[H-RE12] New Developments in Engineering Geology

[HRE12-P05] Geothermal power generation that Japan can take the lead in: Shallow supercritical geothermal power generation

*Hisatoshi Ito¹ (1.Central Research Institute of Electric Power Industry)

Keywords:geothermal resources, supercritical geothermal power generation, Kurobegawa granite

Japan is known to be a geothermal resource-rich country, but as of March 2021, the amount of geothermal power generation is as low as 610,000 kW, which is only about half of the Geysers Geothermal Area in the United States. In order to overcome this problem, NEDO is proceeding with a development plan for supercritical geothermal power generation as a way to carry out large-scale geothermal development in Japan in the future. NEDO appears to have selected four promising areas for priority investigation: Hachimantai and Kakkonda in Iwate Prefecture, southern Yuzawa in Akita Prefecture, and Kuju in Oita Prefecture. In these areas, the target rock body with a temperature of 400℃ or higher (probably granite; hereinafter referred to as a reservoir) is located deep underground, so it is needed to drill through 2 to 3 km overlying layer (mainly volcanic rocks) to reach the reservoir. Although it is necessary to develop technology to excavate deep underground reservoirs, it is also necessary to evaluate the reservoirs. In the past, plans were advanced in Australia to develop a reservoir deeper than 4 km underground, but problems during excavation prevented it from becoming a reality. For this reason, in parallel with the development of deep reservoirs, we should first proceed with the evaluation and development of shallow underground reservoirs, which are considered more likely to be realized. In Japan, the Hida Mountains have high-temperature granite exposed on the ground that is probably unprecedented in the world. The rock temperature of this granite (Kurobegawa Granite) is said to have reached 175℃ when a tunnel was excavated for hydroelectric power generation. In the Kurobegawa granite distribution area, volcanic rocks (Jiigatake volcanic rocks) that are presumed to have overlain the granite are missing due to severe uplift and denudation. Therefore, if this granite is targeted, there is no need to excavate the overlying layer, and it is thought that the reservoir layer can be reached 1 to 2 km underground. In addition, Kurobegawa granite is a Quaternary granite like Kakkonda granite, so fractures caused by tectonics after granite formation are likely to be insignificant compared with granites older than the Quaternary period (for example, the currently known southern part of Yuzawa. However, there is a possibility that Quaternary granite may be present in this area too), and it is thought to be easier to evaluate and develop reservoirs. In nuclear power generation, research and development is conducted on prototype reactors and demonstration reactors before commercial reactors are developed. Supercritical geothermal development targeting the Kurobegawa granite is positioned close to a prototype reactor to a demonstration reactor, and is thought to be able to greatly contribute to the development of elemental technologies for the realization of supercritical geothermal power generation.

References
Ito, H., 2013. Kurobegawa Granite and power generation from engineered geothermal system. Proceedings of the 2013 Annual Meeting of the Geothermal Research Society of Japan, A05.
Ito, H., 2023. Geothermal resources and geochronology: what Quaternary granite indicates. Abstracts 2023 Japan Geoscience Union Meeting. SCG54-01.