The energy production potential, or capacity, of geothermal systems is highly variable. It is primarily determined by the pressure decline caused by mass extraction, but also by heat content. Pressure declines continuously with time in systems that are closed or with limited recharge. The production potential of geothermal systems is, therefore, often limited by lack of water rather than lack of heat. Geothermal resource management involves controlling energy extraction from geothermal systems underground so as to maximize the resulting benefits, without overexploiting the resource.

When geothermal systems are overexploited, production from the systems has to be reduced, often drastically, resulting in an insufficient steam supply to power plants or in loss of wells. Overexploitation mostly occurs for two reasons. Firstly, inadequate monitoring and data collection contribute to poor understanding of the system and lack of reliable modeling and, therefore, the systems respond unexpectedly to long term production. Secondly, overexploitation occurs when many users utilize the same resource/system without common management or control.

The main purpose of tracer testing in geothermal reservoir management is to predict possible cooling of production wells resulting from the long term injection of colder fluid and/or the invasion of natural groundwater. In a geothermal field, the primary resource is water, both as liquid and as steam. A direct measure of its behaviour is thus of obvious importance to field management. Water tracing is the only technique that gives a direct indication of underground flow patterns and velocities.

Information gained from tracer testing of geothermal reservoirs is similar to that obtained from oilfields and includes:

(a) Proper diagnostics of the reservoir comprising evidence of direct connections between the tracer injection point (either within or outside the field) and monitoring wells in the field;

(b) Measurement of direction, speed and mean residence time of water movement;

(c) Determination of the extent to which groundwater downflows intrude into production wells;

(d) Identification of breakthrough (arrival time or first appearance of tracer in the production well);

(e) Quantification of the tracer quantity collected in each production well;

(f) Information needed for calibration or verification of physical models of the geothermal reservoir.

All of this information will aid in gaining an understanding of the nature of a geothermal system, but the measurements, which bear upon injection and groundwater intrusion or cooling potential, have the greatest impact on field management.