Uncertainty Quantification of THC Reactive Transport in the Middle Buntsandstein Fractures: A Numerical Reservoir Simulation Study at the Wörth am Rhein site in the Upper Rhine Graben, Germany

Thermo-hydro-chemical (THC) reactive transport in fractured geothermal reservoirs plays a critical role in optimising thermal energy extraction and ensuring sustainable management. This work presents an integrated modelling framework that couples geochemical simulations using PHREEQC with thermo-hydraulic calculations in COMSOL Multiphysics via the iCP software program. It focuses on the Middle Buntsandstein sandstone unit in the Wörth am Rhein area, where the Dekapalatin project is currently being implemented.

This study investigates the influence of varying brine and mineralogical compositions on a fractured geothermal reservoir. Therefore, a series of scenarios is designed to explore these effects on the reservoir behaviour. These scenarios investigate spatial and temporal changes in the reservoir characteristics. Explicitly, detailed alterations in porosity and permeability resulting from mineral dissolution and precipitation are evaluated, along with their implications for geothermal flow-path development and thermal breakthrough dynamics in the fracture network.

The results indicate that variations in brine and mineralogical compositions can lead to spatially heterogeneous alterations in fracture apertures. These alterations, in turn, modulate local fluid velocity and heat transfer. Secondary precipitation processes, notably of carbonate minerals, are expected to have a relatively significant influence on reservoir porosity and permeability. On the other hand, dissolution processes may promote the development of additional flow paths within the fracture network.

This study improves our understanding of thermo-hydro-chemical interactions in fractured geothermal reservoirs and provides a clear framework for quantifying uncertainty in reservoir management. The coupled approach improves the reservoir performance predictions and informs sustainable decision-making for geothermal exploitation.