In order to study relationship between resistivity structure and reservoir structure of the Sengan geothermal field, especially beneath the Yakeyama and Hachimantai area, two-dimensional analysis using the finite-element method has been applied to the data of Schlumberger soundings of several survey lines obtained by NEDO (New Energy and Industrial Technology Development Organization) and other agencies since 1974. I examined correlation between the resistivity models and other survey results, such as logging data of test wells. Although most of the sounding curves essentially show threelayer structure of which resistivity variation is resistive-conductive-resistive from the surface, four-layer models with resistivities of resistive-conductive-medium-resistive are applied in the 2-D analysis referring to other survey results. Deep test well, SN-7D, drilled at the final stage of the NEDO's projeet in 1987, encountered several large lost-circulations, Succeeding well tests have concluded that they are potential fractures which can produce great amount of geothermal fluid. According to the geologic column of the well, fracture dominated reservoir is located in tuffaceous layers of the Miocene Aniai Formation and Tertiary intrusive rocks. Average resistivity values of those layers are 100Ω·m and 1, 000Ω·m, respectively. They correspond to the third layer and the resistive basement of the resistivity model of the survey line SGN-85B. I Cap rocks are Quaternary lake deposits and the Upper Aniai Formation. They are extremely conductive with the resistivity of less than 5Ω· m, and are obviously analyzed as the second layer of the 2-D model. Dominated clay alteration minerals are montmorillonite for the cap rocks, and chlorite and sericite for the reservoir formation. Since montmorillonite is formed at low temperature environment at borders of geothermal water circulation and it has very low resistivity and low permeability when it contains plenty of water, those layers themselves form electrically conductive cap rocks. On the other hand, high temperature-type clay minerals such as chlorite and sericite seem not to decrease resistivity of host rocks and not to seal fractures. SN-7D is located where the lower boundary of the conductive layer becomes shallow and the temperature distribution shows high anomaly. This is also consistent with distribution of alteration minerals, such as montmorillonite, chlorite and wairakite.
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.
Kuju-iwoyama, the most active fumarolic area in central Kyushu, Japan, is situated at an explosive crater of Kuj u volcano. The natural heat discharge is estimated at about 100 MW and most of it (more than 95%) are from steaming grounds and fumaroles. The temperatures of fumaroles exceed 200°C and the maximum temperature is 480°C. Based on a magmatic steam-meteoric water mixing model, thermal process beneath the active fumarolic area was simulated by computer modelling. The model obtained shows the following features: high temperature magmatic steam (about 580°C and 30kg/sec) is supplied from a depth lower than 2 km. The magmatic steam mixes with the meteoric water (about 10kg/sec) in the high permeable zone (0 to 2 km depth) just beneaththe fumarolic area. The permeable zone is in a state of two-phase. A production process was simulated by extracting the fluid from the shallow part (250-500 m depth). As a result, it was concluded that the heat extraction comparable to 10 MW power generation for more than 10 years is possible without greatly affecting natural geothermal activities. The development of the two-phase reservoir beneath the active fumarolic area is a possible method to utilize volcano energy in the near future.
A geological study on the “Young Volcanoes” in the Matsukawa-Kakkonda area, northeast Japan was carried out to evaluate the heat source in this area. The products from the “Young volcanoes” are divided into the Early stage volcanics (erupted in Matsuyama reversed epoch or older epoch) and the Late stage volcanics (erupted in Brunhes normal epoch) by accumulated paleomagnetie and K-Ar age data in the Sengan field including the studied area. Matsukawa Andesites, Obukadake, Iwate-Ojiromori and Omatsukurayama volcanoes belong to the Early stage volca.nics; and Mitsuishiyama, Iwate, Nyutozan and probably Kurikigahara volcanoes belong to the Late stage volcanics. The results of the K-Ar age determination in this study are as follows: Matsukawa Andesites 1.67±0.12, 1.39±0.08 and 1.29±0.15 Ma, Obukadake volcano 1.57±0.06 Ma, Omatsukurayama volcano 1.07±0.50 Ma, Mitsuishiyama volcano 0.46±0.05 Ma. The estimated volume of products from each volcanoes in cubic kilometers are as follows: Matsukawa Andesites 15.3, Iwate-Ojirornori 0.02, Obukadake 3.8, Omatsukuravama 1.9, Kurikigahara 0.2, and Mitsuishiyama 0.5. It is difficult to evaluate precisely the volume of the magma reservoir from the volume of the eruptive material. In this paper, the youngest eruption age and the total volume of the erupted material for each volcano were plotted on the Smith and Shaw (1975) diagram to compare the relative present temperature of each magma reservoir. The evaluated temperature of the magma reservoir of the Matsukawa Andesites is higher than that of Mitsuishiyama volcano, which shows the youngest K-Ar age data. Futherrnore, the estimated present temperature of the magma reservoir which erupted the Tamagawa Welded Tuffs, 2 Ma and 1 Ma large scale felsic pyroclastic flow deposits, is much higher than those of all andesite volcanoes in the studied area. So it is difficult to restrict the heat source of each geothermal reservoir to each magma reservoir which erupted the andesite volcano in this area. The heat source is considered to be the accumulated compound heat energy from the magma reservoirs in the recent 3 million years including those of Tamagawa Welded Tuffs.
Transient heat transfer from a cylindrical object placed in a saturated porous layer, which is simulating a heat-generating probe vertically placed in an aquifer, has been studied both analytically and numerically. When constant heat flux is specified on the eylinder surface, it is shown that the surface temperature rise with time and the elapsed time to reach steady state can determine the effective thermal conductivity of the saturated porous layer, as well as the groundwater velocity running through it. A present study reveals two distinctive regimes existing during the transient process. For small times, the heat transfer is dominated by conduction, while for large times conduction is balanced with convection. Recognizing these characteristics enables us to develop approximate solutions valid for respective regimes. Complete two dimensional solutions are generated numerically in order to demonstrate the accuracy of the approximate solutions. An application to groundwater velocimetry has been suggested.
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