Hydrofluorocarbons as geothermal vapor-phase tracers

Two hydrofluorocarbons, R-134a and R-23, have been developed for use as a v apor-phase tracer in geothermal systems. These low molecular weight compoun ds are volatile, electrically neutral, nontoxic, relatively inexpensive, an d have detection limits as low as 10(-5) ppm. Data from laboratory and fiel d 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 hig her volatility than water, which affects the rate of transfer of the tracer from the liquid injectate to reservoir steam during boiling. Simple analyt ic models of boiling were used to estimate the effects of the high tracer v olatilities on the outcome of tracer tests in vapor-dominated systems. The results imply that the effects of volatility are exaggerated under conditio ns of high superheat, which promote the continuous removal of steam from th e vicinity of the boiling interface. In contrast, low to moderate superheat reduces the effects of volatility to the extent that the volatile-tracer t est results qualitatively resemble those in which tritiated water is used a s a tracer. Thus, volatile tracers can be used with confidence to qualitati vely describe the distribution of injected water in vapor-dominated systems where superheat is low to moderate. (C) 2001 CNR. Published by Elsevier Sc ience Ltd. All rights reserved.