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2 Geothermal Resources Council TRANSACTIONS, Vol. 16, October 1992 THE RESULTS OF A JOINT SCIENTIFIC STUDY OF THE FLORES ULUMBU GEOTHERMAL AREA *Tony Mahon, **Sobrot0 Modjo and ***Vincent T Radja * Geothermal Energy New Zealand Limited (GENZL) ** Volcanological Survey of Indonesia (VSI) *** Perusahaan Umurn Listrik Negara (PLN) ABSTRACT Indonesia's robust geothermal programme has been gaining momentum in the last few yearswith thesuccessful operation of Units 1, 2 and 3 at Kamojang and the planned developments at Darajat, Dieng, Salak and Lahendong. Many new areas have been investigated and preliminary drilling carried out to test scientificconceptual models and system productivities. The importance of small mini geothermal developnents, early recognised as having potential in the rural and more remote parts of the country, have maintained a prospective role in Indonesia's overall energy planning, with the ongoing investigations in Lahendong in Sulawesi, Kerinci in Jambi and Ulumbu on the island of Flores, East Nusa, Tenggara Since the early volcanological investigations in Flores made by Kemmerling (1 929) to thegeothermal investigations directed by Zen and Radja (1970) the island has been recogrlised as a source of heat and geothermal energy. Scientific exploration of a number of systems on the island was intensified in the 1980's and a concerted effort wasdirected towards Ulumbu, a geothermal area in Manggarai in western Flores. Much of the scientific work was camed out by VSI from their base in Bandung. During 1989 a decision was made to assess the technical and economic feasibility of setting up a small geothermal plant of approximately 3 MWe at Ulumbu. The study was funded by the New Zealand Government. This paper describes the geothermal investigations at Ulumbu in the period 1980 until the presentwith particular attention to the nature ofthe resource and its potential for development. INTRODUCTION Indonesia is rich in geothermal energy and rapid develop ment of these resources is being achieved on the island of Java. Kamojang presently generates 140 MWe and planned developments at Darajat, Salak and Dieng, and extensions to Kamojang could add a further 300 MWe of electricity to the local grid. Major developments of 55 MWeand above are likely to continue into the future in the major centres. Geothermal development in association with rural electrification of Indonesia has been reviewed for a number of years. Serious thought has been directed towards the utilisation of mini geothermal energy schemes in Bali, in Lahendong in North Sulawesi, and at Ulumbu in Flores. A dynamic project is presently underway in Lahendong where electricity will be produced from an experimental binary generating plant of 2.5 MWe capacity, due for completion by the end of 1991 I and a conventional flashed steam plant of 20 MWe capacity. Geothermal areas and their potential for electricity generation were first seriously examined in Flores in the late 1960's and the early 1970s, (Radja 1970) and then off and on through the intervening years until the present. Initial assessment of the geothermal potential of Ulumbu was carried out during the Indonesian Geothermal Study and Evaluation Programme. This programme was conducted by GENZL for the New Zealand Government working in association with the Government of Indonesia (GOI), in Previous to this study the Volcanological Survey of Indonesia (VSI) had been involved in the scientific investigations of the islands geothermal resources. Thus the geology (Setiawan and Suparto, 1984), geophysics (Simanjuntak and Akhmad, 1985) and geochemistry (Kartokusumo and Somad, 1987) had been investigated and reviewed. 97

3 In 1989, KRTA Ltd was engaged by the NZ Government working in association with the GO1 to undertake a technical, environmental and economic feasibility study of a geothermal development at Ulumbu with an anticipated scale of about 3 MWe. As a result of these investigations a decision was made in 1991 to continue with the project to the drilling and resource assessment phase, or phase II. This phase is presently being carried out for the NZ Government, working in association with the GOI, by GENZL and the drilling of the first deep well (1,500-2,000m) is scheduled for around December FLORES AND ULUMBU The volcanic and tectonic history of Flores Island was discussed by Kemmerling (1 929), Radja (1 970) and Setiawan and Suparto, (1 984). This information was summarised by GENZL (1 987) and KRTA (1 989). Figure 1 shows the location of the geothermal prospects, including Ulumbu, in Flores while Figure 2 shows Flores in relation to the regional tectonic