In recent years, the deep geological subsurface gained more and more attention as it offers various reservoirs potentially applicable for different geotechnical use options, e.g. deep geothermal energy, geological storage of CO2 and H2/energy. In this context, knowledge about the occurrence and the controlling processes of fluid-rock reactions in space and time is important to guarantee sustainable long-term reservoir utilization. While fluid-rock reactions occur at pore scale, they can significantly influence both reservoir-scale (transport) processes and regional flow regimes. Based on field observations, exemplary fluid-rock reactions in connection with their wide-ranging effects on geotechnical utilizations will be discussed.