In order to evaluate the effects of water/rock interaction upon a long term reservoir performance in HDR, it is essential to understand the mechanisms and kinetics of dissolution and precipitation of rocks in high temperature water. In this work, these dissolution and scaling kinetics of granite in pressurized high temperature water at 200°C-300°C is investigated experimentally by use of a flow system with once-through type autoclave. Special emphasis is focused upon the effects of temperature upon dissolution rate of minerals composing granite, say quartz, K-feldspar and plagioclase in relation to leaching behavior of elements. Results show that, at a temperature range of 200°C to 300°C, quartz is most stable and shows the lowest dissolution rate. However, at temperature range above 300°C, quartz has the highest dissolution rate and those of K-feldspar and plagioclase show a negative temperature dependence above 300°C. SEM and EMPA analysis reveals that this trend results from the formation of fibrous Xonotlite (Ca6 Si6O17(OH)2) on the surface of K-feldspar and plagioclase. This Xonotlite can be expected as a diffusion barrier and result in rapid dicrease in dissolution rate. On the other hand, at the lower temperature range with rapid temperature down by cooling, amorphous SiO2 is found in a test tube as has been found in actual geothermal plant pipings. Based upon above evidences, scaling behavior can be classified into two categories. One is scaling at high temperature, so called high temperature scaling and the other is one at low temperature or with temperature dicrease. They can be considered as a kind of scaling observed at actual geothermal plant. The reproduction of these scale would be expected to give more effective countermeasure against the scaling problem in existing or future geothermal plant.
The October 1983 eruption of Miyake-jima, about 180 km south of Tokyo, left aa lava flow of 5-lOm thick, which covered the Ako district. The thickness of the upper clinker part of the lava flow, in which air and water circulated easily, ranges from 0.5 m to more than 3 m. Temperatures in the lava flow were monitored by means of thermocouples in steel pipes inserted in the upper clinker part starting 50 days after the eruption and in a drill hole, Miyake-GS-DH-1, penetrating the flow starting 116 days after the eruption. Both monitorings were continued until 3 years later. One year after the eruption, the maximum observed temperature in the pipe holes was 403°C. 81/2 months later, it fell to less than 100°C, Rainfall, which ranged from 2.4 to 3.0 m/year, greately increased the rate of cooling of the lava flow. But the degree of cooling effect by water varied because of the hydrothermal convection system. There are two convection systems in the lava. One is in the narrow cooling joint, generated largely by thermal contraction in the massive part of the lava, and the other is in the upper clinker part just above the previous one. Three dimensional temperature distribution in the latter one is observed directly through pipe holes. The convection system was mantled by the 100°C water saturated zone. The surface of the massive part near the cooling joint wass not affected by the cooling water while it was roofed by the 100°C zone. At the site of Miyake-GS-DH-1, a 5.5 m thick lava flow, which was divided into upper clinker (2.2 m), massive (2.5 m) and lower clinker part (0.8 m), covered the previous ground with 1 m vacant space between them. The maximum temperature was found in the massive part just after the starting of the measurement. And then it fell down to 0.3 m depth in the previous ground at 71/2 months after the eruption and to 1 m depth, 2 months later.
Silica deposition from the high salinity geothermal water of the Fushime area in Kagoshima prefecture occurs within a relatively short time. Therefore, the effect of sodium chloride concentration on the polymerization of silicic acid and the precipitation of silica was studied by spectrophotometry and gel chromatography at 25 and 90°C. The concentration range of sodium chloride was 0.05-2.5 mol dm-3, and the pH range 6-9. At a given pH, the rate of decrease in the monosilicic acid concentration increased by increasing the sodium chloride concentration. It results from an increase in the supersaturated concentration of monosilicic acld due to a decrease in the solubility of amorphous silica. The precipitation rate of silica also increased by increasing the sodium chloride concentration, because the reaction between polysilicic acids was accelerated due to the decrease in electrostatic repulsion among polysilicic acids.
Hydraulic fracturing was successfully applied to a well stimulation for improvement of productivity of the KD-3 production well in the Kakkonda geothermal field. During the stimulation, AE events were localized in the main fracture zone, namely the production zone, and extended downward and upward in the fracture zone. The tonalite intrusive body obstructed the forward extension of fractures. The decrease of AE activity indicates that the extension of fractures has been completed during the stimulation. After stopping the pump, AE occurred intermittently for five hours. This AE activity is considered as an after effect of the hydraulic fracturing and shows a secondary build up caused by shut-in of the opened fractures.
Thermal properties of soil have been greatly concerned in the various fields of science and engineering. Geothermal energy development technology, also, requires detailed information on the dependence of thermal properties of surface soil on depth to evaluate geothermal potential exactly by measuring surface soil heat flow at an early stage of the development activities. Generally, thermal properties of soil have been measured in laboratory with samples gathered from fields. But its values are different from that of in situ, because the samples don't have same soil structure, and temperature and moisture conditions in situ owing to drilling and open air. For these reasons, the authors developed, by modifying the unsteady hot wire method, a field apparatus to measure local temperatures and thermal conductivities at seven points within one meter depth of perpendicular direction from soil surface in situ. The system consists of a prototype needle probe having 26 mm diameter and 1310 mm effective length, a constant power supplier, digital data recorders and a microcomputer system, etc. Consideration has been given to errors introduced by modification from the ordinary hot wire method theoretically and experimentally. The system proved to be able to measure the local thermal conductivvities of surface sod in situ with accuracy within ±8.5 % on the average for the values of the thermal conductivity from about 0.3 to 2.5 W/mK.
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