Current Geothermal Development in Kumamoto Prefecture, Central Kyushu Island, Japan

Abstract

Japanese Government announced an energy policy and showed a plan of the electricity generation for each energy in 2030. Geothermal energy is desired to install power generation more than 1,500MW under this new policy, which is almost three times of the current installed capacity. Kumamoto Prefecture is located in the centre of Kyushu Island, where much of geothermal resources are expected and Oguni geothermal field is located near the Hacchobaru Geothermal Power Plant, which is one of the largest geothermal power plant in Japan. A 2MW geothermal power plant commenced in operation but further development is demanded for the completion of the electric generation plan in 2030. This geothermal field is divided into two reservoirs, the northern and the southern reservoirs, based on the analysis of relationship between altitude and pressure of feed points. The small power plant with a 2MW generator is located in the northern reservoir. In the hydraulic model of the northern reservoir, hot water heated by heat source rises along a fault and moves westward. The fault is clearly found near the mountain from the topographical point of view but it becomes unclear far from the mountain. Therefore, geothermal hot water changes its direction and flows to north. An aerial gravity deviation survey was recently conducted in this area, showing the continuity, which suggests a path of hydrothermal activity. This flow path need not change flow direction on the way, so we formed a model for flow calculation based on this new concept. We have reviewed published models and construct a new model based on the aerial gravity survey and try to answer the question how much of the geothermal energy remains for the development. We will discuss on the results of the simulation study in this paper. We will also show several surface surveys including MT surveys and discuss on the further possibility for the development in the southern reservoir.