This paper describes a calculation method of apparent permeability by using the Monte Carlo Method and its application to a geothermal reservoir. The theoretical results are as follows;(1) The apparent permeability depends on the standard deviation of permeability distribution and not on the difference of distribution functions.(2) The arithmetical average can be used as the representative value, when the standard deviation is less than 0.2.(3) The arithmetical average cannot be used as the representative value of permeability, when the standard deviation is larger than 0.2. The apparent permeablity decreases and approaches zero, as the standard deviation increases. In this case, it is necessary that the arithmetical average must be corrected as the representative value using the standard deviation.(4) When the Prandtl number is larger than 10, the arithmetical average can be used as the representative value of permeability. But when it is less than 10, the arithmetical average must be corrected as the representative value using the standard deviation.
Measurements of seismic noise have been made in the Goshogake geothermal area, northern Japan, in order to study the nature of the waves originated from geothermal activities. Two kinds of the geothermal seismic noise were detected from the amplitude and power spectral analyses: One is that contains high frequency component from 20 to 45Hz with large amplitude. The origin of the noise is surface geothermal activities. Another is that contains rather low frequency component near 10Hz with moderate amplitude. The noise may originate from boiling of hot water below the surface. Generally, the amplitudes and predominant frequencies obtained in the western part of this area in 1982 decreased as compared with those obtained in 1981. This fact is coinsident with the westward migration of geothermal activities in this area. It is also useful to observe the geothermal seismic noise for monitoring geothermal activities.
The strike and dip of fractures such as slickensides, hydrothermal veins and joints, found in drillcores from the Otake-Hatchobaru geothermal area have been estimated after measuring the thermo-remant magnetism of the cores with an astatic magnetmeter. The results show that most of the slickensides and veins dip southward, though their strikes vary considerably from NW-SE to NE-SW. In addition, the analysis of slickensides has revealed that the NW-SE trending faults basically are of left-handed strike slip type, while the NE-SW ones are of light-handed one. It is inferred that both kinds of faults were originally formed by the ENE-WSW compressional stress fields whose minimum stress axis dip NNW with moderate angles. Recently, the normal stress fields, whose intermediate stress axis tends NW-SE, have probably made the NW-SE trending fault planes open and permeable so that they have become the present breeding faults.
Homogenization temperatures of fluid inclusions found in secondary hydrothermal minerals such as quartz, anhydrite and subordinate calcite from the Otake geothermal field, Kyushu, have been measured. The homogenization temperatures in the reservoir have a relatively narrow range of 15-30°C. Except in the case where inclusions may have trapped a gas phase, most of the highest homogenization temperature are on or slightly below the surface boiling point curve. This suggests the state of the maximum subsurface temperature with an activity index of 100 prior to the explorations. However, the subsurface temperature just before the exploration seems to have been nearly equal to the lowest value of the homogenization temperature at each depth. The temperature of the reservoir is estimated to have been 200-220°C at depths of 300-500m, having an activity index of about 90. For the reasons, it appears that the Otake geothermal system had cooled down about 15-30°C from the highest activity level even prior to the exploration and has since undergone further cooling. The contour map of the subsurface temperatures forms a slightly deformed mushroom in shape. This is due to the inclination of the main conduit in a south-east direction. The fluid conduit is also supported by the distribution of alteration minerals.
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