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2 Geothermal Resources Council Transactions, Vol. 26, September 22-25, 2002 The Dieng Geothermal Resource, Central Java, Indonesia Erik B. Layman', lrzawadi Agus2 and Samsudin Warsa3 'Layman Energy Associates, Inc., 1582 Cordova Drive, San Luis Obispo, CA USA 2PERTAMINA Geothermal Division, Jl. Merdeka Timur 6, Jakarta Indonesia 3PT PLN (PERSERO), JI. Trunojoyo Blok M 1/135, Kebayoran Baru, Jakarta Indonesia Keywords Dieng, Indonesia, geothermal resource, geochemistry, gas, resistivity cludes production and injection wells, gathering system, and a 60 MWe power plant. The government plans a staged development of the resource, leading to an installed capacity of approximately 225 MWe in ABSTRACT The Dieng geothermal resource is contained within a hightemperature, volcanic-hosted geothermal system in central Java, Indonesia. The field shows a close relationship with a northwest-trending structural zone defined by an alignment of late Quaternary volcanic vents. A magnetotelluric survey has delineated the extent of an electrically conductive zone which overlies the productive reservoir. The Dieng resource is dominated by a deep, high-temperature, liquid-dominated reservoir in the Sileri sector, which extends over an area of at least 9.75 square-kilometers. Sileri reservoir fluids are moderate r salinity, neutral ph sodium chloride waters with low gas Java Kra katau contents. This sector is highly productive, and over 190 MWe of steam is currently available at the wellhead from 15 wells. At the southeast end of the field, the less extensive Sikidang productive sector extends over 1.3 square-kilometers. The Sikidang sector hosts a moderate depth, high-enthalpy, gas-rich resource of moderate productivity, with average well outputs of about 5 MWe. The Sikidang resource is closely related to a 0.4 square-kilometer area of low- gas but corrosive, magmatic steam located between the Sileri and Sikidang sectors. Steam flows from the magmatic vapor plume along a pressure gradient into the Sikidang vapor-dominated reservoir. Condensation of steam along the flow path results in very high gas levels in the Sikidang sector, especially in the higher-elevation portions of the zone. The government of Indonesia presently owns the Dieng geothermal project, which in- Introduction The Dieng geothermal field is located in central Java, Indonesia (Figure l), about 25 kilometers north of the city of Wonosobo and 90 kilometers northwest of the city of Yogyakarta. The field is situated within the cool volcanic highlands of the Dieng Plateau at elevations of over 2000 meters. The plateau is renowned for its mist-shrouded mountain scenery, thermal features and 7th century Hindu temples. Most of the plateau is intensely cultivated and villages and farms are located in close proximity to the geothermal facilities. I\SUMATRA\ IlboE Sea A = Quaternary volcano Jakarta 1 Guntur Semarang Figure 1. Map of location and regional geologic setting of the Dieng geothermal field in central Java, Indonesia. 5 73

3 History of Development Initial geoscientific surveys and shallow drilling were conducted at Dieng in the 1960 s and 1970 s by UNESCO, the U.S. Geological Survey, and the French agency BEICIP. Between , PERTAMINA, the state-owned firm with responsibility for petroleum and geothermal resources, undertook an extensive deep drilling program at Dieng. During this period, PERTAMINA drilled 27 full-sized production test wells, 24 of which were located in the southeastern sector of the field known as Sikidang (Figures 2 & 3). Of the 27 wells, 14 were demonstrated by testing to be productive with a combined power output of approximately 85 MWe. Many of the PERTAMINA wells have subsequently been plugged for safety reasons or have blockages in the casing. From , PERTAMINA owned and operated a 2 MWe non-condensing monoblok power plant. The plant was supplied with steam from the DNG-2 well (Bachrun, et. al, 1993, and operated intermittently but did not supply power to the Java grid. In late 1994 Himpurna California Energy Ltd. (HCE), an affiliate of U.S. firm CalEnergy Co., commenced operations on the development of the Dieng resource. HCE s involvement was based on Joint Operations and Energy Sales Contracts signed with PERTAMINA and PLN, the state electrical utility, in December of