The Bedugul Geothermal Field, Bali (Indonesia)

The Bedugul Geothermal Field, Bali (Indonesia)

The Bedugul Geothermal Field, Bali (Indonesia)
M. P. Hochstein (Geology Dept., Univ. of Auckland, NZ), R. Mulyadi and E.J. Joenos (Bali Energy, Denpasar, Indonesia)

The 2.45 km deep well, BEL-03, in the Bedugul Geothermal Field (Bali) has been successfully discharged since May 2004. The well (9 5/8 inch diameter) discharges c. 40 t/h fluids at 14 bar separation pressure with a ‘flowing enthalpy’ of c. 2600 kJ/kg. The fluids are produced from a 2-phase zone within a liquid-dominated reservoir lying below 2 km depth (bottom-hole temperature is c. 300 deg C). A second well of similar design and depth (BEL- 02) has been stimulated, also using liquid nitrogen, and it produces fluids with variable flow and a somewhat lower enthalpy since October 2004. Both wells have been brought to discharge more than 6 yr after their completion in early 1998. A third, nearly 2.7 km deep well (BEL-01) was drilled near a small volcanic cone and encountered > 310 deg C at the bottom; however, the well is not productive.

Fig.1. Map of Bali showing the location of the geothermal

The location of the three deep wells, with their well prospect in the Bratan Caldera and preliminary exploration results (until 1980); inset A shows the location of slim holes and large diameter exploration holes in the Bedugul Field drilled in 1997/98. heads at elevations between 1470 to 1570 m, is shown in the inset on Fig.1.

The recent tests mark the end of a 30 year long period of ‘on and off ’ geothermal exploration of the Bratan Caldera prospect, as it was called in 1974, taking its name from a nearby quasi-ephemeral caldera lake (Danau Bratan) , when the prospect was first investigated as part of a New Zealand bilateral aid project. Exploration was continued after 1976 by Pertamina, the Indonesian State Oil company. Until 1981, low resistivity rocks had been located by DC-resistivity surveys beneath the central part of the caldera (Soetantri and Prijanto, 1982). It was inferred that the associated thermal reservoir feeds two concealed outflows with bicarbonate waters that discharge at elevations between 650 and 300 m from numerous thermal springs over the southern and the western flanks (Fig.1). Temperature-gradient holes, down to 200 m deep, were drilled inside and outside the caldera (Fig.1). Extrapolated data from holes B2 and B3 located inside the caldera indicated that temperatures > 100 deg C would occur at depths > 0.5 km in the central part of the caldera and that any high temperature reservoir lies at much greater depth. This setting explains the lack of surface manifestations except for a small patch of steaming ground (SG in Fig.1) near an old temple (Candi Terataibang) at c. 1400 m elevation which is active only during the dry season. The first exploration model was described by Mulyadi and Hochstein (1981).

Further deep resistivity surveys (MT studies) were conducted in 1987; however, exploration drilling was stalled until 1994 when the project attracted investment by a major US developer. In November 1994, Bali Energy, a joint venture between California Energy and a local company signed a Joint Operation Contract with Pertamina and an Energy Sales Contract with the State Electricity Co. PLN to develop a 4 x 55 MW electrical power plant (a contract signed in the absence of any discovery well). The project was re-named after the Bedugul tourist resort, c. 2 km south of Danau Bratan, and became known as the Bedugul project. The project was re-started with additional geophysical surveys (TDEM-MT surveys) leading in 1997 to the drilling of ‘slim holes’ (TCH-1 to TCH-6, see inset on Fig.1) to delineate the temperature- and geological structure of the prospect. The holes had a diameter of 2 and 3/8 inch and were fully cored; they were drilled down to between 1.3 and 1.6 km depth (except for TCH-2 which is only 0.7 km deep). The slim holes confirmed the earlier model that high temperatures only occur at great depths inside the caldera. Low temperatures (<>

As a result of the 1997-1998 financial crisis, the Indonesian Government suspended seven geothermal projects with foreign investments; the list included the Bedugul project. California Energy stopped all operations in Bali and did not test any of the completed three deep wells (BEL-01, 02, 03) which were pressurized and sealed to allow for later testing. The Indonesian government invited investors to continue with the Bedugul Project whose assets were taken over in 2002 by the re-constituted ‘Bali Energy’ company (mainly Indonesian owned). ‘Bali Energy’, as the present developer of the field, started with testing of the deep wells in 2004. In a strict sense, well BEL-03 is the ‘discovery well’ of the Bedugul Field. The well is also the discovery well of one of the deepest high temperature geothermal reservoirs ever explored successfully.

References
Mulyadi, R., and Hochstein, M.P. (1981). Exploration of the Bratan Caldera geothermal prospect (Central Bali). Proceedings NZ Geothermal Workshop 1981, Univ. of Auckland, pp. 189-193.
Soetantri, B. and Prijanto (1982). Assessment of the reservoir potential of Bali Geothermal Field, Indonesia. Transactions (3rd) Circum-Pacific Energy and Mineral Resources Conference, pp.459-464.

(from IGA News #59, January - March 2005)