Darrell Lynn Gallup

Aluminum silicate scale formation and inhibition: Scale characterization and laboratory experiments

Amorphous silica scale enriched in aluminum is deposited from a variety of geothermal brines. Scale deposits examined in the present study are formed by tetrahedrally-coordinated aluminum substitution within an amorphous silica framework. There is no evidence that aluminum in these scales derives from distinct aluminum minerals, such as gibbsite, or from aluminum silicate minerals transported in brine from the reservoir. The formation of aluminum-rich amorphous silica scale is dependent on brine pH, temperature and aluminum concentration. Silica and aluminum exhibit concentration minima in brines at near-neutral pH. Aluminum-rich silica scales dissolve more slowly than pure amorphous silica in water and brine. Lowering brine pH below 5 or increasing it above 9 retards the kinetics of silica polymerization and the formation of aluminum-rich silica. Laboratory studies demonstrate that sequestering agents such as citric acid, acetic acid, and ethylenediaminetetraacetic acid (EDTA) may inhibit aluminum silicate scale formation.

Aluminum silicate scale formation and inhibition (2): scale solubilities and laboratory and field inhibition tests

Abstract The solubilities of pure silica, iron-silicate and aluminum-silicate scales were measured in water from 25–250°C in a laboratory pressure reactor. Iron- and aluminum-silicate scales are significantly less soluble than pure amorphous silica. Aluminum- and iron-rich silica scales at equilibrium conditions are predicted to deposit from near-neutral, low salinity brines at temperatures that are 25 and 75°C above the saturation point for pure amorphous silica, respectively. This laboratory study demonstrates that higher brine injection temperatures are required to mitigate aluminum- and iron-rich silica scaling compared with pure amorphous silica. In a laboratory scale test, pure amorphous silica and aluminum-rich silica deposition rates have been measured at high degrees of supersaturation in the presence of potential inhibitors. Scale deposition was best inhibited by brine pH modification techniques. A commercially available dispersant successfully inhibited amorphous silica scaling, but exacerbated aluminum silicate scaling. Scale inhibition was also achieved in the presence of aluminum complexing\sequestering agents in a patent-pending process. Screening of these potential silica- and aluminum-silicate scale inhibitors in the laboratory will focus the efforts of ongoing field pilot scale testing. The effect of complexing\sequestering agents on scale inhibition was monitored in preliminary field pilot tests. These complexing agents achieved 25% to 80% aluminum silicate scale inhibition.

Reaction Investigation of Metal Salts with Tetrakis(hydroxymethyl)phosphonium Sulfate and Chloride

The interaction of tetrakis(hydroxymethyl)phosphonium sulfate (THPS) and chloride (THPC) with water-soluble and water-insoluble metal compounds was investigated. We evaluated solutions of the phosphonium salts for dissolution of metal sulfides, and complexation of the THP moiety with Fe2+, Fe3+, Cu2+, Zn2+, Hg2+, and Pb2+. Scanning ultraviolet/visible spectroscopy was deployed to monitor complex formation. Applying mole ratios of 1–9 THP:1 M n+ (Fe3+, Hg2+, and Pb2+) revealed the formation of several complexes. THP did not appear to complex with Fe2+ and Zn+2 although isobestic points were maintained in the UV spectra. The results of this investigation did not confirm prior studies that showed complexation of Fe2+ with THP in the presence of ammonium ion. Results suggest that THPS and THPC dissolved metal sulfide precipitates at mildly acidic conditions, and THP often formed stable complexes of variable stoichiometry with the metal cations.