Estimates of geothermal reservoir fluid characteristics: GeoSys.Chem and WATCH

  • Ignacio Salvador Torres-Alvarado
  • Mahendra P. Verma
  • Kizito Opondo
  • David Nieva
  • Füsun Tut Haklidir
Keywords: GeoSys.Chem, GeoChem, WATCH, geochemical modeling, geothermal system


A comparative study of the reservoir fluid characteristics calculation of ten production wells of Los Azufres, Los Humeros and Cerro Prieto geothermal fields using two computer codes GeoChem (GeoSys.Chem) and WATCH is presented. GeoSys.Chem estimates the reservoir temperature and vapor fraction through quartz geothermometry and assuming enthalpy conservation, while the average temperature of quartz and Na/K geothermometers is employed in WATCH and vapor fraction is also calculated through enthalpy conservation. Both programs use the conservation of alkalinity (i.e., proton balance) for pH calculations. The difference in temperature (pressure) causes considerable effects on the calculated geothermal reservoir fluid characteristics of high enthalpy wells and negative (or near to zero) vapor fraction for low enthalpy wells. The calculated high concentration of CO2 in the secondary vapor discharged in the atmosphere at the weir box (up to 11,719 mmole/kg) suggests that the analysis of carbonic species in the geothermal waters is crucial. In the absence of good quality analysis of carbonic species it is suggested to consider the CO2 in the vapor sample at the separator and the total dissolved carbonic species concentration in the water sample (i.e., without considering the liberation of CO2 in the atmospheric vapor at the weir box) for the calculation of geothermal reservoir fluid composition.