Kamojang Geothermal Field (KGF) located in West Java is the oldest developed geothermal field in Indonesia. It is a typical vapor dominated hydrothermal system. From 1983 to 2005, more than 160 million tons of steam has been exploited from the KGF and more than 30 million tons of condensed water and river water were injected to the reservoir system. Regarding to the electricity demand, installed capacity of KGF increased from 30 MWe to 140 MWe in 1987 and 200 MWe in 2007. The evaluation of steam production in 1999 showed the decline of steam flow rate notably occurred at some production wells in the KGF.

The changes of reservoir condition periodically can be measured by the geophysical method, that is, repeat gravity measurement. Repeat gravity measurements and leveling surveys over the KGF to measure the changes in the gravity and vertical ground movement were conducted since 1984. Gravity monitoring between 1999 and 2005 at 51 benchmarks are interpreted in terms of a change of mass. The amount of mass changed was caused by production and injection activities. Based on Gauss’s potential theorem, the 1999-2005 gravity changes in the reservoir area indicate that the system of the KGF has mass decrease of about 3.34 Mt/year. Repeat gravity measurements were conducted in the end of 2008 to monitor the changes of mass balance during production capacity increase. Concerning to the production increase, between 1999 and 2008 the KGF has average mass decrease bigger than between 1999 and 2005. It is very important to balance the mass in the geothermal reservoir to continue the sustainable production.

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