Accurate knowledge of reservoir geometry and flow paths are critical parameters for successful geothermal operations. They are essential for evaluating the long-term behavior and sustainability of geothermal reservoirs. Conventional hydraulic testing and tracer tests are often inconclusive or provide limited information due to complex and challenging reservoir conditions (multiple well systems, complex reservoir geometry, and fracture network, etc.). Recently, a new class of tracer techniques has emerged in order to overcome the major drawbacks of molecular tracers: nanoparticle-based tracers. The main advantages of nanoparticle tracers compared to molecular tracers are their tunable properties and modular structure. Functional and smart nanoparticle tracers such as the threshold-triggered temperature nanotracer enabled the simultaneous evaluation of multiple reservoir conditions (flow paths, temperature distribution, etc.) and created an entirely new field of research. As new areas of research often require detailed insights into fundamental processes, there are still open questions about the interactions between particles, fluids, and rock minerals and their performance in complex geothermal environments. As an example, the application of embedded or surface-bound tracing features (e.g., fluorescent molecules, DNA, etc.) within or on a silica matrix prevents the tracing function from being affected by the environment (e.g., pH changes, salinity effects, redox sensitivity). Although silica has low hydro(thermal) stability and loses its protective function at high temperatures or long-term applications, nanoscience offers a comprehensive set of tools to design and protect the silica matrix. Another advantage is the possibility of surface modifications, which can help to achieve minimum sorption and retention by adapting the ζ-potential of the nanoparticles. In this study, we address recent advances in increasing long-term stability, improving hydrothermal stability of silica nanoparticles, sorption control. Furthermore, we present strategies for the development and functionalization of nanoparticle-based tracers.

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