Between , HCE completed drilling and testing of 18 full-sized production test wells at Dieng, 16 of which are located in the northeast of the field in the sector known as Sileri. Testing has indicated that 15 of HCE s wells are productive. These HCE wells have a combined power output of approximately 193 MWe, which is the present figure for wellhead steam availability for the Dieng field. HCE s initial plan was to construct a 20 MW facility in the Sikidang sector, to be supplied with relatively high-gas content steam from 5 existing PERTAMINA wells. After further economic analysis this plan was modified to a 60 MWe facility, to be supplied by steam blended from PERTAMINA wells in the Sikidang sector and HCE wells in the Sileri sector. The plan to utilize PERTAMINA wells was subsequently abandoned after 4 of the 5 wells proposed for use developed mechanical problems and had to be suspended. HCE completed construction of the 60 MW Unit 1 plant in the Sikidang sector; a pipeline to deliver steam from 7 HCE production wells in Sileri sector; and a brine injection pipeline system to connect to 5 injector wells. Of the 5 injectors, 4 are located in the Sikidang sector and two of these are PERTAMINA we1 Is. The plant was commissioned in early I998 but never went into cornmercial operation. This was the re- sult of a contractual dispute between HCE and the Indonesian government related to the Asian financial crisis which began in late Current Project Status Ownership of the Dieng geothermal project was returned to the government of Indonesia (GOI) in August 2001, as part of an agreement reached with the U.S. Overseas Private Tnvestment Corporation (OPIC). OPTC had assumed ownership of the project from HCE in November, 1999 after payment of an insurance claim to HCE. GO1 intends to develop the Dieng project as a joint venture ( NEWCO ) between PERTAMINA and PLN, under amended Joint Operations and Energy Sales Contracts. PERTAMINA will sell power to PLN at a price to be determined in the amended contracts. Participating interests in NEWCO are set at 67% for PERTAMINA and 33% for PLN. NEWCO shareholders will provide paid-in capital, working capital and guarantees for debt payment, and will also maintain individual, as opposed to joint and several, liabilities. NEWCO is proposing to develop 225 MWe of capacity at Dieng with a total new investment requirement of US$443 million. Re-commissioning of the 60 MWe Dieng Unit l facility is currently underway and anticipated to be completed by July, Additional units are planned to come on line in 2005 (55 MWe) and in 2006 (110 MWe). NEWCO is encouraging proposals from potential investors in the Dieng project. Geologic Setting The Dieng geothermal resource is set within a volcanic mountain range composed of Quaternary andesitic volcanic wellhead bottom of dlredlonal wall 8 hmarola I, hotspring A mountain peak stream drainage elevation contour 2% (meters e.s.1.) 1 rim of early stage caldera Late Quaternaty volcanic vents: 3 * crater dome OOOmeters Figure 2. Map of geologic and thermal features and well locations, Dieng geothermal field. The location of the 60 MWe Unit 1 power plant is also shown. t N s 74

4 rocks, which rises to elevations of over 2500 meters (Figure 2). Remnants of an early-stage volcanic center is marked by the arcuate summit of G. ( Gunung or Mount) Prau, which appears to be the northeast rim of a large caldera. A northwest-trending belt of late Quaternary volcanic vents passes through the inferred center of the older caldera feature. This volcanic axis, which includes the craters and domes of G. Sipandu, G. Pangoman, and G. Pakuwaja, is closely related to the Dieng geothermal resource. The fumarolic areas ( kawahs ) of Sileri, Sikidang and Pakuwaja are all located within this late-stage vent trend, as is the area of the productive geothermal resource. A system of northwest-trending fractures is inferred to control the intrusion of the magmatic heat source at depth, as well as the volcanic vent trend, the fumarolic discharges, the location of the geothermal reservoir. A conjugate system of northeast-trending fractures may play an important role in truncating the reservoir in the southeast portion of the field, and in controlling the northeast extension of the zone of fumarolic discharge at Kawah Sikidang towards Telaga Warna ( colored lake ). Rocks penetrated by the geothermal wells include a sequence of interbedded lavas, tuffs and breccias which extend from the surface to depths of meters. Below the volcanic sequence is a finegrained, micro-diorite intrusive ( andesite complex ), which is generally encountered at elevations between -300 to +600 meters a.s.1. The intrusive may represent the upper, solidified portions of the magmatic heat source for the Dieng geothermal system. Productive fracture zones in the geothermal reservoir are encountered in the lower portions of the volcanic sequence and in the micro-diorite intrusive. Dieng wells intersect the classic zonation of hydrothermal alteration associated with hightemperature geothermal systems. This includes a zone of argillic, clay-rich alteration in the upper portions of the system, associated with conductive geothermal gradients in the low-permeability cap rock above the geothermal reservoir. Clay Resistivity Anomalies and Relation to Resource Areas of high total conductance (or low resistivity) at Dieng are associated with clay-rich, electrically conductive hydrothermal alteration overlying the productive reservoir. Results of a 126-station magnetotelluric (MT) resistivity survey conducted in the Dieng region (Geosystem, 1998) are summarized in a map of total conductance (log of conductance value in mhos) to 1 kilometer depth (Figure 3). An elongate, northwest-trending area of highly conductive rocks (log total conductance >2.0), with dimension 3 x 10 kilometers, encloses the three main fumarolic areas and the axis of late stage volcanic vents. Within this broad conductive region, three separate and more intense anomalies (log total conductance >2.4) are associated with the fumarolic areas at Kawahs Sileri/Sipandu, Sikidang and Paku w aj a. In general the MT total conductance anomaly shows a good correlation with the high-temperature reservoir at Dieng. The sea level isotherm map (Figure 4) shows that the high-temperature reservoir is elongate in a northwest direction, and extends for over 6 kilometers from the Kawah Sileri area to the southeast past Kawah Sikidang. This high-temperature zone coincides well with the axis of the total conductance anomaly. However, the separate, more II 1.6- \ / log of MT total conductance (in mhos) to 1 km depth, contour with value fumarole Figure 3. Map of log of magnetotelluric total conductance in mhos to 1 kilometer depth, Dieng geothermal field. hot spring mountain peak low conductance high conductance t A N meters 5 75

5 proximately 11.5 square kilometers (Figure 5). Additional drilling is likely to expand the productive area to the southwest, northwest and northeast, because non-productive wells have not yet been drilled in these areas to define the production limit. The productive resource at Dieng has been divided into two main sectors, Sileri and Sikidang, based on geographic location and common production characteristics. Production characteristics for representative wells from the Sileri and Sikidang sectors are summarized in Table m \ isotherm with temperature at sea level. C A mountainpeak t fumarole \ geothermal well b hot spring - - I I, 00- I G. Pakuwaja Figure 4. Map of isotherms at sea level elevation, Dieng geothermal field. eas of higher vertical permeability. Increased vertical permeability allows local upbowing of isotherms above the productive reservoir, fumarolic discharge of steam to the surface, and development of thicker, more intense argillic alteration extending to the surface. This is supported by the deep isotherm pattern which shows two distinct thermal bulges at Sileri and Sikidang (Figure 9). NW! In contrast to the above, drilling has shown that the area of lower conductivity between the Sikidang and Pakuwaja anomalies does indeed reflect a zone of lower temperatures, which is characterized by non-productive wells. Limited drilling in the area of the Pakuwaja conductivity anomaly indicates that temperatures may be higher there relative to the cooler zone between the Pakuwaja and Sikidang anomalies. However, commercial productivity has not yet been established by drilling in the Pakuwaja area and it is not known if a r. Pulosorl separate resource area is present. Production Characteristics of Field Sectors A Sileri Sector The Sileri sector resource is characterized by relatively deep, high-temperature