The authors have been investigating a history of the physico-chemical condition of the Ohnuma geothermal field during 20 years since the start of operation. Concentration of CI and dilution of tracer, I, are characteristic in its history. Several geophysical parameters of the geothermal reservoir are estimated by simulating it, based on a lumped parameter model, to fit to the observed chemical data. Equations required for this model are energy and mass balances. In this model, a part of reinjected hot water and fluid from the exterior of the reservoir are mixed with the geothermal fluid in the reservoir, and the reinjected water is heated up during its residence in the reservoir by heat extraction from the ambient hot rock. The results of estimation are summarized as follows; 1) Mean residence time of fluid in the reservoir is approximately 1 year. 2) Effective porosity of the reservoir is about 1%. 3) 75% of the reinjected water is discharged again through production well. 4) Heat extraction from the reservoir rock to fluid is effective to reach 60% of rock mass. 5) Temperature of water reinjected to the reservoir rises more than 100°C during residence in the reservoir. 6) Average reservoir permeability is approximately 0.069 kg/(s.kPa) as in-flow index.
Some geothermal fields can be effectively developed by high angle drilling. This method can minimize the number of drill sites and consequently help minimizing the environmental impact for construction of geothermal power plants. The high angle drilling method which uses MWD and High Torque Down Hole Motor, has become widespread for oil field development in Japan. We have applied the method to drill and complete two wells at Takigami geothermal field in Oita Prefecture. These two wells with maximum hole angles of 61° and 56°, were accurately controlled. One of these wells succeed to turn the hole azimuth 40° while it was keeping the maximum hole angle 56°. By analyzing borehole data while drilling to total depth, it is concluded to be very important to remove cuttings completely to the surface. Moreover, the total well cost of these wells was dramatically cut by shortening the drilling program due to effective utilization of this method and by increasing penetration rates.
A self-potential (SP) survey was carried out in Aogashima volcano in Izu-Ogasawara arc, which erupted about 200 years ago from the center of Ikenosawa crater. A terrain related SP distribution of about -1.2 mV per meter of elevation was observed. In the crater, positive SP anomalies were detected over western half of Maruyama pyroclastic cone and along the inner side of the crater rim. Negative SP anomalies were found over the eastern half of the cone and in the central area north of the cone. A post-processor to calculate electrokinetic potentials from histories of underground conditions (temperature, pressure, salinity, flow rate etc.) computed by geothermal reservoir simulation was applied to two-dimensional models of the volcano. The main features of the SP distribution were reasonably well reproduced in the models. The terrain related SP distribution is reproduced as caused by downflows of meteoric water. If impermeable structure beneath the crater rim interrupts the meteoric water downflow, a positive SP is produced along the inner side of the crater rim. Beneath the western half of the cone, continuous heat supply from the deep magma is required to keep the high temperature upflows at present, which also produce positive SP anomalies.
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