setting. The island is situated on the inner of two concentric ridges which form part of an active subduction system which exrendsfor about 2000 kilometres east from the island of Java. There is a predominance in the island of north west trending faults associated with an east west trending fold axis which is consistent with subduction along an east west axis. Most of the faults are linear along their surface traces suggesting that they are steeply dipping. Similarly most of the mapped faults occur in the Tertiary volcanics in the north of the Island. This has been suggested as indicating that little faulting has occurred in the Quaternaryrocksinthesouthorthatit hasoccurred but has been masked by erosion of the younger volcanic landscape. During the course of the 1987 geothermal evaluation programme three geothermal prospects in west Flores were investigated and researched. Of the three areas, Wai Pesi, Wai Sen0 and Ulumbu, only the latter was considered as promising enough for immediate development. The Ulumbu prospect, Figures 2 and 3, occurs 11 km to the south of Ruteng the regional capital of western Flores. Most of the thennal activity is centred on the Poco Rii - Leokvolcano caldera and flanks (area about 28 km2). The activity consists of hot springs, fumaroles, sinters and altered ground. According to Setiawan and Suparto (1 984) the hottest of the thermal features are the springs and fumaroles in the upper Wai Kokor Valley at an elevation of m. These features, extending some two hundred metres up the valley, are known as the Ulumbu geothermal field. Other significant areas of altered ground occur east of Ulumbu within the Poco Leok crater at elevations of approximately900 metres and some 3 kilometres north of 'I L 1 I I I HGUREf Geothermalprospeds ofnusa Tenggara, showing the boundaries of provincesand Kabupatens, and the location ofgeothermal prospects (approximateonly in somecases)that have beenfullyidentified. 98

4 Flores Sea KEY Trench - Teelti on overriding side km. 200 km Regional Tectonic Setting FIGURE2 Ulumbu in the Wai Mantar Valley. The altered rock is of Quaternary age. The Wai Wara and Wai Engal altered ground within the Poco Leok crater and the Wai Mantar altered ground are associated with hot and warm springs respectively. Sulphur sublimates are present in the vicinity of the Kokor fumaroles. The Ulumbu Resource Figure 3, modified from KRTA (I 989), shows the Ulumbu geothermal area and its location relative to Ruteng, Iteng, Ponggeokand W Kokor. As indicated earliera reasonable amount of surface scientific survey work has been completed. However, one of the technical philosophies underlying this type of rural development is that initial costs should be kept as low as possible thereby improving the economics ofdevelopment. To fulfil this, the initial size of development should be small compared with the possible ultimate size of the prospect reducing the risks involved in siting initial production wells. The extent of scientific work completed at Ulumbu and the depth of knowledge of the prospect, exemplifies these comments. Geology Figure 4, taken from KRTA (1989) representing the geological map of Setiawan and Suparto (1984) shows the major geological features of the area. According to the above authorsthe Ulumbu geothermal field occurson the flanksofthe Poco Leokvolcaniccomplex at an elevation of some 650 metres above sea-level. The Quaternary rocks rise to an elevation of approximately 1,600 metres RSL and rest on a Tertiary basement comprising mainly lavas, breccias and tuffs, and pssibly calcareous sediments. It is considered that the Tertiary basement rocks have beendown-faulted to thesouth along an eastwest trending fault known as the Cancar Fault. Its trace passes Ruteng some kilometres to the north of the town. The resulting Cancar Depression is assumed to have been partially infilled successively by massive dacitic tuff 99

5 Geophysics Ulumbu Geother Thegeop3ysical model of Ulumbu has been based on the work of Simanjuntak (1982) and Simanjuntak and Akhmad (1 985). These workers carried out resistivity Schlumberger traversing and vertical electrical soundings. The soundings with a maximum AB/2 spacing of 1000 to 2000 metres were interpreted by the above authors and reviewed by KRTA (1 989) in terms of layered resistivity models and projected in vertical cross sections. KEY ---- Formed 10ad... T,*c* Cameale ncl/rlalian m a The interpreted resistivity data showa low resistivity layer sandwiched between an upper shallow layer and basement, of higher resistivity. The low resistivity layer has a modelled thickness of approximately 600 to 800 metres and stretches from the Wai Meseh to Wara (Ref Fig. 