Hydrochemical evolution and environmental implications of geothermal reservoirs under transient surface water injection

Geothermal energy has expanded rapidly worldwide, but declining reservoir pressures pose a challenge to its long-term sustainability. Supplementing reservoirs with surface water offers a potential solution. However, the environmental and operational impacts of such injections remain poorly understood, limiting effective management strategies. In this study, we developed a multiphysics modeling framework integrating heat transfer, fluid flow, and geochemical reactions to simulate the hydrochemical and porosity evolution in geothermal reservoirs under transient surface water injection. We identified distinct thermal, hydraulic, hydrochemical, and porosity influence zones around the injection well and quantified their spatiotemporal evolution. Results revealed time-dependent expansion of impact zones, shifts between dominant mineral dissolution and precipitation regions, changes in water quality, and accumulation of carbonate scales that reduced porosity by up to 0.03 % within 0.15 m of the well. These insights can guide water source selection, injection management, and production strategies, improving injection efficiency while mitigating clogging risks. This study provides a framework for assessing environmental and operational consequences of surface water injection in geothermal reservoirs, supporting sustainable geothermal resource management.