Geothermal energy using as electricity is generally recognized as a sustainable energy, but there is vagueness for a meaning of the “sustainable energy” from a quantitative point of view. If geothermal power generation is operated in a comparatively small geothermal reservoir, how will the electrical power generation change with long time extraction of large quantities of geothermal energy? It is expected that characteristics on the sustainability of geothermal power generation an its energy strategy among many natural renewable energy resources will be recognized by evaluating this mechanism. The behavior of the hypothetical geothermal reservoirs which have been modeled on the basis of the data related to reservoir engineering on the eight geothermal power plants in Japan, has been examined with numerical simulation. The eight geothermal power plants as reference fields are (1) Mori district, (2) Kakkonda district, (3) Sumikawa district, (4) Okuaizu district, (5) Hatchoubaru district. (6) Takigami district, (7) Kirishima district, and (8) Yamakawa district. By analyzing the data on the above eight fields, three type reservoir models have been classified according to sizes of reservoir volume - three type modes are named as a large scale reservoir (4000m×4000m(plain area)×1500m(thickness)), a middle scale reservoir (2000m×2000m×1500m) and a small scale reservoir model (1500m×1500m×1500m) respectively. Reservoir volume of many domestic geothermal power plants are laid from a middle scale reservoir model to a large scale reservoir model, and these reservoir types are liquid-dominant. Numerical simulation has been carried out as variable parameters on specific enthalpy (800kJ/kg to 1550kJ/kg) and flow rate (11kg/s to 200kg/s) of mass flux as a heat source. The simulation study showed that the enthalpy and flow rate of mass flux as a heat source are less than 1300kJ/kg and less than 100kg/s to correspond to reservoir characteristics in many domestic geothermal power plants. As results, it is expected the period to maintain 30MWe power generation may be almost 50 years to 100 years as the sustainability of geothermal power generation.
Alteration minerals focused on alunite and other acid alteration minerals were examined in the northern part of the Hatchobaru geothermal field. Alunite minerals are widely distributed with more than 300 meters thickness. Alunite minerals have a wide range of δ34S from 0.5 to 23.7 per mil, indicating several different origins: supergene and hypogene origins. APS (Aluminium phos-phate-sulfate) minerals in the core of the alunite minerals and the newly found acid alteration minerals such as zunyite, topaz, and andalusite suggest that volcanic acid sulfate solutions were once present to have formed the thick acid alteration zone at the Hatchobaru geothermal held.