Aqueous Speciation Calculation of Geothermal Waters -Its Application to Geothermal Well Discharges and Limitations

An aqueous speciation code using a personal computer is developed and applied to some Japanese geothermal well discharges. The calculated activities of component ions are used to plot the compositions of discharges on activity diagrams and to check the saturations of some alteration minerals. Many minerals are saturated at temperatures close to the reservoir temperature estimated by silica -and alkali- geothermometers. The temperature and mineral assemblage of a reservoir can be estimated by plotting temperature vs. saturation index. Factors introducing the uncertainties for the calculations are also discussed.