A series of water injection was practiced in the Matsukawa vapor-dominated geothermal field, northeast Japan, and the necessity of a tracer test emerged to evaluate the returns of injected water. Because production wells produced superheated or saturated steam, vapor-phase tracers (sulfur hexafluoride and hydrofluorocarbons) and two-phase tracers (alcohols) were examined in terms of their practical use. We selected the alcohols because (1) they were significantly soluble in water and thus handled simply during injection, sampling and analysis, and (2) they were expected to boil and flow as with injected water in the reservoir. Five tracer tests were conducted on the four production wells from 2000 to 2003, and the returns of tracers were successively detected for each test. The results of the tests using mixed solutions of ethanol and i-propanol showed variation in the ethanol/i-propanol ratios and different peak times between the alcohols, which may depended on the difference in volatility and the mechanism of twophase flow in the vapor-dominated reservoir. The tracer test using mixed alcohol may have potential to give some information about the boiling process of injectate and properties of two-phase flow in the reservoir.
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