The high-temperature and liquid-dominated Mahanagdong geothermal field supplies steam since 1997 to power plants with total installed capacity of 180 MWe. A geochemical assessment of the field is made based on analytical data of fluids sampled at the wellheads of wet-steam production wells. Using speciation programs, individual species activities in the initial aquifer fluids were modeled. Results indicate that “excess enthalpy” discharged by some wells is mostly caused by phase segregation of the vapor and liquid phases in producing aquifers. Aquifer fluid component concentrations were calculated taking the phase segregation to occur at 30°C below the aquifer temperature. This condition yields a liquid saturation of ~20% which is reasonable in a typical fracture-dominated reservoir rock. The modeling results are slightly affected when the inferred aquifer temperature is varied within ±20°C. On the other hand, the selected phase segregation point and the discharge enthalpy greatly affect the results. For non-volatile components, calculated concentrations can vary up to ~20% lower if discharge enthalpy is >2000 kJ/kg and phase segregation is selected within the temperature range that gives maximum enthalpy of steam (200-250°C). For gases, their aqueous concentrations in the aquifer fluids can widely vary depending on the choice of phase segregation temperature. This variation involving the volatile components increases proportionally with the discharge enthalpy of the well thus considerable uncertainty is involved when discharge enthalpy approaches that of dry steam.

The modeled compositions of aquifer fluids were used to assess how closely equilibrium is approached between solution and various minerals. At inferred Mahanagdong aquifer temperatures (250-300°C), the concentrations of H2S and H2 of purely liquid aquifer fluids are slightly higher than those at equilibrium with hydrothermal mineral assemblages incorporating (1) hematite+magnetite+pyrite and (2) grossular+pyrite+magnetite+wollastonite. Departure from equilibrium is high for the Fischer-Tropsch and NH3-N2-H2 reactions. The observed distribution of data points for the gases is attributed to the presence of equilibrium vapor in the aquifer fluid. Initial aquifer vapor fractions were derived assuming equilibrium between H2,aq and the hematite+magnetite assemblage. If equilibrium vapor fraction in the initial aquifer fluid is considered, H2Saq concentrations are significantly above the equilibrium curves for the considered mineral assemblage whereas CO2,aq concentrations closely approach equilibrium. Some derived aquifer vapor fractions are high (1-4% by mass). Using earlier data and those produced for this study, some areas in the field indicate losses in equilibrium vapor.

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