Modeling Neutralization of Acid Fluids in the Biliran Geothermal Field, Philippines

Abstract

Using numerical geochemical modelling through FORTRAN programs GEOCAL, SOLVEQ-XPT and CHIM-XPT (2016), the hydrogeochemical model (neutralization of magmatic-acidic fluids by extensive water-rock interaction) of Biliran geothermal field was tested. Water-rock interaction (WRI) was simulated by incrementally titrating an andesitic wall rock into 1 kg of the magmatic-acidic water at the reservoir temperature (350°C). Results of the modeling showed significant change in pH (1.5 to 5.6) at the end of the rock titration with the neutralization process resulting from the precipitation and/or dissolution of alteration minerals minerals including rutile, quartz, calcite, epidote, feldspars, chlorite end-members, apatite and pyrite. Alteration assemblages of rocks intersected by the Biliran drill holes using petrographic analysis and XRD conform well to the predicted alteration minerals while differences in the observed and predicted mineralogy may be attributed to the oversimplification of the modeled scenarios.