Scale Precipitation During Circulation at the Hijiori HDR Test Field, Yamagata, Japan

Abstract

Several kinds of scale precipitated in the pipeline and production well during two years long circulation test in the HDR geothermal system of Hijiori, Yamagata, Japan.
The ratio of silica and calcium carbonate depends on fluid temperature and chemical composition. In the case of HDR-2 nearer to the injection well, wellhead temperature is about 120? after thermal breakthrough and the major scale is calcium carbonate, calcite and aragonite, changed from amorphous silica. And the lower reaches of the pipeline, calcite became dominant instead of aragonite.
In the case of HDR-3 further from the injection well, wellhead temperature is kept about 160? during the circulation test and amorphous silica scale precipitated slightly.
At the deep zone of the production well, anhydrite scale precipitated at the reverse temperature zone shown by PTS survey. Injection of cool water dissolved the anhydrite in the reservoir rock and anhydrite precipitated at the higher temperature zone because of the reverse solubility curve of anhydrite.
This anhydrite dissolve and precipitation influence scaling in the pipeline. In the pipeline of HDR-2, high content Ca reacted with CO2. High content Ca came from dissolved anhydrite in the reservoir and kept during the lower temperature of production well. However in HDR-3, further from the injection well and higher in temperature, slight amounts of amorphous silica precipitated. Ca content was too low for scaling and SiO2 was high enough to precipitate slightly.