| Title | Review of Baseline Geochemical Model and the Impact of Production at the Darajat Geothermal Field, Indonesia |
|---|---|
| Authors | R. Purwantoko Mahagyo, Phil A. Molling and Abu Dawud Hidayaturrobi |
| Year | 2010 |
| Conference | World Geothermal Congress |
| Keywords | Darajat, geochemistry, condensate, injection, upflow, production |
| Abstract | The geochemical-based initial-state conceptual model at the Darajat geothermal reservoir was largely delimited using distribution of non-condensable gas (NCG) chemistry, gas geothermometry and boron. Changes in chemistry to the initial-state distribution due to exploitation are reviewed through 2008. Chemical constraints of early well fluid samples allow characterization and location of predominant reservoir processes at near initial-state conditions of the Darajat Geothermal Field. Three end-member compositions were determined: (1) Central Upflow (basal recharge); (2) the southern Condensation Cell (active shallow heat pipe); and (3) Eastern and Southern Edge Field (marginal recharge-MR). Best estimates of an upflow NCG geothermometer temperature (liquid/steam equilibrium) is 275-310o C. The upflow is producing between 10-20% equilibrium steam-cap steam and 80% equilibrium steam from flashed liquid from gas grid estimates. Superheat values at initial-state range from 1.0 to 9.0o C. The major chemical distinctions of the different portions of the Darajat Geothermal reservoir are (1) an H2S-enriched Central Upflow, (2) a CO2-enriched Condensation Cell, and (3) elevated N2/Ar signal at the Edge Field wells. This edge field characteristic is corroborated by the meteoric signatures of stable isotopic values for early samples. Each compositional-type is located within the structural framework of the Darajat Field. The Central Upflow Zone is confined to between Gagak and Cibeureum Faults. The Condensation Cell is located between the Cibeureum and Cipandai Faults. Edge Field fluids (“air-related” groundwater) appear to use the Gagak Fault to migrate from the NE edge to the center of the Darajat Field. Similar groundwater fluids appear to mix with the compositions of the Condensate Cell along the Ciakut Fault. The early boron (B ppmw in steam) contained in the Central Upflow and Edge Field Zones was ~1.0 ppmw. The range of values in the Condensation Cell was higher than both the Central Upflow Zone and Edge Field Zones with a range of B between 1.5 and 5 ppmw. Boron has increased to up to 50 ppmw in the reservoir steam and ~100 ppm in the condensate injectate during the current production stage. This suggests that B is being concentrated (re-cycled) along specific flow paths as the condensate (liquid) injected is boiled and concentrates to many times the reservoir steam values. Continued monitoring of boron is also conducted regularly at surface manifestations for comparative purposes to the shallow reservoir. NCG in steam (NCGstm) has systematically decreased in production wells until 2001 and was followed by small increases in NCG (wt. %) from 2001 to 2006. In general, the NCGstm in production wells from initial-state and NCGstm values in producing wells in 2006 indicates the NCGstm has decreased. This is not the normal historical trend of NCGstm in steam cap portions of reservoirs. Both at the Geysers and Larderello gas concentrations in steam initially increased with the onset of production (Beall and Box, 1993; D’Amore and Pruess, 1985). The current low NCGstm at Darajat is believed to result from the generation of steam from low gas liquids, such as groundwater, surface water injectate, or condensate injectate. |