Amorphous iron silicate scales in surface pipelines : characterization and geochemical constraints on formation conditions in the Miravalles geothermal field, Costa Rica

Abstract

Scales, formed in surface pipelines as a result of mixing of neutral and acidic fluids at the Miravalles geothermal field, have been characterized by chemical analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS). They consist of a chemically homogeneous amorphous iron silicate with a stoichiometry similar to the mineral minnesotaite ((Fe++,Mg)3Si4O10(OH)2). With the aid of computer programs for geochemical aqueous speciation (WATCH and EQ3NR) and for reaction path modelling of aqueous geochemical systems (EQ6), the geochemical constraints of the formation of the scales were modelled. The mixing experiments simulated with EQ6 between neutral and acidic fluids, determined a range of log K for the dissolution reaction of the scale phase, 6H+ + Fe3Si4O10(OH)2 ↔ 3Fe2+ + 4SiO2 + 4H2O, lies between 11 and 13 at 195°C. Above this range, the probabilities of over saturation of this compound are very high. On the basis of a series of mixing scenarios, acidification of neutral fluids to a pH value of 6.0 is suggested in order to prevent the formation of these scales.