Calibrating Geochemical Software Programs with Respect to Fe-Sulfides and Fe-Oxides Saturation Indices

Abstract

Saturation indices of common Fe-sulfides (pyrite, pyrrhotite) and Fe-oxides (magnetite, hematite, goethite) in geothermal setting are calculated by geochemical software programs WATCH and SOLVEQ/CHIM and then calibrated with actual documentation of petrologically identified scale minerals in surface facilities. This study aims to determine which geochemical modeling software yield reasonable saturation indices that are consistent with field data and can therefore be used to assess and predict scaling potential of geothermal fluids. Results show that SOLVEQ yield lower and more reasonable saturation indices relative to WATCH. However, there is a discrepancy of saturation indices yielded by the software programs with actual data. In order to appreciate both the potential and extent of Fe-sulfide or Fe-oxide deposition, it is more useful to model the fluids using reaction path software CHIM-XPT that will yield predicted amount of supersaturated Fe-precipitates that are more consistent with actual observations. Due to significant variation of results, sensitivity of the calculated saturation indices to key parameters (temperature, pH, Fe and H2S levels) is also explored. The saturation indices given by WATCH are very sensitive to the input solution pH and reference temperature and should therefore be validated and interpreted with caution to account for variation in these parameters that are common consequences of sampling, analytical procedures and assumed reference temperature.