Summary

Volatile chemical tracers have been developed to track the flow of injected water into vapor-dominated (steam) geothermal fields. Electrical production from geothermal fields requires that the cooled water be injected back into the reservoir. The purpose of the injection-production loop is to maintain water pressure within the field and to avoid the environmental problems entailed in surface disposal. Unlike liquid tracers, which are added to the injection water and continue on through the liquid reservoir, vapor-phase tracers are added in the liquid injectate but boil before entering the steam reservoir. This means that the tracers must be soluble in both the liquid and vapor phases as well as stable at high temperatures, highly detectable, and nontoxic.

The first compounds tested as geothermal gas tracers were the chlorofluorocarbons dichlorodifluoromethane (R-12) and chlorotrifluoromethane (R-13; Adams, 1991). They were injected into The Geysers, a large geothermal field in Northern California. The tracer R-12 was detected in 13 out of the 16 wells sampled one day after injection. By the end of the test at 51 days after injection, R-12 had been detected in 38 of the 49 wells sampled. Peak concentrations ranged from 3.2 parts per million-million (ppt) to 31.1 parts per million (ppm) for R-12, and from 29 ppt to 1.7 ppm for R-13. Transit times were very rapid for some flow paths. Tracer was detected in fluid from wells as far away as 0.8 km from the injection well on day 1. By day 5, the tracers were detected 1.6 km from the injection well.

The utility of the chlorofluorocarbons as tracers was examined by comparing the results of the field test to a simple model that included boiling and first-order chemical decay. This comparison indicated that R-12 was less stable than R-13. Additional field tests of R-13 recovered up to 90% of the tracer, demonstrating that it is stable at reservoir temperatures of up to 240°C for periods of at least a few weeks (Beall et al., 1994).

Although the chlorofluorocarbons performed well as tracers, they became expensive or simply unavailable during the 90's because of their ozone depletion potential. Efforts are currently underway to replace them with compounds that do not contain chlorine, such as sulfur hexafluoride (SF₆) and low molecular-weight aliphatic hydrofluorocarbons. Sulfur hexafluoride has performed well in exclusively liquid-phase geothermal systems (Bixley et al., 1995), but has proved problematic in vapor-phase systems due to its high volatility (Voge, 1994). As demonstrated by Adams (1995), a high volatility can lead to early separation of the tracer from the liquid injection plume. Where early separation occurs the tracer may take a different fracture path than the bulk of the injectate. Hydrofluorocarbons are considerably more soluble than SF⁶ because of the polarity induced by the hydrogen atoms, although they are not as stable. We are currently testing the hydrofluorocarbons in our autoclaves for their stability under geothermal conditions, and will perform several field tests at The Geysers during 1998.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah