The Appi field is one of the promising geothermal fields in Japan. A high temperature reservoir is present below thick impermeable layers and vapor-dominated zone is found in the southwestern portion of the reservoir. Based on various geological, geochemical and geophysical surveys and deep well drilling and testing, Mitsubishi Materials Corporation (MMC) has developed a three-dimensional numerical model of the reservoir (“MMC2014” model) to examine the optimum development scale. In this research, we extended the MMC2014 model, the upper surface of which is located at ~500-m depth, to the ground surface to include shallow groundwater systems and reproduced a steam-heated hot spring in the area. We need to assign highly anisotropic permeabilities to shallow formations to reproduce relatively low feed point pressures of deep wells (~8 MPa at depth of 1,500 meters). Based upon the natural state model, we carried out performance prediction and calculated its effects on the thermal conditions of the hot spring. Simulation results indicate that present production plan enables stable power generation of 15 MW for 30 years, and have little effect on the thermal conditions of the hot spring.

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