4). There are indications of deeper low resistivities east of Ulumbu. The low resistivity layer has an area of around 50 km2 and is deepest under the area of thermal activity at the Wai Kokorfumaroles. Boundaries to the north and the west are indicated but the anomaly lies open to the south and east. I I I./ FIGURE3 from pyroclastic flows and andesite lava flows erupted during early Quaternary volcanism centred north of Ulumbu probably in thevicinity ofthe Mandasawu VolcanicRanges. Volcanic activity is believed to have then moved further south where successive eruptions of andesite lava and breccias formed the large Rii stfato-volcano which eventually collapsed to form the Rii Caldera. Renewed volcanism nearthe centre ofthe caldera produced andesite lava and tuff breccia partially infilling the caldera. The youngerof these formations, the Wara andesite occupies the centre of the crater. There is considerable discussion on the displacement of the Tertiary basement within the caldera structure. KRTA assumed a displacement of 500 metreswhich put the top of the basement within the caldera at a depth of some 300 metres below sea level which corresponds to the high resistivity basement. ltwas suggested that the Tertiary volcanic basement was responsible for the modelled high resistivity 'basement' and that it was largely impermeable except where it has been cut by faults which serve as channels for circulating geothermal fluids. Geochemistry The geochemical model of the resource has been obtained by the collection and analysis of steam, water and gas samples from the local fumaroles and hot springs. These were interpreted by Kartokusumo and Somad (1 982) and KRTA (1 989). No deep hot sodium chloride water is discharged from the local springs, the discharged waters being of the acid sulphate and near neutral sodium, bicarbonate sulphate types. Both waters are formed by the heating of meteoric (surface) waters by steam containing carbon dioxide and hydrogen sulfide and generally occur within 500 metres of the surface. Gas geothermometry and molecular gas ratios of the steam discharged from the fumaroles at Wai Kokor indicate the steam is derived from a deep fluid source whose temperature is of the order of 250 C to 300 C. However, there is no indication as towhetherthe fluid isderived from a source vertically below the fumaroles or some distance horizontally offset from them. The water chemistry suggeststhat the warm springs in the west are further from the source than those at Kokor or Lungar (Ref. Fig. 4) and togetherwith the steam and gas results, suggest that the major upflow zone is beneath Kokor or towards Lungar. This is also suggested from the deeper resistivity pattern. Gas concentrations (mainly C02 and H2S) in the 100

6 fumaroles were around 2-3% wt with C02/H2S ratios of around 20 to 30. There was no evdence of magmatid volcanic type gases like SO2 or HCI in the steam disc h a rg ed. Natural Heat Discharge Several estimates of the heat discharged by the steam and water from the local thermal manifestations have been made. These range from 20 MWtto 100 MWt. The Wai Kokor fumaroles are remarkably similar to features at Ketetahi on Mt Tongariro, New Zealand and the heat output from this area is between 70 to 100 MWe. It is therefore considered that the original estimate made in the Indonesian Geothermal Evaluation Study of 68 MW1 is of the right order. Reservoir S t ru ct u re Evidence from both geophysical work and isotopic geochemistry, (natural isotopes) suggests that some of the fluids discharged from the surface at Ulumbu are derived from some form of storage reservoir. The nature of this reservoir remains uncertain although it has been suggested that it takes the form of a permeable rock formation which is sandwiched between a relatively impermeable basement rock and a capping rock whose low permeability has resulted from mineral alteration and mineral deposition created by the ascending geothermal fluids. Certainly the amount of natural heat escaping from the system appears to indicate a reasonably large storage area capable of supporting a geothermal development of modest proportions. BGURE4 [m CESBRECCIA ;m WAR4 ANOESITE 1 s MANTAR BRECCIA ANOESITE &A DOME MANOASOW ANOESITE GIRlT ANDESITE TERTIARY TERTIARI SISTEM FAULT OASMED WMERE.e' 0- APP!l?)XlYATLLI LOCATE0 *.e-:. CALDERA OR CRATER SHAPED /"' LIT9LOGY BOVN5ARI MOT SPRING 8 fumaltola 9 SPRING 'y' SECTION LINE GEOLOGIC MAP OF ULUMBU GEOTHEItMAL AREA MANGOARAI 01 WEST FLORES AFTER SETIAWAtl AND SUPARTO (1984) 101

