日本地熱学会誌

In order to remove supersaturated silicic acid from geothermal brines, a seeding method using silica gels has been proposed. In this study, the effects of pH and temperature on silica precipitation on the silica gel seeds were experimentally and kinetically examined in model geothermal brines. The apparent activation energy of silica precipitation on the silica gel seeds ΔE [J·mol^-1] is presented as a function of the hydroxide ion concentration:ΔE=(5340+275ln[OH^-])Rwhere R is gas constant. The apparent rate constant of silica precipitation K [s^-1] is presented as a function of the hydroxide ion concentration and the temperature T [K] and further is combined with two important characteristics of the silica gel seeds ; the surface area A_s and the particle diameter D_p50K=θA_sD_p50^-0.72[OH^-]^1.06e^-ΔE/RT.where θ is constant, 7×10^-4 for the silica gel seed. By combining the equation for activation energy with the reaction rate equation for silica precipitation, the removal rate and removal quantity of silicic acid can be calculated.

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The Kakkonda geothermal field, northern Japan is a typical hot water dominated type geothermal system. Recently, the deep well named WD-la penetrated the shallow reservoir (at the depth of about 1500 m and above), the deep reservoir (at the depth between 1500 m and 3100 m) and the heat source (at the depth of about 3100 m and below). The temperature profile was obtained to the bottom at the depth of 3729 m where the temperature was above 500°C. The rock type of the heat source is young granite. The Kakkonda hydrothermal system is understood as follows; that is, the hot water convection has developed in the upper permeable formation located at the depth of 3000 m and above after the intrusion of the young heat source at such a shallow depth of 3000 m. A numerical simulation is carried out based on a simple two-dimensional hot water convective model with a conductive cooling heat source below the permeable formation. As the result, it is estimated that the Kakkonda geothermal system has developed during tens of thousands of years after the time of intrusion and is in the most active stage at present. This means that the age of the Kakkonda geothermal system is much younger than those of other well-known geothermal systems such as Wairakei and Kawerau in New Zealand. This simple model may explain the temperature profile down to the bottom of the deep well and the flow pattern of hot water in the permeable formation.

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