Quantitative Interpretation of Tracer Test Data

Geothermal reinjection is an important part of sustainable management of geothermal resources. Reinjection started out as a method of waste-water disposal, but is now also being used to counteract pressure draw-down and to extract more thermal energy from reservoir rocks. The possible cooling of production wells, or thermal breakthrough, is one of the main disadvantages associated with injection. To minimize this danger while maintaining the benefit from reinjection requires careful testing and research. Tracer testing, which is used to study flow-paths and quantify fluid-flow in hydrological systems, is probably the most important tool for this purpose. The main purpose in geothermal studies and management is to predict possible cooling of production wells due to long-term reinjection of colder fluid. Comprehensive interpretation of geothermal tracer test data, and consequent modeling for management purposes, has been rather limited, even though tracer tests have been used extensively. Their interpretation has mostly been qualitative rather than quantitative. A simple and efficient method of tracer test interpretation is presented, which is based on the assumption of specific flow channels connecting injection and production wells in geothermal systems. Computer software (TRINV) based on this method uses an automatic inversion technique to simulate tracer return profiles quite accurately, and to estimate (invert for) flow characteristics of the flow channels. The results of the interpretation are consequently used for predicting thermal breakthrough and temperature decline during long-term reinjection. This method has been used successfully in a number of geothermal fields worldwide and examples from Laugaland in N-Iceland and Ahuachapan in El Salvador demonstrate its’ effectiveness.