This paper examines the advantages and disadvantages of chemical tracers for use in geothermal reservoir monitoring. Tracers are used to determine the magnitude of connectivity between injection and production wells in order to estimate the likelihood of premature fluid breakthrough. Even though chemical tracers are generally less environmentally sensitive than radioactivematerials, quantities injected need to be much larger to be distinguishable by chemical analysis. As a result, a nonequilibrium concentration of tracer material is injected into the reservoir, and the tracer is susceptible to retention within the reservoir by ion exchange, diffusion into the solids or immobile reservoir fluid, adsorption or dissolution. These various reactions lead to changes in the tracer concentration as the traced fluid flows through the reservoir, and therefore reduce the capability of the experiment to distinguish concentration changes due to purely mechanical effects. Experimental observations reported here show that substantial fractions of K I tracer were retained under reservoir conditions, even though it appears that the retained material was subsequently released in to more dilute fluid. release" mechanism that will distort the later response of a tracer breakthrough.

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