| Title | Precipitation Mechanism of Carbonate and Sulfate Scales in Enhanced Geothermal System (EGS) at Hijiori, Japan |
|---|---|
| Authors | Norio Yanagisawa |
| Year | 2011 |
| Conference | Western Pacific Regional Branch |
| Keywords | EGS, calcite, anhydrite, inhibitor |
| Abstract | Several kinds of scale precipitated in the pipelines and productions well during two years long circulation test in the Enhanced Geothermal System (EGS) of Hijiori, Yamagata, Japan. The ratio of silica and calcium carbonate depends on fluid temperature and chemical composition. In the case of HDR-2a nearer to the injection well, wellhead temperature is about 120Ž after thermal breakthrough and the major scale is calcium carbonate. In the case of HDR-3 further from the injection well, wellhead temperature is kept about 160Ž 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. Injected water dissolved the anhydrite in the reservoir rock and anhydrite precipitated at the higher temperature zone because of the reverse solubility curve of anhydrite with temperature. This anhydrite dissolution and precipitation behavior influences scaling in the pipeline. In the pipeline of HDR-2a, 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. Chemical scale inhibitor, sodium polyacrylate (C2H3COONa) injection trials into the reservoir were done to decrease scaling. After inhibitor was injected, production fluid chemistry changed. HCO3 concentration increased with inhibitor suggesting that the inhibitor can be used and precipitation of calcite can be prevented in many geothermal fields. And inhibitor prevented to solve anhydrite as source of calcium at lower temperature. This suggests that this inhibitor effect for scale prevention in EGS. |