Enhanced Geothermal Systems (EGS) offer promising potential for expanding geothermal energy access, particularly in regions without conventional resources. However, their growth is limited by the risk of premature thermal breakthrough or “short-circuits” in fracture-dominated reservoirs, which reduce production well temperatures and shorten operational lifetimes. Monitoring and mitigating these issues require better understanding of reservoir conditions, especially the spatial distribution of fluid flow, which remains challenging due to subsurface uncertainties. This thesis investigates two approaches for reservoir characterization and thermal-hydraulic flow control.The first approach focuses on thermally degrading tracers for characterizing reservoir thermal evolution. Phenyl acetate, a tracer analog, was tested in bench-scale microfluidic experiments to study its hydrolysis kinetics in the absence of silica. A reactive transport model was fitted to experimental measurements of its reaction product, phenol. Results showed the hydrolysis rate without silica was 4–10 times slower than previously reported study by Hawkins et al. (2021) with silica present, underscoring the influence of surface interactions on tracer performance. The second approach explores the use of temperature-responsive “active” tracers, poly(N-isopropylacrylamide)-sodium acrylate (pNIPAM-SA) hydrogel microspheres, for managing flow in short-circuited fractures by forming yield stress fluid resistant to the shear rate of a flow. Two studies in this thesis examined their transport behavior. One study analyzed their transport properties in dilute conditions using a microfluidic EGS reservoir model. An adsorptive transport model was fitted to observed concentration profiles, showing that swollen hydrogel microspheres experienced hydrodynamic chromatography effect and adsorption, while shrunken ones exhibited reversible adsorption. Another study aimed to demonstrate jamming behavior by injecting concentrated hydrogel microsphere suspensions into a simulated short-circuited fracture. Measurements of pressure drop, density, and flow rate were used to evaluate flow resistance and jamming potential. Although jamming was not observed, swollen hydrogel microspheres notably increased flow resistance, suggesting such increase happened by affecting the bulk apparent kinematic viscosity. Together, these studies contribute new insights into thermally reactive tracer kinetics and hydrogel microsphere transport under geothermal-relevant conditions, offering potential tools for EGS reservoir monitoring and flow control.