The extraction of geothermal energy from HDR (hot dry rock) in the earth's crust has received wide attention. An abundant amount of geothermal energy could be recovered from HDR by circulating fluid through crack-like reservoirs which are created by a hydraulic fracturing technique. During the extraction of geothermal energy, the surface of the crack-like reservoirs are cooled by the fluid and the thermal contraction of the rock occurs. As a consequence, secondary cracks may initiate from the crack-like reservoir perpendicular to the reservoir surface. In order to discuss the initiation of the secondary cracks, the present paper is concerned with the stresses around the reservoir including thermal stresses on the basis of theory of quasi-static thermoelasticity. The surrounding rock is assumed to be a continuum which is homogeneous and isotropic with respect to thermal and elastic constants. The singular equations were derived from the boundary conditions and they were solved by means of the combination of the collocation method and the inversion formula. The thermal stresses around the reservoir were shown in graphs and the initiation of the secondary cracks was discussed.

View full abstract

The electrical resistivity of drilled core from the Hokuroku district, northern part of Akita Prefecture, were measured by the Four-Electrode method. The measurements were carried out under the condition that the sample had been super-saturated with water.The electrical resistivities of the rocks show such characteristics as follows;1) A close correlation is found between resistivity and water content.2) The relation between resistivity ρ (ohm cm) and water content X (weight %) is given by a formula log ρ=A-BX, were A and B are the constants related to rock type respectively and they are very sensitive to the change of rock facies.These fundamental relations may suggest a useful measure to respect of rock body condition.

View full abstract

This paper proposes the fracture-matrix system permeability for the simulation study of a fractured reservoir. This permeability can be incorporated into a developed numerical model for a conventional reservoir to deal with fluid flow in a fractured reservoir approximately. The merits and demerits of this model are also compared with those of other two models developed for a fractured reservoir.

View full abstract

This paper presents some results obtained by a history matching in the Takinoue geothermal field. The numerical model, used as a simulator, is a three dimensional, two phase model, into which the concept of fracture-matrix system permeability is incorporated so as to deal with fractures in the reservoir. Acceptable matches on pressure and temperature performances were obtained, by taking an equivalent width of fractures as a matching parameter.

View full abstract

Since FY 1980 the hydraulic fracturing experiment has been operated at the Ekakedani site near Mt. Yakedake, Gifu, as a part of the Sunshine Project. In the two series of pumping-water-experiment in FY 1981 and FY 1982, an estimate is carried out for induction and growth of fractures in slate and sandstone rocks by observing variations of water level and temperature in the several wells surrounding an injection well. The results obtained indicate that better response for possible fructuring behaviors is found in the ascending rate of water level in the observation wells rather than in mere variation of water level. Because of some restrictions of acoustic emission sonde operations, less remarkable features are also found on the variation of water temperature.

View full abstract

A research program was carried out to develop mist-cooled condensers for binary-cycle power plants using organic fluids of low boiling points as the working fluid. In order to improve mist-cooling heat transfer performance on the outside surface of tubes, the surface configurations which extend the water film on tubes were developed. The heat transfer coefficient of wavy grooved finned tubes increases by almost 30 % from that of a conventional smooth finned tube. In-tube condensation heat transfer coefficient of internall-ygrooved tubes was found to be about 50 % higher than that of a smooth tube. A computer program was developed to determine the heat transfer performance of mist-cooled condensers and optimization analysis was conducted. The optimum values of the geometry factors and the operating conditions under constraints of manufacturing process were found for a 10 MW binary-cycle power plant. A tube bundle which serves as the minimum unit in the field operation was manufactured and the condensation performance was measured. The experimental data and the predictions by the computer program agree to within 10 %. It is proved that the computer program serves well to the task of designing optimum mist-cooled condensers in response to different design requirements.

View full abstract