Investigation of a Rapid Geothermal Aluminosilicate Scale Formation Mechanism: Preliminary Results of Kinetic Experiments

Abstract

Amorphous aluminosilicate scales have been observed to form in geothermal pipelines at numerous locations internationally, with examples reported from geothermal fields in Iceland, El Salvador, Nicaragua, New Zealand, Japan, Indonesia, and the Philippines. These scales frequently accumulate at higher rates and temperatures than would be expected for pure amorphous silica scales, resulting in the unexpected fouling of surface infrastructure, and injectivity declines in geothermal brine reinjection wells; both of which present significant problems for the utilisation of geothermal energy. Yet, though this phenomenon has been studied for over 40 years, a mechanism responsible for the formation of aluminosilicate scales has yet to be unequivocally identified. In this work, aluminosilicate colloids were synthesised and captured under controlled conditions during time constrained aqueous experiments. The rapid formation of aluminosilicate colloids was found to coincide with the supersaturation of aluminium (oxy)hydroxide, producing filterable aluminosilicate colloids within 120 seconds; with the initially Al rich colloids becoming increasingly Si rich with reaction time. Aluminosilicate colloids synthesised under neutral to alkaline conditions sequester both Na and K during their earliest formation; with Si/Al, K/Na, and Al/K+Na) mole ratios approaching those of real geothermal scales formed at the San Jacinto-Tizate field study site, as well as others reported in the literature. Similarly, these synthesised colloids demonstrate a near identical [K/Na]solid / [K/Na]solution enrichment to that of real geothermal aluminosilicate scales. The onset of aluminosilicate formation appears to be independent of aqueous silica concentration, and the saturation state of silica with respect to amorphous silica; although the solution Si/Al mole ratio does influence the stoichiometry of the colloids produced. These latter two features are congruent with a formation mechanism identified for amorphous aluminosilicates formed at low temperatures under acidic conditions within recent hydroxyaluminosilicate (HAS) research. Although these preliminary results require further testing, the successful synthesis of artificial geothermal aluminosilicate colloid analogues demonstrated by this work presents an invaluable opportunity to both better resolve geothermal aluminosilicate-scaling processes, and more rigorously test methods of aluminosilicate scale mitigation and prevention.