Tracer Test in Hot Spring Fluid Layer at Minami-Izu Geothermal Field, Shizuoka, Japan

Abstract

Tracer test is carried out in high temperature hot spring fluid layer at Minami-Izu geothermal field, Shizuoka, Japan. In Minami-Izu field, the temperature of several hot spring wells is about 100 degree C at a depth around 150 m. And the west area of this field, hot spring fluid product without pump and the Cl concentration is about 10,000 mg/l. The Cl concentration decreased in east area. Then, to survey the flow direction and rate of hot spring fluid, we carried out tracer test. Firstly, we drilled monitoring well until 600 meter depth. Secondly, we drilled injection well injection well until 150 meter depth as same as feed point of many hot spring wells. At middle of September, 2012, the injection of hot spring fluid started until end of September. And 500g uranine tracer injected at 16 September. We monitored tracer appearance at 4 wells using optical fiber system and lab spectrometer. After 9 hour from tracer injection, first tracer appeared at K-13 well about 50 meter distance from injection well and next to monitoring well. The tracer concentration rapidly increased and showed peak at several days after injection. The return ration at K-13 is estimated about 30%. This tracer response shows the possibility of existence of large fracture between injection well to K-13. And in other well, at K-11 about 150 meter from injection well, tracer appear at early October. And this tracer response shows the possibility of natural flow of hot spring fluid. Then, we discuss about tracer response of hot spring field compare with my previous survey at Hijiori EGS and underground water layer.