A self-potential (SP) survey was carried out at the Mataloko geothermal field as a part of geothermal exploration for a future exploitation in Flores Island, Nusa Tenggara, Indonesia. As a result the highest positive SP anomaly was observed at the main geothermal manifestation area. The extent of positive SP anomaly zone has a good correspondence with a hot-spring area. In order to estimate the extent of the high permeability zone around the surface manifestation zone, a two dimensional numerical model was developed with a coupled fluid flow and SP simulator. A rough estimation of the permeability distribution of the Mataloko geothermal area was thus executed by matching of the calculated SP with the observed data.

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In order to characterize the Ogachi reservoir, four tracer tests have been carried out successfully, twice in 1994, one times in 1995 and 1997, respectively, at the CRIEPI's Ogachi HDR site in Akita prefecture, Japan. Since the response curve obtained from the tracer monitoring involves two curves due to the two reservoir, the upper and the lower, a numerical simulation of two flow path model was done to separate the response curve each other.
Using the response curves obtained by monitoring and calculation, the characteristics of the Ogachi reservoir was estimated. The results are as follows ;1) Considering the fitting of the synthesized and observed curves, the Ogachi reservoir is evaluated to have multiple paths.2) Modal volumes of the upper reservoir are estimated 9.5 m³ in 1994's 1st test, 12.5 m³ in 1994's 2nd test, 18.7 m³ in 1995, and those of the lower are estimated 298 m³, 249 m³, 106 m³, respectively. Through the circulation tests, the modal volume of the lower reservoir decreases gradually, and in 1995 test, the modal volume decreases remarkably, because of permeability improvement of the lower reservoir performed before the circulation.3) Values of width at 1/2 height which is assumed to represent dispersion of the reservoir decreases between 1994 and 1995, but the value increases in 1997. This increase is supposed to be based on the plugging to the fracture of the sand which is used before circulation, or the precipitation in the fracture because of the cooling down in the reservoir due to huge water injection to the reservoir before circulation.

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Two types (aril and ester) of nitrogen bearing cationic surfactants were applied to recover silica from fluids supersaturated with respect to silica at the Sumikawa geothermal plant, northeast Japan. The reagents were mixed with two kinds of fluids ; one was fluids immediately after flashing under an atmospheric pressure (no retaining time ; NRT) and the other was fluids keeping in teflon bottles for 15 minutes at 90°C (15 minutes retaining time ; 15RT) after flashing. Total silica concentrations in both decreased with increasing the reagent concentration independent of the cohesion types and the molecular weight except for a starting material for the aril type one. Total silica concentrations in fluids with 15RT were lower than those in fluids with NRT and reached a nearly constant value of 380 mg/l (more than 50 mg/l reagent concentration), the solubility of amorphous silica at 90°C estimated in this study. Zeta potentials of silica slurries were increased with increasing the reagent concentration and crossovered the zero potentials around 75 mg/l reagent concentration for fluids with 15RT and around 100 mg/1 for fluids with NRT. From these results, the following model is demonstrated as a deposition mechanism ; in fluids with 15RT, 20 monomer silicas became one polymer silica by polymerization after flashing and reacted with one nitrogen ion in one cation cohesion.

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