Chemical Characteristics of Acid Fluids in Some PNOC Geothermal Wells

Abstract

Wells drilled above and within the margins of upflow regions in most PNOC-EDC geothermal fields produced acid fluid during initial discharge tests. The acid wells commonly discharged SO4 rich waters with varying amount of CI and dissolved cations. Waters from very hot (>28OoC) reservoir contain excess CI concentrations, suggesting that their acidity is partly caused by HC1 from, most likely, a volcanic source. In most wells, the dissociation of HSO; results to a sharp decline in pH as the fluids ascend. Boiling models also show that for reservoir waters with substantial amount of HCOr- pH increases as a result of boiling. This is due to exsolution of C02 into the steam phase. In general, except for the magmatic reservoir waters which have inherently low pH at depths, fluid pH at reservoir conditions is about 5.0 which is near-neutral at high temperature conditions (T>250oC).