Heavy metal precipitation from geothermal fluid of 87N-15T production well in the Okuaizu geothermal field, Tohoku district, Japan
1991 Volume 41 Issue 228 Pages 231-242
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1991 Volume 41 Issue 228 Pages 231-242
Feasibility studies on the Okuaizu geothermal field have been carried out to test the possibility of generating 65, 000kW of electricity. A production well, 87N-15T, drilled to a depth shows the highest productivity in the geothermal field. It produces geothermal fluid of high temperature (300°C at the well bottom ) and high salinity (Cl-: 15, 300-18, 500 mg/l).
A heavy metal precipitate containing high amounts of gold and silver(max: Au=116ppm, Ag=3.49%) was discovered inside the two=phase (vapor and water) pipeline in front of the separator where pressure and temperature drop from 60kgf/cm2 and 275°C to 8.4kgf/cm2 and 171°C.
The texture and mineralogy of the precipitates were examined as were the chemical properties of the fluid to the surface from the underground reservoir. The precipitates deposited in the pipe before the separator are dominated by sulfides, mainly tetrahedrite with subordinate amounts of sphalerite and galena. These minerals are minute in size and occur in intergrown aggregates. The gold is submicroscopic in size but seems to be included in the tetrahedrite and/or sphalerite. The silver is contained in tetrahedrite and galena. Computer calculations for the chemical properties of the reservoir fluid based on the data gathered from the separated water at the surface is as follows: Temperature=280°C, pH=4.08, logf(H 2)=-3.11, Au=0.06ppb, HS-=0.245 mg/l, Cl-=13, 400 mg/l.
