Abstract

The Pertamina Kamojang geothermal field, located in west Java, 100 km southeast of Bandung, produces high temperature (250°C) steam from fractured volcanic reservoirs. Detailed knowledge of the lithofacies, fracture density distribution and the orientation of the fracture systems have been primary considerations during field appraisal and development.

The high cost of conventional coring to obtain this information has proven restrictive, and only limited core sampling has been performed. In addition, orientation of cores is also expensive and sometimes unreliable and there is always a potential for lost recovery (especially across the more fractured intervals).

Continuous and depth controlled, high resolution (0.2" to 0.3") electrical imaging (from the Formation MicroScanner™) was first performed in 1991 under the adverse/hostile wellbore conditions of the Kamojang field. Geological interpretation of the images on an interactive workstation has allowed the identification of lava flows, pyroclastic and tuffaceous zones. Depositional surfaces and local alteration may be recognized in the tuffaceous zones.

Fractured zones are also identified and open fractures can be distinguished from healed features or from cooling joints and formation cracks induced by the drilling process. The dip magnitude and strike direction of the open systems can be accurately determined and this data is proving essential in optimizing the direction of subsequent development wells to assure they penetrate maximum fracture densities. A combinable continuous temperature measurement is used to indicate which of the fractures are productive.