters was judged necessary for the temperature observation wells. The exploration program was initiated in 2000. deeper levels, however, the conductive zone was found to be located further to the southwest, and to extend along an arcing NW-SE trend (Figure 5). Based on the hypothesis that the conductive zone is a result of hydrothermal alteration of original rocks to hydrated clay minerals near the top and lateral margins of the geothermal reservoir, the resistivity structure led to a working model in which the reservoir is located mainly to the northeast of the fumarole area, and extends northwest and, to a lesser degree, southeast of the fumaroles. With this model in mind, much-reduced primary and secondary areas of interest for further investigation could be defined (Figure 5). AMT Survey An AMT survey with a relatively dense nominal station spacing of 200 meters was planned to cover as much of the primary area of interest (about 40 km 2 ) as logistically possible, with coverage also of selected parts of the extended area of interest. Geosystem srl was contracted to perform the survey under the supervision of GeothermEx. Survey techniques and coverage were modified during the planning and execution of the survey, as opportunities to improve the results were identified. A total of 624 AMT stations were occupied and data collected, using a frequency range of 0.1 Hz to 10 khz. In addition, 9 MT stations were acquired. The AMT survey succeeded in clarifying most of the uncertainties left by earlier surveys about the resistivity structure. As expected, a conductive zone was revealed at shallow levels (near 250 meters above mean sea level) in the vicinity of Furnas do Enxofre (Figure 4). At Figure 4. Area of AMT survey, and contour of 6-ohm resistivity at +250 m elevation. Figure 5. Contour of 6-ohm resistivity at +100 m elevation, and updated area of interest based on AMT survey results. 347
A second significant conductive anomaly, near Agualva, to the northeast of the Pico Alto area, was also identified (Figure 5). It represents another area of interest for further investigation, but, lacking direct evidence of high temperatures, it merits a lower priority than the Pico Alto area. The high quality of the AMT results, in combination with other data, provided a sound basis for selecting locations for temperature observation wells. Ten potential well sites were identified and assigned priorities for the next phase of work. Temperature Observation Wells The temperature gradient drilling program needed to be designed carefully to meet its objectives. The relatively deep holes would need to be drilled in a difficult environment of mixed lithologies and variable permeability. In addition, to prevent downflows that might obscure temperature profiles, the wells would need to be completed with tubing cemented securely over the entire length of the hole. Problems of site access had to be faced, and a more-rigorous-than-expected environmental review process was required, because of the sensitive nature of the project area. The central part of Terceira contains various habitats and species (both native and endemic) that are protected within the European Natura 2000 network. It is also considered to be an important recharge area for fresh-water aquifers that supply a substantial fraction of the island s population. These factors led to delays in the start-up of the program, and required modifications to some hole locations and drilling methods. Drilling began in the summer of 2003. The original plan was to use a rotary drilling rig to drill the top-hole portion of each well and a coring rig for the deeper interval. Experience in the first well showed that coring was slow in many intervals, while the rotary rig performed better in most rock types. After the first well, therefore, the rotary rig was used exclusively for drilling all intervals. The well design consisted of a first section drilled to about 200 meters and cased with a 7-inch casing; the second and final section was completed with a cemented string of 1.66-inch tubing. The upper section of each hole was drilled as much as possible with an air hammer in order to protect the water quality of the shallow aquifers. The drilling program was adjusted as it proceeded, in light of initial results, costs and project objectives. Although a program of 5 to 6 holes had been envisioned, it was eventually determined that 4 holes would provide sufficient information while avoiding delays in the overall development program (Figure 6). The last of the temperature observation wells was completed in early April 2004. Temperature profiles measured in the wells confirmed that both their design and completion were satisfactory. Except in well TG-J, which is nearest to the fumaroles at Furnas do Enxofre, near-isothermal conditions are present in the first several hundred meters, reflecting the influence of shallow groundwater aquifers or other non-geothermal phenomena (Figure 7). Beginning near about 300 meters depth, a change to very high temperature gradients occurs, providing a direct indication of geothermal conditions below. The temperature profiles indicate little, if any, impact from flow of water behind the tubing, and are reliable for extrapolating temperatures. Deep temperatures can confidently be estimated to exceed 200 C over a substantial area (Figure 6). In well TG-H, a convective profile in the bottom interval and a maximum measured temperature of 233 C demonstrate the presence of a high-temperature reservoir (Figure 7). The temperature gradient data have confirmed and strengthened the conceptual model suggested by the AMT and other exploration results, and have provided the information needed to proceed with confirmation drilling, having reduced exploration risk to an acceptable level. GeoTerceira is currently Figure 6. Locations of temperature observation wells, and inferred temperature distribution at mean sea level. Figure 7. Stabilized temperature profiles measured in observation wells. 348
undertaking the preparations needed to drill the first deep, full-diameter wells on Terceira. It is anticipated that drilling activities will start in the second half of 2005, after executing the necessary civil works for drilling sites, the tender process for the drilling services, and the preparation and submission of the reports demonstrating compliance with environmental requirements. Conclusions GeoTerceira s exploration program on the island of Terceira has succeeded in identifying a high-temperature geothermal system in the Pico Alto area, and has provided enough information about the position, extent and depth of the system to design a confirmation-drilling program with a relatively low level of exploration risk. The program to date serves as an example of how an exploration effort can be tailored to meet the technical, economic and scheduling requirements of a commercial geothermal development project. Essential elements of such a program are: clearly defining of the objectives of the program, with an eye toward risk reduction in successive stages; selecting the methods that contribute directly to the program objectives, rejecting or minimizing activities that contribute only marginally to risk reduction by providing extraneous or ambiguous data; adapting the selected methods to the logistical, regulatory, budgetary and other constraints under which the project must be carried out; and providing the flexibility to make adjustments as work proceeds, orienting the work at all times to the objectives of the program. References Geothermal Energy New Zealand Ltd. and Mitsubishi Corporation, 1981. Geothermal Prospection, Ilha Terceira, Açores: Geological Report. Report prepared for Região Autónoma dos Açores, Laboratório de Geociências e Tecnologia, July 1981. Geothermal Energy New Zealand Ltd. and Mitsubishi Corporation, 1982. Geothermal Prospection, Ilha Terceira, Açores: Geophysics Survey. Report prepared for Região Autónoma dos Açores, Laboratório de Geociências e Tecnologia, May 1982. IDROGEO srl and 3R Research snc, 1994. Terceira Geothermal: Identification and Characterisation of a New Deep Seated High Temperature Geothermal Reservoir Phase I Final Repot. Report for Commission of the European Communities, D.G. XII, Non Nuclear Energy Programme 1991-94 (Joule II), July 1994. Johnston, D. A., 1980. Geothermal Resources in Portugal and the Azores. Unpublished report prepared for the U.S. Department of Energy, 51 p. Nunes, J. C., J. L. Alves and V. H. Forjaz, 1990. Sismicidade instrumental dos Açores no período 1980-89: implicações neotectónicas. In Encontro 10 anos após o sismo dos Açores de 1 Jan 80. Angra do Heroismo, October 1990. Self, S., 1973. Recent Volcanism on Terceira, Azores. PhD thesis, Imperial College, London, December 1973. Zbyzewski, G., A. Cândido de Medeiros, O. da Veiga Ferreira, and C. Torre de Assuncão, 1971. Carta Geológica de Portugal, Noticia Explicativa da Folha, Ilha Terceira, Escala 1:50,000. Serviços Geológicos de Portugal, 43 p. 349
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