PROCEEDINGS, 3 rd International ITB Geothermal Workshop 2014 Institut Teknologi Bandung, Bandung, Indonesia, March 3-7 , 2014 SATURATION INDICES OF PROGRADE AND RETROGRADE SOLUBILITY MINERALS IN UBL-11 FLUID DURING TEMPERATURE DROP OF UBL-12 Jodhi P. Giriarso, Hendy Sujarmaitanto, Dradjat B. Hartanto PT. Pertamina Geothermal Energy Menara Cakrawala 15th Floor Jl. MH. Thamrin No.9 Jakarta e-mail: jodhi.pramuji@pertamina.com ABSTRACT During 2012-2013, UBL-12 had experienced cooling because influx of surface water. This diluted reservoir fluids in cluster D, include UBL-11 as a production well. Dilution effect seen in the decline of chloride, silica, and NCG, increase of bicarbonate and sulfate. WATCH simulation was conducted during this cooling. Saturation indices of prograde minerals such as amorphous silica, chalcedony, and quartz decreased. On the other hand, saturation indices of retrograde minerals such calcite and anhydrite relatively increased. Allegedly, addition of water volume due to dilution caused increase solubility of chloride and silica which led to decrease of the saturation indices of prograde solubility mineral, and also increase solubility of water-soluble gases which led to NCG decline. Increase of soluble gases in water led to the rise of bicarbonate and sulfate concentration as precursor of calcite and anhydrite formation. Keywords: minerals, saturation indices, dilution ULUBELU Ulubelu Geothermal Area is located in district of Tanggamus, Lampung province, about 100 km west of Bandar Lampung (Figure 1). Since August 2012 this area had been producing 1 x 55 MW, followed by second unit the next 55 MW in September of the same year. Figure 1: Map location of Ulubelu Geothermal Field Contributed wells of unit 1 and 2 are from three clusters: B, C, and D. Not all wells in those three cluster contributed. Some of non-active well functionalized as monitoring wells to measure pressure and temperature periodically. One of the non-active wells in cluster D was UBL- 12. Monitoring of this well indicated significant temperature decrease. UBL-12 Date Temperature (°C) Depth (mTD) Feb 2011 285 2100 Apr 2012 277 Jul 2012 123 Table 1: Temperature decrease in UBL-12 UBL-11 well is located in Cluster D nearby UBL-12 well. Chemical composition of UBL-11 was monitored to see its affection during cooling period in 2012. Apparently, the decrease of temperature of UBL-12 affected to this well. Chemical changes such as sulfate and bicarbonate increased, chloride decline occurred in UBL-11 (Giriarso, 2012). Kotabumi Manggala Metro Ulubelu BANDAR LAMPUNG Blambanganumpu Liwa G. Sekincau G. Ratai G. Rajabasa 105° 104° 5° 6° 106° Lp. Barat Tanggamus Lp. Selatan Lp. Timur Lp. Tengah Lp. Utara Way Kanan Tulang Bawang 4° G.Rendingan Kotaagung Muchsin/UBL Lokasi Proyek Tl. Semangka G.Tanggamus PETA LOKASI ULUBELU G.Rendingan