production. Reservoir temperatures range from deg C, with first production typically encountered at or below sea level, at depths between meters. Reservoir permeability in Sileri is rather high, allowing average well power outputs of about 13 MWe. Wells produce a moderate enthalpy ( BtuAb) two-phase discharge, with steam fractions at the wellhead ranging from 30-50%. Reservoir waters are moderate salinity, neutral-ph Na-C1 fluids with low gas content. Chloride content in total well discharges ranges from ,000 ppm, while gas contents in produced steam range from weight %. Downhole surveys indicate Sileri wells when shut-in stand with a water column across the depth interval of the reservoir production zones, indicating a liquid-dominated resource. Upward projection of hydrostatic pressure gradients measured in the reservoir zone indicate the regional deep water table lies at a depth of approximately 400 meters below the surface in the Sileri area, or about meters a.s.1. Reservoir boiling is indicated in some central Sileri wells at elevations of approximately meters a.s.1. Most shut-in wells develop a gas G. Rau A ZONE OF CORROSIVE STEAM / FLUID, PRODUCTION SI KI DANG t N A oaow meten Drilling to date at Dieng has defined a total proven productive area of ap- Figure 5. Map of boundary of proven productive area with resource sectors identified, Dieng geothermal field. Dashed line shows location of section in Figure

6 Table 1. Production characteristics of representative wells, Sileri and Sikidang sectors, Dieng geothermal field. the vapor-dominated resource is shown in Figure 5. The westernmost wells in the Sikidang sector stand with a water column across the depth interval of most or all of the production zones. This is consistent with lower production enthalpies from these wells and a transition to liquiddominated conditions. Corrosive Steam Zone cap, with wellhead pressures between psig. Safety procedures must be carefully followed during initial well openings, due to high H2S concentrations in this "dead" gas which accumulates in the shut-in wellbore. Sikidang Sector Production from the Sikidang sector is at shallower depth and at lower temperatures than at Sileri. Reservoir temperatures range from deg C, with first production in most wells encountered at depths of meters, or about +500 to +750 meters a.s.1. elevation. Reservoir permeability is low to moderate, resulting in average well outputs of about 5-6 W e. Wells produce high enthalpy discharges ( Btunb) of either dry steam or steam with a small water fraction of up to 15% at the wellhead. Accurate chloride measurements in produced liquids are often difficult to obtain in high enthalpy wells due to evaporative concentration. However, chloride levels in total flow appear to be in the range of ppm for wells which produce some liquid and for which good quality data is available. This is significantly lower than chloride levels in the Sileri sector. Gas levels in produced steam can be very high in Sikidang wells, with stabilized values after sustained flow in the range of 4-18 weight %. In the early stages of testing, gas levels of over 30 weight % have been measured in some wells. After longterm tests and production during the 1980's through 1994, gas levels generally declined. However, after testing was initiated on two Sikidang wells after about a 2-year shut-in period, gas levels had returned to initial high levels. One Sikidang well (DNG-20) provides an exception, in that the stabilized gas level in produced steam is 1.6 weight %. This appears to be the result o the 2100 meter depth to fist production for this well, which is unusually deep for the Sikidang sector. Downhole surveys indicate several wells in the eastern portion of the Sikidang sector stand when shut-in with a vapor column across the interval of the reservoir production zones, indicating a vapor-dominated resource. These same wells produce dry steam discharges. Pressures in the vapor-dominated zone range from psig, generally decreasing to the south. Pressure profiles indicate this vapor zone extends to a depth of at least 2200 meters, or -200 meters a d. The inferred area of At least one, and probably two wells in the transition area between the Sileri and Sikidang sectors produce acidic, corrosive steam. During a 21-day test in 1995, production from DNG-24 was