In order to clarify the hydrothermal structure in the Takinoue Geothermal Area, Iwate Pref., Northeast Honshu, Japan, 415 hydrothermal veins developed in the area are examined. These veins are classified into five systems:NNW-SSE to NW-SE strike and moderate dip system (NW-moderate dip system), NW-SE strike and steep dip system (NW-steep dip system), NE-SW to E-W strike and steep dip system (NE to E-W system) and horizontal system. The latter two fracture systems, which are the youngest, consist of quartz, calcite, aragonite, prehnite, K-feldspar, wairakite, yugawaralite, laumontite, analcime, stilbite, pyrite, marcasite and kaolinite. On the basis of their occurrence and distribution, eight mineral zones are established, that is, prehnite zone, wairakite zone, K-feldspar zone, yugawaralite zone, laumontite zone, stilbite zone, analcime zone and calcite zone.The minerals
In order to evaluate long-term geothermal reservoir performance, it is important to know how permeability of rocks changes with time under hydrothermal environments. This paper presents experimental results of permeability tests performed on two granites under the conditions of temperature up to 350°C and confining pressure up to 45MPa. Two types of spesimen were used for the experiments; a thick-walled cylinder of the intact rock and a thick-walled cylinder with a throughgoing artificial fracture perpendicular to the cylinder axis. The permeability was measured under the hydrothermal conditions or after the specimen was coolled down, by passing water radially through the specimen. The permeability of the intact samples under the hydrothermal conditions increased with time, and the increasing rate strongly depended on test temperture and flow rate of water. From both porosity distributions and the relations between permeability and porosity, it has been revealed that the water flow occured only through the selected microcracks, and that the width of the microcracks increased during the flow of hot water. Thus the increase of the permeability was caused mainly by the widening of the microcracks due to the rock dissolution. Furthermore, when the pore fluid pressure was greater than the confining pressure, as in the case of radial flow from inner wall, stress corrosion cracking caused the extensive growth of the microcracks, which eventually formed fractures along the rift plane of the granite. The permeability of the artificial fracture decreased initially, and then increased, during the flow of hot water through the fracture. From the reduction of the surface roughness (rms. value) by the test, it was infered that the initial decrease of the permeability was caused by the creep fracture of the surface asperities. In addition, SEM observations of the surfaces showed that some channels developed in the surfaces due to the rock dissolution by hot water, which caused the later increase of the permeability.
The pressure drop through model fractures has been studied experimentally. The model fractures are made of two parallel acrylic plates whose dimensions are 2m (length)×0.44m (width), and the created aperture has the dimension of 4mm, which is common to all model fractures. In the midportion of the opening space (lm in length) rectangular obstacles whose cross sections have the dimension of 10mm in length and the three different heights of 1, 2 and 3mm are glued to both upper and lower plates with the interval of 30mm. The obstacles are oriented normal to the flow direction. The water is pumped up from the reservoir and led to the one end of the model fracture. The pressure drops along the flow direction are monitored. Using the streakline visualization technique the nature of water flow in the model fracture, for example laminar or turbulent, is also determined. An equation to predict the friction coefficient for a given Reynolds number, a number of the obstacles and their heights has been proposed; the experimental results are compared well with the theoretical prediction.
Thermoluminescence of quartz in volcanic and pyroclastic rocks from the Kakkonda geothermal area was investigated. Thermoluminescence glow curve of quartz in the Tamagawa Welded Tuffs (reference sample) collected from a point 10km away from the Kakkonda geothermal power plant shows single peak which is in the range between about 280 to 330°C. Approaching the Kakkonda geothermal power plant, low temperature region of glow curve is depleted, and total emission of thermoluminescence decreases. Total emission is no relation to the stratigraphic boundaries. Five percent isopleth of relative emission compared with the total emission of the reference sample is located at a point about 2km away from the Kakkonda geothermal plant in a northwest to southeast direction along the Kakkonda river, and 1 to 2km in a northeast to southwest direction. The area enclosed with the isoplethes of relative emission below 40% is almost overlapped with sericite/ montmorillonite mixed layer mineral zone. These features mean that the paleodose in quartz was decayed by geothermal activity and relative emission could be an indicator to estimate temperature and its duration. These results suggest that thermoluminescence of quartz from geothermal area is applicable to the evaluation of thermal influence and the prediction of the subterranean heat source in geothermal systems.
Recently it has become important to develop a deep, high temperature and low permeable ge-othermal reservoir in the Kakkonda geothermal field. Thus, it is important to develop new techniques for the efficient well stimulation. This paper describes an example in which we succeeded to increase the production of a geother-mal well by hydraulic fracturing using AE (Acoustic Emission) measurement and temperature logging. The depth of the stimulated well is 2126m, to produce steam from high temperature reservoir. In this field, temperature of the reservoir deeper than 1500m is higher than 300°C. Therefore, casing covers the well from the surface to 1700m depth, to avoid low temperature fluid. From 1700m to the bottom, this well is an openhole. The well encountered four fractures that induced lost circulation during drilling in the high tem-perature reservoir. However this lost circulation gradually ceased. Temperature logging after the drilling did not show temperature anomaly between 1700m and the bottom. Therefore, permeability of the well was estimated to be low. To increase the permeability of the well, hydraulic fracturing was carried out using water for five days. Pressure brake-down occurred on the 2nd day of the fracturing, and AE was observed. On the 3rd day of the fracturing, temperature logging indicated that the injection point created by the fracturing was at the depth of 1798m. Then, we tried to increase permeability of the fracture by increasing the pumping rate. On the 5th day of the fracturing, as AE's activity and wellhead pres-sure became low, then we stopped pumping and finished the hydraulic fracturing job Finally, we could get a steam amount of 50t/h from the well. This course of fracture treatment suggests that the AE measurement and the temperature logging are useful to control and optimize the hydraulic fracturing. These techniques also give us information for exploration of fractures because they disclose fracture distribution around wells.
We describe a downhole AE measurement performed at Ogachi Hot Dry Rock field in Akita prefecture during a hydraulic fracturing. Two types of AE signals, i.e., high frequency events and low frequency events containing of both P-and S-waves, were measured. Kaiser effect was clearly observed when total flow exceeded that of a previous experiment. The AE sources located by the triaxial hologram analysis show dynamic extention of the created subsurface fracture. Moreover, precise AE source location of AE doublets reveals a subsurface fracture plane on which the propaga-tion of the AE sources are concentrated. Differences of distribution of the AE sources and the AE activity between the two AE types suggest that two types of the subsurface extention occured during the hydraulic fracturing. The AE source location employing the downhole AE measurement well agrees with a result located by a microseismic surface network (which monitored AE in a following large scale hydraulic fracturing.) The high frequency events were too small to detect by the surface network.
The evaluation of extent of geothermal reservoirs is very important to develop geothermal energy extraction systems. Tracer tests are one of the most useful ways to evaluate geothermal reservoirs. In this study, the models of underground fractures were made in the laboratory, and we performed tracer tests by using the models. Also, numerical simulation on tracer response curves was carried out. Good agreement was found between the test data and the numerical simulation. On the basis of the results obtained, a new method to identify the extent of underground fractures by use of tracer response has been proposed.
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