7 There has been considerable discussion about the structure of the Wai Kokor Valley and whether the lineation following the direction of the fumaroles represents the surface traces of a NNE SSW trending fault orwhether it represents an erosional feature. Similarly the steep escapement to the east and south east of the valley remains somewhat of a mystery as regards its ultimate origin. Irrespective of the nature of these structures there is evidence of relatively high deep permeability along a lineation on the western side of the valley. Conclusions The Ulumbu geothermal resource has beendefined tothe extent where certain positive conclusions can be drawn about its further exploitation. Assuming that energy extraction can be achieved through the drilling of wells to economicdepths and that there is sufficient permeability in the local rocks to permit thisthen the amount of energy available is sufficient, to support a 3MWe develapment. The amount of steam required to support a 3 MWe back pressure modular type turbine is approximately 50 to 55 tonnes per hour ( kg/sec.) at 6ba inlet pressure. This is commensurable to a heat flow of around 40 MWt and of the order assessed as being discharged from Ulumbu. It is apparent that the planned development is less than cr similar to the natural energy discharged, a very positive result. SITING PRODUCTION WELLS This is possibly the most critical decision-making process in a small scale geothermal development which ultimately determines the economic viability of the project. The reason for this is that one of themajor costs incurred in such a development is the drilling costs. A well drilled to 1500m in depth could cost two million US dollars. It is therefore essential to narrow and reduce the risk margin in the siteselection as much aspossible. At the completion of their review in 1989, KRTA chose five possibledrilling sites based on the model ofthe resource. These are shown in Figure 3 as A, B, C, D and E. Much discussion has taken place on the sites during the interim period. In particular their locations relative to the Kokor fumaroles, their viability asdetermined by rig and equipment access and the environmental and sociological considerations and restrictions concerning theirdevelopment. Based on the assessment that access to sites A, B, C and D are about equivalent and that initial concern about the development of Site C has been overcome, Site C has been selected as the location forthe first well. The overriding consideration has been the proximity of this site to the known zone of sub-surface permeability defined by the lineation or fault underlying the Kokor fumaroles. Where as both sites C and E would fulfil this requpr?ment Site C has the easier access and was selected ahead of Site E. ENVIRONMENTAL CONSIDERATIONS As part of the 1989 KRTA Feasibility Study the University of Cendana, based in Kupang on the island of Timor, carried out an environmental impact study of the project. This was in compliance with the regulations covering the issuing of a Preliminary Environmental Information Report (PIL) or an Environmental Impact Analysis (ANDAL). An executive summary of this study was included in the KRTA Feasibility Report. Following the publication of this report a further study was carried out on the social impacts and implications of the development to the local population, by the New Zealand Department of Scientific and Industrial Research. The study was commissioned by the NZ Government in agreement with the GOI. Both reports drew attention to the major issues and problems with the development but agreed in principle that these could be mitigated in a reasonable and satisfactory manner. It was suggested to the developers that a Steering Committee, including at least some local village representatives, should be formed to ensure the maintenance of an equitable balance between develop ment and the environment. This was agreed to. ENGINEERING CONSIDERATIONS D ri I I i n g A decision has been reached that a drilling rig capable of reaching a depth of 2000m should be employed. This allows fluid production to come from any depth below about 800m or below the safe limits of the setting of solid production casing. To reduce well damage as much as possible air or an aerated fluid will be utilised during drilling, wherever possible. Number of Wells Although somegeothermal wells have a capacity considerably higher than 3 MWe the exploratory nature of the 1 02