dominated by dry, superheated steam. Steam condensates collected during superheated steam production exhibited ph levels under 3.0 with unusually high chloride levels. Analyses indicated up to 90 ppm gaseous hydrogen chloride was contained in the superheated steam. During the last 36 hours of the test, the well began to produce a small fraction (6-7%) of moderately acidic fluid, with extremely high chloride levels probably resulting from evaporative concentration. It appears likely that this fluid production resulted from a shallow casing leak in the well, although a deep source for the acid fluids has not been ruled out. Production test data from the adjacent DNG-23 is incomplete but is suggestive of superheated, acidic steam production. Downhole surveys indicate DNG-24 when shut-in stands with a vapor column across the depth interval of the production zones in the well, indicating vapor-dominated conditions. Production zones occur below 1800 meters depth (below +450 meters a.s.l.)the production zone temperature is 330 deg C, with steam zone pressures of over 1800 psig. Variations in Resource Chemistry Across the Field Significant variations in brine and gas chemistry are observed across the 6-7 kilometer extent of the Dieng resource. Chlorides in total well discharge (Figure 6) show a smoothly varying pattern in the Sileri area, with a central zone of highest chloride levels (1 2,000-14,000 ppm) between G. Sipandu and G. Pangoman. Chloride levels decrease gradually in all directions away from the central high chloride zone, which is elongated in a northwest-southeast direction. In the high-enthalpy discharge wells of the Sikidang sector, chloride levels are quite variable and do not show any consistent pattern. However, the range of values of ppm is significantly lower than that observed in the Sileri sector. Wells in the vapordominated sector at Sikidang produce no liquid and chloride levels are nil, with the exception of the acidic wells described above. Total gas levels in produced steam (Figure 7) also show a smoothly varying pattern in the Sileri sector, with a central 577

7 area of low gas contents (<OS weight %) extending from south of G. Sipandu to the north end of the Sikidang sector. The low gas zone includes the area of acidic steam production. Gas contents increase gradually in all directions away from this central low, which is elongate in a northwest-southeast direction. Gas levels remain relatively low in the eastern portion of the Sikidang sector (1-2 weight %), but increase markedly in the western portion to between 5-20 weight %. Gas levels in one poorly productive well in the Pakuwaja sector was over 25 weight %. The ratio of H2S to C02 in produced steam (Figure 8) shows a smoothly varying pattern across the entire field. An elongated, central high H2S/C02 region (1000 *H2S/C02 > 200) extends from south of G. Sipandu to the north end of the Sikidang sector. The very highest H2S/C02 region (1 000 *H2S/ C02>300) coincides with the zone of acidic steam production. H2S/C02 ratios decrease smoothly away from the central high in all directions. In general there is a close correspondence between the central area of lowest gas content in steam and the area of highest H2S/C02 ratio in steam. The very highest H2S/C02 ratios are at the southeast end of the low total gas region, in the area of the acidic steam wells. The area of highest chlorides in the Silieri sector is included within the central low gas and high H2S/C02 regions. I A mountain peak iso-chloride contour (ppm) fumarole vapor-dominated zone.'b hot spring \ geothermal well chloride concentration (ppm) for selected well Conceptual Model Sileri Sector The Dieng geothermal resource is dominated by a deep, hightemperature, liquid-dominated reservoir in the Sileri sector, which extends over an area of at least 9.75 square-kilometers (Figures 5 & 9). Sileri reservoir fluids are moderate salinity, neutral ph sodium chloride waters with low gas contents. A central area of thermal upwelling in this sector is indicated by a region of higher temperatures (>330 deg C) and higher chloride levels in total well discharges (12, ,000 ppm). Reservoir fluids cool and undergo dilution as they flow radially away from this central zone of upwelling. Relatively high H2S/C02 ratios in the upwelling zone probably reflect proximity to a sulfur-rich magmatic source. H2S/C02 ratios decrease along the flow path away from the central zone, due to the greater reactivity of H2S with reservoir rocks and fluids compared to C02. The upwelling zone at Sileri is also characterized by lower total gas contents (<OS weight % in steam) compared to peripheral, somewhat cooler areas of the Sileri reservoir. Lower gas levels in the zone of upwelling are somewhat unusual in high-temperature systems. For example, gas levels are highest in the upwelling zone at Tongonan, Philippines (Lovelock, et. al., ;i E I i S.L lo00 Figure 6. Map of chloride levels in total well discharge, Dieng geothermal field. 