Potassium bromide (KBr) and potassium iodide (KI) tracers were individually injected into two reinjection wells at Sumikawa: KI with steam condensate into one well and KBr with separated waste water at high temperature (140 °C), into another well 1000 m away from the KI well. The tracer concentrations were then analyzed in all production wells for 16 months. The iodine tracer was first detected in two wells (wells S-4 and SB-1) within 1000 m of the injection well within 47 h and in other wells (with the exception of well SC-2) up to 1500 m away within 9 days after injection. Before background correction as mentioned below, most of the injected iodine tracer was recovered from the production wells within 6 months. The bromine tracer was also observed first in well S-4 within 5 days and in other wells within 1 month, except for well SC-2. These observations show that vapor condensate quickly returns into wells S-4 and SB-1 compared to geothermal fluids. These results are consistent with the previous calculations examined through a box model using chlorine concentrations and hydrogen isotope ratios of fluids from production wells. The results of the present tracer tests were examined using a multi-path model. First, the background concentrations of both tracers were recalculated because the total recovered amounts of I and Br exceed the amounts that were originally injected. The variations of the tracer concentrations were then fitted by setting two or three paths between the injection and production wells. Our calculations show that the recovered fractions of I and Br are 36 and 40%, respectively, and the velocities of fluids in the reservoir are estimated to be 0.4 to 14.9 m/h in the case of both tracers.
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