| Abstract |
The past 6 years saw rapid changes in the Tongonan geothermal reservoir as the field underwent full commercial operations starting in 1996 from 112 MWe to 550 MWe gross generation. The most prominent among the changes is the transformation of the discharge of production wells, mostly from the center of the field, from being liquid-dominated to steam-dominated. In order to properly manage and sustain the reservoir, a geochemical evaluation was made of the deeper resource untapped by the production wells. The result showed that the reservoir is sustainable as the deeper part of the resource still hosts more than 90% of the hot geothermal liquid. The gas-mineral equilibria tool (FT-HSH2) developed by DÆAmore gave an estimate of the steam fraction and temperature at the central region at 1-5% steam (or 95-99% liquid) and 290-310░C, respectively. A consistent temperature of 280-300░C of the same region was extrapolated from the Na/K geothermometer. At the outflow region of the reservoir where production wells still have a liquid fraction in their discharge, a steam loss of 1% is estimated in areas with inflow of cooler peripheral waters and injected brine while a 5% steam fraction was noted in areas having higher-enthalpy production wells. The same gas-mineral equilibria method gave good agreements with the quartz geothermometer for the low-enthalpy wells signifying its fast re-equilibration. In dry wells, the method reflected higher temperatures indicating that the last re-equilibration of the participating species is far from the wellbore, probably at the point where water containing the reacting minerals is still present. |