Geothermal Resource Risk in Indonesia - A Statistical Inquiry

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Conference Paper: Geothermal Resource Risk in Indonesia - A Statistical Inquiry


This paper presents a statistical study of the geothermal resource risk in Indonesia, specifically, that the resource base and well productivity are adequate and that the drilling cost per well is reasonable. This paper is timely because the Government of Indonesia is now embarking on an ambitious plan to develop up to 4,000 MW of geothermal power capacity by 2014-2015 and a longer-term target of 9,500 MW by 2025. This study relies on the resource base estimates made by the Indonesian Government-owned enterprise P.T. Pertamina (now “PGE”) for nearly 100 sites, and productivity data on 215 wells in the country in the GeothermEx archives; these wells comprise some 80% of the production wells drilled in Indonesia. Pertamina’s assessments are shown to be consistent; such a thorough national inventory of the geothermal resource base is available from very few countries. The resource base (proved-plus-probable-pluspossible) at a site ranges from 10 MW to 800 MW with a log-normal distribution. More than 70% of the known Indonesian fields have a resource base greater than 50 MW and nearly half of the fields (about 40) offer a resource base of 100 MW or more.

Commercial wells in Indonesia vary in capacity from 3 MW to more than 40 MW, with a median value of 9 MW. The mean, median and the maximum well productivity encountered in Indonesia are larger than seen in most countries; we believe the most likely range of well productivity worldwide is 4 to 6 MW. The commercial wells in Indonesia tend to fall in one of four groups in terms of capacity: (a) 3 to 5 MW representing “tight” wells; (b) 7 to 9 MW representing “typical” wells; (c) 15 to 19 MW representing wells that usually produce from a 100% steam-saturated but otherwise moderate-temperature reservoir or from a “steam cap”; and (d) greater than 27 MW representing “ultra-high” temperature and/or highly permeable reservoirs. The last two types of wells, which occur in relatively few countries other than Indonesia, represent 40% of the 215 wells studied in this paper.

Geothermal wells in Indonesia are mostly in the 1,000 to 2,800 m depth range and the drilling success rate ranges from 63% to 73%, which are also typical in most countries. Using an existing correlation of drilling cost versus well depth from several countries, and the defined statistics on well depth and productivity in Indonesia, we estimate that the cost per MW well capacity in Indonesia is statistically less than seen in many countries, the most probable value being in the range of $300,000 to $400,000 per MW. The average drilling success rate and well capacity in Indonesia show a clear “learning curve” effect; both success rate and well capacity tend to increase with the number of wells drilled, eventually reaching a plateau.

Given that (a) more fields with large resource bases are encountered in Indonesia than in most other countries, (b) well capacity in Indonesia typically is larger than in other countries, and (c) drilling cost per well in Indonesia is smaller than in most countries, the overall resource risk in geothermal projects in Indonesia should be lower than in other countries.

Subir K. Sanyal, James W. Morrow, Migara S. Jayawardena, Noureddine Berrah and Shawna Fei Li

Thirty-Sixth Workshop on Geothermal Reservoir Engineering; Stanford, CA; 2011/01/31

Stanford University, Stanford Geothermal Program, 2011

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Subir K. Sanyal, James W. Morrow, Migara S. Jayawardena, Noureddine Berrah, Shawna Fei Li. 2011. Geothermal Resource Risk in Indonesia - A Statistical Inquiry. In: Proceedings. Thirty-Sixth Workshop on Geothermal Reservoir Engineering; 2011/01/31; Stanford, CA. Stanford, CA: Stanford University, Stanford Geothermal Program; p. SGP-TR-191