Proceedings World Geothermal Congress 2010 Bali, Indonesia, 25-29 April 2010 1 Geologic Conceptual Model Update of the Darajat Geothermal Field, Indonesia Sri Rejeki 1 , Dave Rohrs 3 , Gregg Nordquist 2 , Agus Fitriyanto 2 1 Chevron, 100 Northpark Blvd, Covington, LA, 70433 USA; 2 Chevron, Sentral Senayan II, Jalan Asia Afrika 8 Jakarta, Indonesia; 3 Independent Consultant, Santa Rosa, CA, USA srejeki@chevron.com, drohrs@sonic.net, gnordquist@chevron.com, agdv@chevron.com Keywords: Darajat, geothermal, reservoir, geologic model ABSTRACT Darajat is a volcanic geothermal reservoir located in West Java, Indonesia, that currently supplies steam to generate 260 MWe from 3 power plant units. Thorough reservoir characterization and modeling was conducted in 2001-2004 as part of the Darajat Unit III reserves assessment. Since that study, additional resource information was obtained from a Magneto Telluric / Time Domain Electro Magnetic (MT/TDEM) survey conducted in 2004, a micro-earthquake (MEQ) array that was established in 2005, and a make-up drilling campaign conducted in 2007-2008. The new information obtained from the geophysical surveys and drilling results has led to an improved understanding of the geology of the reservoir, including the distribution of low porosity reservoir rock, the permeability distribution, and reservoir boundaries. This new information, together with down-hole pressure, temperature, and production logs, has been integrated into an updated geologic conceptual model and incorporated into a 3D earth model through the application of geostatistical modeling tools. The 3D earth model provides the geologic input into the numerical simulation model which is being used to evaluate reservoir performance and to assess the capacity of the reservoir to support additional generation. 1. INTRODUCTION The geologic evaluation of the Darajat field began in the 1970’s. Resource assessment work was conducted by Pertamina with assistance from the New Zealand government. Three exploration wells were drilled by Pertamina in the late 1970’s, leading to the discovery of the resource. Amoseas Indonesia, a subsidiary of Chevron, was awarded the project in 1984 and continued exploration and development drilling in 1986. During 1987-1988, Amoseas drilled four wells to confirm reserves for the initial development. The 55 MWe Darajat Unit I, owned and operated by PLN, began operations in late 1994 after Amoseas drilled 6 additional development wells. To support the installation of a second power plant, geophysical and geochemical surveys were conducted in 1996-1998 together with the drilling of twelve development wells and six slim-hole step-out exploration wells. Unit II, which is owned and operated by Chevron, started commercial operations in May 2000 and currently produces approximately 95 MWe. Following the success of the 1996-1998 drilling campaign, detailed reservoir characterization and 3D geological modeling was conducted in 2001-2004 to support the reserves assessment for the Darajat Unit III expansion project. The reservoir geology was characterized with a volcanic facies model, which served as the basis for a 3D geologic model (Harrison, 2004). The 3D geologic model was incorporated into a numerical simulation model in order to forecast reservoir performance. The numerical simulation demonstrated that the reservoir could support additional generation. Unit III, a 110 MW plant owned and operated by Chevron, was commissioned in July 2007, bringing the total installed capacity of the field to 260 MWe. Following the start-up of Unit III, 7 make-up wells were drilled in 2007- 2008 raising the total number of wells drilled at Darajat to 37. The geologic model has been updated with new information and interpretations derived from the latest drilling campaign, the most recent geo-scientific studies and field performance data. 2. GEOLOGIC SETTING The Darajat field is situated along the eastern side of a range of volcanic centers nearly 30 km in length which includes the active volcanoes Gunung Papandayan (last erupted in November 2002) and Gunung Guntur (last erupted in 1840). Darajat is at an elevation of 1750–2000 meters above sea level, and lies about 9 kilometers southwest of the producing Kamojang geothermal field and 10 km east of Wayang Windu geothermal field (Figure 1). Figure 1: Physiographic map of Darajat and the surrounding area. Darajat is spatially associated with an eroded andesitic stratovolcano, Gunung Kendang. The volcanics found in the reservoir section are predominantly of intermediate to mafic composition, but younger formations found in the central part of the area consist of hornblende andesite lava, rhyolite obsidian, and related ash tuffs. Major geologic structures, which are identified as lineaments on remote sensing images and aerial photography, include the Kendang, Gagak, Cibeureum, Cipanday and Ciakut faults. Other geologic structures are mainly inferred from the alignment of MEQ events and entry distributions. Structures trend predominantly in NE-SW and NW-SE directions (Figure 2).