Hydrofluorocarbons as geothermal vapor-phase tracers

Two hydrofluorocarbons, R-134a and R-23, have been developed for use as a vapor-phase tracer in geothermal systems. These low molecular weight compounds are volatile, electrically neutral, nontoxic, relatively inexpensive, and have detection limits as low as 10 5 ppm. Data from laboratory and field tests indicate that they are stable enough to be used as tracers in vapor-dominated systems such as The Geysers. However, these compounds have a higher volatility than water, which affects the rate of transfer of the tracer from the liquid injectate to reservoir steam during boiling. Simple analytic models of boiling were used to estimate the effects of the high tracer volatilities on the outcome of tracer tests in vapor-dominated systems. The results imply that the effects of volatility are exaggerated under conditions of high superheat, which promote the continuous removal of steam from the vicinity of the boiling interface. In contrast, low to moderate superheat reduces the effects of volatility to the extent that the volatile-tracer test results qualitatively resemble those in which tritiated water is used as a tracer. Thus, volatile tracers can be used with confidence to qualitatively describe the distribution of injected water in vapor-dominated systems where superheat is low to moderate. # 2001 CNR. Published by Elsevier Science Ltd. All rights reserved