The CaSil technology at the He Ahi/Tauhara geothermal field: Silica scale elimination in the presence of a calcite inhibitor

Abstract

Silica scaling is a major problem in the utilisation of geothermal resources worldwide. It blocks valves, heat exchangers, and reinjection wells, severely limiting the energy that can be extracted. Current mitigation efforts are not wholly successful in preventing scale from forming, so costly and time-consuming cleaning and maintenance efforts are still required.
CaSil Technologies Ltd has developed a novel and transformative technology that approaches the problem of silica scaling in a different way. Instead of sequestering or delaying the precipitation of silica, the CaSil technology chemically captures dissolved silica and transforms it into a new calcium silicate hydrate (CaSil) material. With rapid reaction kinetics, the Silica Saturation Index is lowered significantly below 1, effectively eliminating silica scaling. The CaSil material forms discrete and finely dispersed particles which can freely flow through heat exchangers and pipework, without sticking to metal surfaces.
The CaSil technology has been previously proven at automated pilot scale at three different New Zealand geothermal resources (Wairakei, Kawerau, Mokai). Contact Energy Ltd invited CaSil Technologies to demonstrate the technology at the He Ahi Eco-Business Park, located on the Tauhara geothermal resource near Taupo. The field is designed for the supply of direct-heat to industry and is serviced by a single well. To prevent calcite scaling in the production well, a calcite inhibitor is dosed downhole. This was the first time the CaSil technology was trialled in the presence of a calcite inhibitor, which may interfere with the calcium containing treatment agent dosed for the CaSil reaction.
This paper presents performance data of the pilot plant and complementary batch experiments performed on-site which demonstrate the robustness and effectiveness of the CaSil process. The Silica Saturation Index was reliably reduced << 1 across the trial period and a good compatibility between CaSil and the calcite inhibitor established.