Reaction of pH-Adjusted Reinjection Brine and Geothermal Reservoir: Simulation by Chemical Equilibrium Approach

Abstract

The geochemical effects caused by reinjection of a separated water mixed with sulfuric acid (pH-adjusted brine) was studied in the water-rock interactions of a reservoir. In Japanís Oguni geothermal field, the pH increase of the pHadjusted brine (pH=5) is initially large, then approaches steady state due to the reaction of one kilogram of brine with several grams of rock. The pH is buffered by the mineral assemblage of albite, K-feldspar, K-mica and quartz. A small amount of wall rock is sufficient to neutralize the pH-adjusted brine due to the large buffering capacity of the wall rock. The chemical changes in aqueous species and alteration in mineral assemblage produced by the pH-adjusted brine are similar to those produced by the brine without added acid, independent of rock-types (andesite, dacite and tuff) and alteration degree (fresh and partially altered). Anhydrite would not precipitate from the pH-adjusted brine in the Oguni field, even when sulfuric acid was added to the system. The pH-adjusted brine would prevent the precipitation of calcite as well as amorphous silica in fractures around the reinjection well. These results suggest that the geochemical environment in the reservoir is little affected by reinjection of a small amount of brine with added sulfuric acid solution.