Geothermal field dynamics

Measurements in geothermal reservoirs used for power production indicate any penetration of recent water, and help in exploring the potential of geothermal systems.

The deep, high-temperature geothermal waters used for electricity production contain negligible amounts of tritium. As these waters have been isolated from atmospheric influences many years (in New Zealand between 100 and 12,000 years), the tritium has often decayed to below measurable limits.

This makes it a straight forward exercise to determine whether the geothermal water/vapour being used in a producing field is pure geothermal water, or if it contains surface water contamination. Surface water contamination, particularly from reinjection bores drilled too close to the production well, will lower the reservoir temperature and hence the production efficiency.

Stable isotopes and radioactive tracers have been used in regular monitoring programmes of geothermal production fields, with the results being able to support decision making on geothermal reservoir management. Tracers are compounds that are added to water that is being injected back into a geothermal system, so that the water may be traced through the geothermal reservoir. Tritium is particularly useful as it will fractionate to both the liquid and the steam. This technique can also be used to determine the circulation time for high temperature geothermal waters and explore the potential of new geothermal systems.