| 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. To properly manage and sustain the reservoir, a geochemical evaluation was made of the deeper resource untapped by the production wells. The results show 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 (FT-HSH2) by D'Amore gave estimates of the steam fraction and temperature in the central region of 1 - 5% steam (or 95 - 99% liquid) and 290 - 310?C, respectively. A temperature of 280 - 300?C for the same region extrapolated using the Na/K geothermometer shows the consistency of both methods. In the outflow region of the reservoir, production wells still have a liquid fraction in their discharge. Here, a steam loss of 1% is estimated in areas having inflows of cooler peripheral waters and injected brine while a 5% steam fraction was noted in areas producing from higher-enthalpy 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. |