ALUMINIUM-RICH SILICA SCALING: SAN JACINTO-TIZATE GEOTHERMAL ENERGY PROJECT, NICARAGUA

Abstract

Geothermal pipeline scale sampled from four locations along the San Jacinto North Reinjection Line was found to consist of metal-rich amorphous silica containing up to 16 wt. % Al2O3, 6.9 wt. % Fe2O3, 4.1 wt. % K2O, and 2.7 wt. % Na2O; with some zones of the scale significantly enriched with Au-Ag mineralised Cu-Fe-Zn-Pb sulfides. X-ray Diffraction (XRD) patterns and Field Emission Scanning Electron Microscope energy dispersive spectroscopy (FESEM EDS) in-situ compositional analyses confirm that Al, Na, K, Ca and some Fe are contained within the amorphous silica molecular structure, with remaining Fe as sulfide minerals and corrosion products. Al is the most abundant metallic element in all scale samples, with Na, K, and Ca concentrations closely correlating with Al. Fe is concentrated in the earliest formed scale yet is rare to absent in the most recently deposited scale, and is likely derived from corrosion of steel pipe and well casings. The concentration of Al in silica scale reduces with distance from the production well heads, and is most concentrated in the earliest deposited base of the scale, which contains 4-12 wt. % Al2O3 (bulk area analyses) and 10-16 wt. % Al2O3 (in-situ colloids). The most recently deposited silica contains ~11 wt. % Al2O3, both bulk zones and colloids. The contrast between bulk and in-situ metal concentrations reflects the observation (via FESEM) that the scale is largely an admixture of primary metal-rich silica colloids, and pure SiO2 mantling this earlier deposited matrix. The broad XRD peaks are typical of amorphous silica yet they also show a systematic 2? angle shift positively correlated with aluminium concentration. These characteristics closely resemble those both of aluminium-rich silica scales from other geothermal fields, and hydroxyaluminosilicates (HAS) precipitated in lab experiments. A novel reaction pathway based on these observations, along with recent experimental and computational chemistry research into the formation of HAS, is proposed. This mechanism accounts for both the lack of apparent silica polymerisation induction period in cases of aluminium-rich silica scaling and its deposition in solutions undersaturated with respect to amorphous silica, observed both in scaling experiments and in the field at San Jacinto, and several other geothermal power stations worldwide.