Modeling of Formation of Acid Water Discharged from High Temperature Geothermal Reservoir

Abstract

The Fe-rich acid geothermal waters (pH = 3-5) associated with large enthalpy fluids are discharged from the high temperature reservoirs (over 300oC) in the Fushime (Yamagawa) and Kakkonda geothermal systems. In contrast, the moderate temperature reservoirs in these geothermal systems produce neutral to slightly alkaline waters with a little or no excess enthalpies. The geothermal water in Fushime is derived from seawater, while that in Kakkonda is believed to be of meteoric origin. In order to study the formation processes of these saline and dilute acid waters, numerical modeling based on chemical equilibrium for liquid-solid-gas phases was carried out. The models involve boiling in the reservoir caused by production-induced pressure decrease near the well and consequent reactions with wall rocks. The calculated fluid compositions through the models closely agree with the compositions of the fluids discharged from the high temperature reservoirs in Fushime and Kakkonda. The calculation results indicate that the H+ supply from sphalerite precipitation during boiling is the most probable cause of the observed acidity in the saline Fushime waters. On the other hand, the H+ supply from the reactions with pyrite and magnetite and from H2 redox reaction possibly contributes to the observed acidity in the dilute Kakkonda waters. The models demonstrate that production-induced boiling and consequent chemical reactions in the reservoirs are able to produce the observed acidity, without any contribution of acidic volatiles such as HCl and SO2.