To avoid silica scaling, three different pH conditions (5.0, 6.5 and 7.3) were examined by passing Sumikawa geothermal fluids (SiO2 concentration ; 775 mg/L) through columns (50 mmφ, 55 cm in length) packed with alumina beads (1 or 2 mmφ). The flow rate decreased from 5 to 2∼3 L/min with time (within 72 hours) in all pH conditions. Silica was found to deposit mainly around the top of the column (within 20 cm from the top), where Al concentration in the silica deposits increased with time. The total amount of silica deposited during experiments increased drastically with increasing amount of fluid after some induction periods where no significant amount of silica was deposited. In the most acidic condition (pH 5.0), silica deposition was also observed instead of an iron material such as SS400 was corroded. These results indicate that around pH 6.5 is a better condition to avoid silica scaling instead of a small amount of silica is still deposited for fluids with high SiO2 concentration (>700 mg/L).
Kuju Volcano in central Kyushu, Japan has an active fumarolic field in the central part. Resistivity measurement was applied in order to detect the steam reservoir beneath the fumarolic field. As the result, an extremely resistive zone higher than 1000Ωm was detected at a depth of 10 to 50 m. A short directional hole (27 m long and 18m deep below the surface) was dug toward the resistive zone in May 1991. We obtained superheated steam from the margin of the resistive zone and then used it for material testing. The temperature of the steam was 233°C at the well head and was estimated to be 278°C at the bottom of the hole. In November 2001, we planned to get much hotter steam in order to conduct another experiment. Then a longer directional hole (47 m long and 30 m deep below the surface) was dug toward the center of the above-mentioned resistive zone. However, we obtained only saturated steam and the temperature at the well head was about 98°C. The saturated steam was discharged for about one month but stopped discharging because of deposition of sulfur inside the pipe. Repeat resistivity measurement was conducted in order to clarify the change in the thermal state in September 2002. The resistivity model showed that the resistivity of the above mentioned resistive zone decreased to several tens Ω m, that is, the resistive zone has disappeared. Such a drastic change would be explained if we assume that the superheated steam reservoir turned to be the liquid dominated reservoir, that is, the change in resistivity means cooling of the reservoir. The cold meteoric groundwater around the fumarolic field may be supplied to the steam reservoir. Phreatic eruptions occurred at about 300 m south of the active fumarolic field in October and December 1995. Geophysical monitoring after the eruption shows quick cooling of the central part of the volcano and also indicates a large amount of meteoric water supply to the central part of the volcano. The change in resistivity observed in this study may be one of the phenomena associated with the 1995 Phreatic eruptions.
X-ray powder examination on the alteration of the drill hole cuttings revealed a mineralogical zoning of kaolinite -α-cristobalite (I) -, zeolite (II) - and kaolinite (III) - zones toward the bottom of the well MT-1 in the Mataloko geothermal field, central Flores, eastern Indonesia. Zone II is further subdivided into the upper heulandite (ha) -, middle laumontite (IIb) - and lower wairakite (IIc) - subzones in descending order. A temperature profile in the crystallization process deduced from the phase equilibria of zeolite minerals and the kaolinite crystallinity index dramatically increases toward the deeper part exceeding the hydrostatic boiling-point curve of pure water and reaching 236°C at a depth of around 180m. This depth is ascribed to the maximum enthalpy point on the two-phase region of water, dividing the deeper vapor-dominated reservoir and shallower vapor condensation zone. The vapor condensation zone generally is 360m thick, but in the Mataloko area it was only half as thick, owing to lithostatic pressure. Abundant clay and zeolite minerals in Zone II might have formed an extremely impermeable zone, and the continuing excess heat supply might have generated a lithostatically-pressurized clay cap. The lithostatic pressure has since been released.
In order to evaluate the applicability of sodium fluorescein as geothermal tracer, in addition to the effect of pH on the fluorescence intensity of sodium fluorescein, the time dependence and the effects of dissolved ions on fluorescence intensity were examined. The fluorescence intensity decreased with decreasing pH when the pH of samples was lower than 9. On the other hand, the intensity was showed stable value when the pH was higher than 9. Therefore, for the samples without dissolved particular chemicals, the fluorescein concentration can be certainly measured when the pH of samples is adjusted to over 9 before measured using a fluorescence spectrophotometer. The fluorescein in the transparent and the brown glass bottles were easily decomposed under lights of . fluorescent lamps. On the other hand, the fluorescein in the bottle covered with aluminum foil was hardly decomposed during 1 month. As a result of examining the effects of dissolved ions on the fluorescence intensity, the effects of potassium, calcium, chloride, sulfuric acid or carbonate ion hardly could be observed. On aluminum ion, it was proven that the fluorescence intensity was not almost affected in the aluminum ion concentration of the degree that is also included for general geothermal brine (about 1 mg/l). Though the magnesium ion affects fluorescence intensity, adjusting pH over 12 could ease the effect. As the ferrous ion concentration is high, the accurate measurement becomes very much the difficulty since the fluorescence intensity seems to lower by the adsorption of the fluorescein on hydroxide as similar form of colloidal.Keywords : sodium fluorescein, fluorescent tracer, fluorescence intensity, peak wavelength, pH effect, salt effect
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