This paper describes a new interpretation for relative permeabilities obtained by the experimental data already reported. This estimation of apparent relative permeabilities is based on a pre-determined distribution function of saturation. The conclusions are as follows:(1) The relations between saturation and the apparent relative permeabilities of wetting and non-wetting phases depend not only on the average of saturation, but also on the standard deviation.(2) The apparent relative permeabilities are conspicuously larger than those calculated by the arithmetical average value of saturation, when the standard deviation of saturation is larger than about 0.2.(3) When distribution functions of saturation have single peaks and their average and standard deviation are fixed, the apparent relative permeabilities based on those distribution functions have almost the same value.(4) The distribution function with two or more peaks may yield apparent relative permeabilities lager than those obtained using the function with a single peak.(5) If Corey type equations are used to describe relative permeability curves, nonlinear relations between the apparent relative permeabilities and saturation a re obtained in all cases.
Two new heat flow values were determined at Houfu in Yamaguchi Prefecture, and at Kunisaki in Oita Prefecture, Southwest Japan. These two heat flow stations are located in Palaeogene and Cretaceous granitic rock regions. We determined a relatively low heat flow value of 39.4±8.1 mW/m2 at Houfu and a relatively high value of 79.1±3.5 mW/m2 at Kunisaki. The latter value is anomalously high in comparison with the geologic are-heat flow dependency (Polyak and Smirnov, 1968), suggesting a possible deep magmatic activity in recent ages, although other explanations are possible. Quantitative analyses are necessary to clarify the origins of such high heat flow observed in the old granitic region.
A three-dimensional steady state liquid phase simulator for hydrothermal systems was developed. It is called KYU88-1. In this simulator, thermal water flow through a porous medium is assumed to obey Darcy's law. The governing equations are approximated by the finite difference equations and calculated by the successive overrelaxation method. Using this simulator, a base model of a real field (the Takenoyu-Sugawara geothermal area in the middle Kyushu, Japan) is constructed. From this model, it is inferred that the hydrothermal system of this area extends to more than -3000 m above sea level and the Takenoyu fault, along which Takenoyu and Hagenoyu hot springs appear, has high permeability at the shallower part but not so high permeability at the deeper part. Moreover, a low resistivity zone near the well DW-7 detected by ELF-MT surveys is explained by this model, and the low resistivity zone is regarded as not a well fractured high permeable zone but a low permeable alteration zone which acts as a barrier to the hot water flow from eastern side.
An abundant amount of geothermal energy could be recovered from hot dry rocks by circulating fluid through geothermal cracks created by a hydraulic fracturing technique. During the extraction of heat, the surface of geothermal cracks are cooled by circulating fluid and the thermal contraction of the rocks occurs. Therefore, to control the geothermal cracks, it is necessary to make a fracture mechanics study including thermoelastic effects. In this paper, thermoelastic analysis is made for an artificial geothermal crack in a hot dry rock. Discussion is focused on the behavior of an artificial geothermal crack during extraction of heat. First, we concern ourselves with the effect of the initial rock temperature gradient on the fluid temperature at the outlet and the behavior of the geothermal crack. Next, the effect of the change of injection flow rate is considered. Finally, the time limit for the stable geothermal crack is discussed. The results may be useful to achieve stable control of geothermal cracks in hot dry rocks.
In order to fully utilize the geothermal resources which abound in Japan, technology for the exploitation of such resources as low productivity geothermal reservoirs, super hot formations adjacent to magma bodies and magma itself should be developed. The downhole coaxial heat exchanger (DCHE) system is thought to be the most suitable for the exploitation of these geothermal resources. The authors have carried out a preliminary study on power generation using a DCHE system. The purpose of this study was to clarify the thermal output levels per DCHE or the effective thermal conductivity of the formation, at which the power generation system becomes economical. In addition, we wanted to clarify the geothermal resources favorable for this system. The following conclusions have been drawn in this paper: (1) For specified geothermal conditions, there are optimum operation conditions, e.g. inlet water temperature and flow rates, which give maximum net power output, in the case of power generation using the DCHE system. (2) when used with the DCHE system, the two-phase turbine type power generation system is superior in cost performance to the multi-flush type power generation system under the conditions specified in this study. (3) The net thermal output levels of the DCHE under optimum operation conditions were estimated for the four assumed effective thermal conductivities of the formation. The estimated net thermal output levels after 15 years of power generation were 1.4, 4.6, 9.0 and 13.0 MW/DCHE for the effective thermal conductivities of 2.7, 10, 20 and 30 kcal/mh°C, respectively.
Two massive hydraulic fracturing experiments were conducted in 1983 and 1984 by Los Alamos National Laboratory to create man-made geothermal reservoirs for the Hot Dry Rock geothermal energy development. In the two experiments, water was injected at almost the same depth of about 3, 500 m, but at a distance of about 200 m. In spite of the short distance between the water injection points, there was a great differece in the created fracture progression. In this study we analized AE (Acoustic Emission) observed during both experiments to find the cause of the fracture progression difference. The fault plane solutions of AE showed that there were clear differences in the source mechanisms between the two experiments. The directions of the maximum and minimum stresses in the fault plane solutions in both experiments were almost the same and these directions were consistant with the regional tectonic stress distribution estimated around the experiment site. This suggests that the fracture progression differences might not be caused by the hauge in the stress distoribution with depth. Using the modified Hill's earthquake occurence mouel in rock with cracks, the fracture progresion difference was explained by the effect of the distribution pattern and density of cracks in rock.
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