This paper presents possible algorithm which can be used for a conventional geothermal well logging interpretation. This algorithm consists of the basic equations in formation evaluation, empirical equations often used in oil reservoir evaluation, and equations derived from double-porosity model theory. To show the validity of this algorithm, sample calculations arepresented in this paper. The total porosity distributions and the distributions of the fraction of the pore volume made up of fractures to the total pore volume of the system are obtained. The fractured zones estimated from these results agree approximately with the fractured zones estimated from drilling charts. This algorithm may be one of effective methods for the estimation of porosity distributions in a fractured geothermal reservoir.
The groundwater motion under a geothermal condition is driven by the potential force originated from the water table gradient and by the buoyancy originated from the temperature distribution. Numerical experiments suggest that three types of groundwater flow system are possible; (1) the system in which potential and convective flows are balanced each other, (2) the system in which convective elements are transported to downstream by the potential flow, and (3) the system in which the potential flow is predominant. When the convective effect is relatively significant, such as systems of (1) and (2), some local flow systems are formed in the aquifer, and recharge and discharge areas of groundwater appear alternately along the surface of the aquifer. Thermal convection has an effect to narrow the area of heat distribution near by the heat source, while the potential flow can distribute heat over a wide area to downstream far from the heat source.
An observation of telluric current was carried out on 27 November 1978 in the Kurobe Sennindani hot dry rock, utilizing the Kurobe Jobu railway tunnel known as a hot tunnel penetrating through the hot dry rock. The results obtained are as follows: (1) Observed variations of telluric current do not show any characteristics of natural origins but of artificial electric noises or stray current originating probably from some electric power plants within 1.5 km distance and DC-electric cars operating at about 4 km away. (2) Variations with frequency of 4.4 Hz in earth current were found at a low temperature zone (about 15°C), but it disappeared at hot a rock zone (about 70°C). It may follow that earth current of 4.4 Hz-frequency is unable to penetrate into the hot rock zone due to skin effect. Taking 300 to 500 m for the depth of the hot rock zone below the ground, 2 Ω-m of low resistivity of the hot rock zone is obtained by Cag niard equation. The result was discussed, referring to rock characteristics of the high temperature zone. (3) Irregular variations of earth current were in opposite directions at the low and high temperature zones. (4) Long period currents by streaming potential were observed at the hot rock zone. It is estimated that streaming potential was induced by warm and humid wind forming from hot rock walls and fumaroles.