A comprehensive review of tracer tests in enhanced geothermal systems

Tracers are materials injected into reservoirs of enhanced geothermal systems (EGSs) for reservoir characterization and thermal performance assessment. This work presents a comprehensive review of tracer test techniques in EGSs to provide a useful guideline for tracer test implementation based on 134 tracer tests at 14 EGS sites. This review investigates what information is possible to obtain through using tracers, what types of tracers can be used, and how to design a tracer test. An illustration of specific methods for determining inter-well connectivity, reservoir volume, heat transfer surface area, and thermal drawdown is presented. Based on the investigation of physicochemical properties, tracers are classified into inert and novel types. Furthermore, tracer test design techniques are explained, including tracer selection, quantification of tracer injection mass, hydraulic flow regime, and sampling collection. Based on the global tracer test results, the following information was obtained: (1) tracer tests could predict the thermal drawdown more robustly with the addition of novel tracers; (2) when selecting tracers, the factors of the test target and reservoir conditions (pH, temperature, and rock type) must be considered; (3) a well-designed test, considering all the design techniques, guarantees the tracer test quality; (4) challenges remain in estimating heat transfer surface area, and combining inert- and novel tracer tests could potentially solve this problem; therefore, further investigation is required to fully understand the behavior of novel tracers in geothermal reservoirs.