| Abstract |
The Hatchobaru power station has the largest rated capacity of geothermal power generation in Japan. It is well known that the first double-flash system was adopted here. In this area, the whole of geothermal brine, about 1,500 t/h, is reinjected through a retaining tank for silica scale prevention, which is one of the extra features of this power station. However, the present reinjection system is not always adequate for scale prevention. Therefore, adoption of pH modification is considered as an alternative scale prevention method. In order to clarify the effects and impact of pH modification on the Hatchobaru field, brine transmission experiments were carried out using a flow-type autoclave. Silica scale deposition in the inlet of the passage was restrained, and the permeability decline was improved by adjusting brine pH to 5.5. Furthermore, it was proven that the acid brine was neutralized by interaction with rocks. According to these results, it is considered that reinjection of pH adjusted brine will prevent the reinjection capacity decline due to concentrated silica scale deposition, and will not give adverse environment impacts. The effect of pH modification on the prevention of declining reinjection capacity was estimated by numerical simulation. It was estimated that lives of reinjection wells would be prolonged around three times of present situation by pH adjusted to 5.5 in the Hatchobaru field. |