DECLINED MASS TRENDS OF UNBALANCED PRODUCTION TO RECHARGE ACTIVITY IN KAMOJANG GEOTHERMAL FIELD, INDONESIA: A CONTINUOUS MONITORING WITH HYBRID GRAVIMETRY

摘要

Repeated gravity measurements have been applied at the Kamojang geothermal field in Indonesia. Previously, we used only relative gravimeters to measure precise gravity change. We detected gravity changes caused by the production and injection. However, we cannot assess the gravity change at the reference station, because we only used the relative gravimeter. Hence we introduced the A10 absolute gravimeter (Micro-g LaCoste, Inc.) in 2009 for not only the assessment of the gravity changes at the reference station, but also the detection of the gravity change caused by the underground fluid flow changes. Mass variations of these microgravity data since 1984 in Kamojang geothermal field described a relatively steady natural recharge around 2.7 MT/year during increased production capacity from 30 MWe to 140 MWe in 1987 and 200 MWe in 2007. Fluid recharge to reservoir that come from injection activity is limited around 2 MT/year while increased production rate. This unbalanced production to recharge activity leads a declined mass trend in Kamojang Geothermal Field. Monitoring data between 1999 and 2008 by relative gravimeter displayed declined mass trend in southern area and the most negative of gravity changes have been around 20 (mu)Gal in some area. A continuous monitoring using absolute gravimeter in 2009, 2010 and 2011 revealed a higher negative gravity changes in some area more than 40 (mu)Gal and more declined mass trend in southern area. The declined mass trends also supported by declined pressure and steam flow rate of production wells.