Characterization of Silica Precipitation at Geothermal Conditions

Abstract

Geothermal fluids in high enthalpy geothermal systems are often strongly enriched with dissolved silica, because these waters quickly ascend from hot geological formations and are, therefore not at chemical equilibrium on the surface. Especially the dissolved silica concentration would still resemble the amount of silica dissolved the reservoir thus being highly oversaturated at surface conditions. This silica might precipitate uncontrolled at various locations in the geothermal plant, in the reinjection well, or in the reservoir at the injection site thus causing severe damages of the plant or the reservoir. To prevent such uncontrolled silica precipitation, this study aims to understand the precipitation process by characterizing both the solid silica precipitates (using geochemical and mineralogical methods) and the polymerization of silica in solution at well-defined lab conditions (using measurements of ultrasonic velocity). Precipitation reactions have been investigated in dependence of temperature, pH value and ionic strength. The results can be used for understanding the polymerization and precipitation process as well as to develop methods that can be applied at geothermal sites for for controlled and accelerated silica precipitation in ponds or vessels before fluid re-injection. This eventually would prevent silica precipitation at the reinjection site which would severely damage the reservoir by permeability reduction.