8 development at Flores suggests that two wells could be necessary to fulfil the requirements of the project. Similarly a well may be needed for reinjection purposes. In the economic assessment carried out by KRTA a two well programme was allowed for. An initial dud productjonwell could be subsequently used as a reinje~ion well, thereby maintaining the economic viability of the project. Civil E~g~neer~n 9 A detailed examination of access roading and bridging has been undertaken and a full programme of road upgrading is presently underway. A water supply for d~~l~ng and other purposes, includ~n~ a new supp~y to the local villages, will be obtained by damming the Wai Kokor upstream of the drilling and development site forming a reservoir of size commensurable to the requirements of the project. Generating Units This was reviewed by KRTA and sensibly no final decision was made unti~ the nature of the resource had been assessed by drilling. A number of alternatives are available. These include non condensing back pressure modularturbines of 3MWe capacity, modular condensing turbines of similar size and binary cycle turbines also of a~~rox~~ate~y the same size. The final selection will be based on the enthalpy of the fluids discharged from the well and the various economics determining the utilisation of the resource. Similarly any incremental capacity increases to the initia~ 3~We deve~opmen~ sho~~d be considered at this early stage, when reviewing turbine options. and Lybrand Associates Ltd. It is not the intention of this paper to review the economic results but simply to indicate that the larger the generating capacity becomes so does the economic viability of the project increase. CONCLUSIONS Scientific investigations of the Ulumbu geothermal prospect have defined an area of anomalous rock resistivity and heat flow of approx~mate~y 50 km2. The natural heat flow from the main areas of hot spring and fumarole activity has been estimated at around 70 MWt which would be more than sufficient to produce around 3 ~~e if it can be harnessed and used in a modu~ar~ur~ne. The nature of the resource, vapour dominated or water dominated, is still uncertain but there is no chemical evidence indicating the present of a large reservoir of hot sodium chloride water. Gasgeothe~ometry indicates a resource fluid temperature in excess of 250 C. A deep well to 1500 to2000 m will be drilled as close to the fumaro~es in the Wai Kokor Val~ey (Site C) as possible, in the near future. This will define the nature ofthe resource, its temperature and productivity and enable the type of turbine and power plant most suitable forthe resource, to be defined and selected. The necessary civil engineering work for rig disemba~atio~, road~ng, b~dging and site preparation is undeway. An environmental and sociological study of the project has been completed. This study shows that the development will enhance the quality of life for both the local inhabitants of Ulumbu and Manggarai and Flores. Waste Effluent Removal and Disposal During drilling waste drilling fluidswilf be ponded in an area close to thedrilling site. These will be released slowly into the Wai Kokor which is not used for irrigation or drinking purposes with^^ a distance of some five kilometres downstream of the fumarole activity. large volumes of waste water derived from the plant, if the system turns out to be a water dominated system, will be removed by reinjection back into the system using a reinjection well. Economic Analysis A detailed economic analysis of the project was carried out as part of the feasib~iity study by KRTA, by Coopers The authors would like to acknowledge the Indonesian and New Zealand Government who gave permission for this paperto be publish~d. Similarly acknowledgement is offered to KRTA and Coopers & Lybrand Ltd for the indirect use of their material. REFERENCES GENZL, (1987); Indonesia Geothermal Study and Evaluation Project. The Zealand Ministry of Foreign Affairs. 5 Volu~es. 103

9 Kartokusumo, W. and Somad, (1 987); Geochemistry Report on the Ulurnbu Geothermal Area in Flores, East Nusa Tenggara. VSI Internal Report. Kemmerling, G.L.L. (1 929); Vulkanen Van Flores in Vulkanologische En Seismologische Mededeelingen, No 10. KRTA, (1 989); Flores Mini-Geothermal Power Project Ulumbu Geothermal Field Flores Indonesia. Feasibility Study Report. In Association with Coopers & Lybrand Associates Ltd. New Zealand Ministry of External Relations & Trade. Rxija, V.T. (1 970); Preliminary Evaluation of the Geothermal Energy Potential of Flores, Monograph No. 08-ER-70. Setiawan and Suparto, (1984); The Geology of the Ulumbu Geothermal Area, West Flores, Indonesia: VSI Internal Report. Simanjuntak, (1 982); Geophysics of Ulumbu. VSI Internal Report. Simanjuntak and Akhmad (1985); Report on the Geophysical Study of Ulumtu-Ruteng Geothermal Field. VSI Internal Report. Zen, M.T.and Radja, V.T. (1970); Result of the Preliminary Geological Investigation of Natural Steam Fields in Indonesia. UN Symp.Development and Utilisation of Geothermal Resources. Pisa, Italy. 104