CHEMICAL EQUILIBRIUM MODELS FOR THE FLUIDS IN THE FUSHIME GEOTHERMAL SYSTEM: ISOENTHALPIC AND ISOTHERMAL BOILING

Abstract

Numerical modeling for fluid-mineral reactions during production-induced boiling in geothermal reservoir was carried out. The isoenthalpic and isothermal boiling processes for the geothermal waters derived from seawater were tested. The temperature decrease in the isoenthalpic boiling process causes precipitation of sphalerite and galena. In contrast, the isothermal boiling process results in dissolution of sphalerite and galena from wall rocks driven by the H2S escape from liquid to gas phase at a constant temperature. The degassing in the isothermal process also causes continuous dissolution of pyrite from wall rocks, precipitation of magnetite and the incerases in H2S and H2 concentrations. As a result of these reactions, the decrease in pH with increasing enthalpy is computed. The isothermal boiling process would play an important part for the changes of fluid compositions in the high temperature reservoirs under production.