Conservative tracers can be used to characterize flow processes between injection and production wells, since, by definition, they react neither with the aqueous medium nor with fracture surfaces as they are advected through a geothermal formation. In contrast, reversibly adsorbing tracers interact electrostatically with the fracture-rock surface and this interaction serves to retard slightly their arrivals at a production well relative to those of conservative tracers. In combination with numerical simulation modeling, this slight retardation can be used to constrain the adsorptive-surface area, which is related to the surface area for heat transfer—a critical parameter for predicting reservoir performance. We examine a range of molecular and colloidal candidates for use as reversibly adsorbing tracers in Engineered Geothermal Systems (EGS), including fluorescent organic anions and cations, Group 1A metals, and fluorescent nanoparticles.

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