Assessment of Geothermal Resources of the Hakkoda Geothermal Area

Abstract

For an assessment of the Hakkoda geothermal area, we gathered temperature, structural, and water discharge data. The temperature data estimated from 99 boreholes are expressed as the temperature gradient map. The subsurface structure is divided into four formations of lava dominant formation, pyroclastic and sedimentary dominant formation, intrusive rock dominant formation, and pre-Tertiary dominant formation. The three-dimensional distribution of each formation is constructed by the modeling of MT, gravity, and aeromagnetic survey data. The water discharge model estimated by the lost circulation data of 16 boreholes is expressed as a probability density function. Water discharge expresses water quantity at a certain temperature per time unit from one borehole, so that electric power, defined as energy per time unit, can be given . by the water discharge multiplied by the temperature of water. The electric power of two systems of high-temperature (higher than 150°C) and of intermediate-temperature (between 15°C and 90°C) is estimated through the Monte Carlo method and given by the cumulative probability curve and the mean or expectation values. The final result is the expectation value map of the electric power extracted from one borehole ranging from the surface to a 3km depth. The map of high temperature systems indicates that high potential areas, higher than the expectation value of 2, 500 kW, occur in and around the Hakkoda volcanoes, the Okiura caldera, the Shimoyu spa, and the Ikarigaseki caldera. The total electric power of this area will be calculated through the sum of the expectation values. Assuming one bore-hole per square kilometer to avoid any interference between two boreholes, the total expectation value of electric power of the area higher than 2500 kW is about 330 thousands kW. The expectation value is expected for the early stages of the recovery and does not ensure that the rate of water discharge will remain constant for an extended period. The life of reservoir, however, will be ensured by a more detailed survey, not by the data presented here. The result will be criticized after new data are collected and new experiments are made. But the model is constructed by quantitative values and the algorithm is a simple equation, so that it will be easy to change the model and the algorithm and to revise the result again.