Small-scale hydraulic fracturing was conducted in the laboratory to create a tensile fracture parallel to bedding plane in a block of three marbles with different mean grain size, ranging from 0.093mm to 2.55mm. To determine the initial aperture distributions of the fracture, height distributions of a pair of surfaces of the fracture were measured along matched paths from a common reference surface with a profllometer which had a stylus tip of 0.025mm in radius. Spectral and statistical analyses were carried out for surface profiles and the initial aperture distributions of the fracture to correlate geometrical characteristics of the fracture to grain size of calcite.
Main results obtained in this study are summarized as follows:
1) The height of the surface profiles was fractal in the range of this study. The fractal dimension determined from power spectral density ranged from 1.392 to 1.425. The fractal dimension slightly increased with the mean grain size of marble.
2) The initial aperture distributions created by the two fractal surfaces were not fractal because the power spectral density was flattened in wavelengths greater than the mismatch wavelength of each marble.
3) The roughness of surface profile, the mean value of the initial aperture, the standard deviation of the initial aperture and the mismatch wavelength increased with the mean grain size of marble.

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A method of calculation of heat loss from a production well to the surrounding formation has been developed and implemented into a wellbore flow simulator WELCARD-III which is a new version of WELCARD-II (Takahashi et a1., 1998). A comparison between computational results using WELCARD-III and the measured values of borehole temperature and pressure during production in the Kakkonda and the Mori geothermal fields in Japan was carried out and the computational error was within 5%, including cases with dissolved CO₂ gas. The influence of parameters such as heat loss, temperature and pressure at the feed point and total mass flow rate on appearance depth of annular-mist flow was investigated. The summary of the results are as follows:
1) As heat loss becomes larger, the flashing point and the depth at which annular-mist flow appears become shallower. This is due to the decrease in fluid temperature.
2) As feed point temperature becomes higher, the flashing point and the depth at which annular-mist flow appears become deeper. The distance, furthermore, between the flashing point and the appearance depth of annular-mist flow becomes longer. This is due to the increase in saturated pressure.
3) The feed point pressure shows little influence on the depth of annular-mist flow appearance.
4) As total mass flow rate becomes larger, the flashing point and the depth at which annular-mist flow appears become deeper. The distance, however, between the flashing point and the depth at which annular-mist flow occurs becomes shorter. This is due to the increase in pressure loss.

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In order to evaluate the temperature conditions of geothermal wells and surface manifestations, geothermal index (GI) is proposed here. GI is given by the equation of GI = kRAI, where RAI is the revised activity index and k is a coefficient calculated by the highest temperature found in a well. Japanese steam production wells have GI values higher than 35, whereas non-productive ones in the Mizuwake-toge geothermal area, Kyushu, have those smaller than 40. Types of the depth-temperature curves of the Mizuwake-toge wells are also discussed in relation to GI: the upflow types are higher than 20, the heat conduction types are smaller than 20, and the down flow types are smaller than 5. Accordingly. this GI makes it possible to compare the comprehensive thermal conditions for shallow to deep wells as well as for surface manifestations.

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