1982). In the central Sileri sector, reservoir boiling at shallower levels may produce exsolution of gas and discharge at fumaroles, resulting in gas depletion in the production zone. Alternately, low gas levels in the upwelling zone in central Sileri WOm 2 X verlicel exaggeration A approx. we11 location 9 fumarole / approximate top of pmdudlve reservoir oo 0 probable 2-phase boiling ' 0 zone end gas exsolution fl steam flow and condensation.:.-.:.: vapor dominated zone *. /fs fluid flow 1 moderate enthalpy. low-gas neutral ph fluid with 45,000 PPm CI 2 high enthalpy, mostly high-gas O resource; p= HCCbearhg. corrosive magmatic steam w/ low gas content; p+1800 psig Figure 9. Cross-sectional model of Dieng resource running from northwest to southeast. Location of section line is shown in Figure

8 G. Prau ~ m '\ G. Pakuiaja. -- A mountain peak 8 fumarole b hot spring \ geothermal well \ contour of total gas in produced steam (wtx: ' ' area of high-gas steam, wt%. _/' Figure 7. Map of total gas levels in produced steam, Dieng geothermal field m &/: G. Pakuwaja contour of weight ratio HZS I COz X 1000 in produced steam mountain peak 8 fumarole geothermal well % hot spring Figure 8. Map of weight ratio of H2S to C02 x 1000 in produced steam, Dieng geothermal field. may be related to the low gas levels in the high-temperature, acidic steam encountered at the north end of the Sikidang sector. Both areas have more or less magmatic affinities, as evidenced by high temperatures and high H2S/C02 ratios. Further analysis is required to investigate this relationship. Sikidang Sector The 1.34 square-kilometer, high-enthalpy Sikidang resource appears closely related to an adjacent 0.4 squarekilometer zone of corrosive, magmatic vapor at the north end of this sector. The magmatic character of the corrosive steam zone is indicated by the high temperature (>330 deg C), very high H2S/C02 ratio in the gas (up to 0.4), and presence of gaseous HCl in the steam. A continuous vapordominated zone extends from the magmatic vapor plume into the southern Sikidang resource. Pressures in the vapor-dominated zone decrease along the southeastward flow path, from over 1800 psig in the magmatic zone to under 1000 psig in southern Sikidang. Gas levels increase markedly along the flow path due to steam condensation and gas accumulation in the higher elevation portions of the Sikidang reservoir. Gas levels increase from 0.4 weight % in the magmatic steam zone to over 7 weight % in the vapor zone in southern Sikidang. In the western portion of the Sikidang sector, additional condensation results in wet- ter wells and very high gas contents, approaching 20 weight % in steam in some wells. Acknowledgements The authors would like to thank the management of PERTAMINA and PT PLN for allowing us to publish this paper. The senior author (Layman) developed many of the ideas included in this paper while working as an employee of Himpurna California Energy Ltd., and acknowledges the contributions of Elliot Yearsley, Will Osborn, Batara Simanjuntak and Kifle Kahsai to this effort. The authors retain sole responsibility for any errors of interpretation. Alison 0. Layman prepared all graphics included in the paper. References Bachrun, Z.I., Soeroso, and Suwana, A, 1995, Twelve years of exploitation history of well Dieng-2, Dieng geothermal field, Indonesia: hoc. World Geoth. Cong., p Geosystem SRL, 1998, Interpretation report of MT soundings in the Dieng area, central Java, Indonesia: unpublished report submitted to Himpurna California Energy Ltd. Lovelock, B.G., Cope, D.M. and Baltasar, A.J., 1982, A hydrogeochemical model of the Tongonan geothermal field: Roc. 4th New Zeal. Geoth. Workshop, p