To elucidate the effect of polyacrylic acid (PAA) on the formation of silica scales from geothermal water at geothermal power plants, two different experiments were carried out using geothermal water supplied from a production well, TP-2, at the Takigami geothermal power plant in Oita Prefecture, Japan. Polymerization of silicic acid in geothermal water and adsorption of silicic acid and aluminum on the surface of silica gel powder as a model compound of pre-formed silica scale were investigated in the presence and absence of PAA around 90℃. The polymerization of silicic acid which causes changes in monosilicic acid concentration with time and in hydrodynamic size of polysilicic acid particles was not affected by PAA. On the other hand, the adsorption behavior of aluminum was considerably different in cases between the absence and presence of PAA. Only a small amount of aluminum was adsorbed in the absence of PAA, but most of aluminum was adsorbed in the presence of PAA. The amount of silicic acid adsorbed in the presence of PAA was significantly larger than that in the absence of PAA. Consequently, the Al-PAA complex may accelerate the formation of silica scales from geothermal water.
小浜温泉バイナリー発電所において付着したスケール中の鉱物相を組織解析した。小浜温泉の温泉水における炭酸塩スケールとシリカ系スケールの析出傾向を熱力学的に解析した。スケール中の鉱物相と比較した結果，非晶質シリカ系スケールの析出を予測するためには，非晶質シリカだけでなくMg2+ の影響を考慮した非晶質マグネシウムシリケートについても解析する必要があった。一方，炭酸塩スケールの形成にはMg2+ の影響はほとんど見られなかった。その解析手法を用いてスケール付着を抑制する地熱流体の輸送・熱交換の方法について考察し，非晶質マグネシウムシリケートの付着が問題となりうる熱水の化学組成を検討した。イオン強度が0.1-0.2 程度の地熱熱水では，pH が9 以上になると非晶質シリカは溶解傾向にあっても1 ppm オーダーのMg2+ によって非晶質マグネシウムシリケートが析出するリスクがあった。地熱熱水の温度が100˚C 近傍では非晶質マグネシウムシリケートの析出傾向は大きく，75˚C 近傍で熱水運用することにより大幅にそのリスクが低減されることが示唆された。
A criterion to evaluate possible influence of geothermal development onto nearby hot spring aquifer, based on geochemical characteristic of the aquifer, was developed in a form of decision tree. Hydraulic and thermal connection of a geothermal reservoir to nearby hot spring aquifer may be expressed by five types of simplified structure models, which are, identical thermal water-, seepage thermal water-, steam heated-, heat conduction- and independent- systems. Since interference of geothermal fluid production differs depend on such types, required monitoring items for protection of hot spring aquifers also differ for each type. The type of relationship between a specific geothermal reservoir and a nearby hot spring aquifer can be roughly estimated by temperature and anion characteristics of the hot spring waters for volcanic geothermal systems. In a former study, a criterion was developed based on five types of geochemical characteristics of hot spring waters. However, such a categorization was not quite successful when volcanic gas or CO2 gas was supplied to the aquifer or geometrical setting was not appropriate. Although hydraulic relationship is well described by five types, geochemical characteristics of hot springs are not well represented by five types only. Therefore in this study, a new criterion was developed by adding two new types, CO2 gas supply and volcanic gas supply types, resulting in seven types in total. Spatial arrangement of the target aquifer and geothermal reservoir is also taken into consideration. The threshold values in this criterion are based on data sets obtained from hot spring wells and natural geothermal manifestations all over Japan. Then the new criterion was applied to hot spring waters in Kuju region, Oita, Japan where three geothermal power plants are in operation. As a result, all seven types were identified for this region. It was found that hot springs of seepage thermal water type and steam heated type densely exist near geothermal power plants, suggesting that such hot spring type identification may be an effective tool for geothermal exploration. The new criterion, which helps to estimate effects of geothermal development onto each nearby hot spring aquifer and enables to protect hot springs against excess geothermal development, may be applied for other volcanic geothermal regions.