Numerical Modeling of Geothermal Heat and Lithium Co-Production in Fault-Hosted Reservoirs

The dual production of geothermal energy and lithium from fault-controlled reservoirs, such as the Rittershoffen doublet in the Upper Rhine Graben (URG), presents a significant opportunity for the energy transition. However, long-term feasibility depends heavily on the complex interplay of fluid flow and chemical transport. We developed a numerical model using a control volume finite element method with embedded discontinuities, calibrated against comprehensive field data (pressure transients, tracers, and thermal profiles).Our results reveal a highly heterogeneous flow field: a rapid primary path through the major fault/damage zone creates hydraulic "short-circuits," while slower secondary paths sweep the surrounding fractured reservoir. While thermal energy production remains remarkably stable over a 50-year forecast, lithium concentrations are more sensitive to these flow dynamics.We show that in the absence of active lithium leaching, concentrations decline as lithium-depleted brine recirculates. However, we demonstrate that even modest leaching rates (0.3 g/m3/yr) can sustain concentrations above 100 ppm. These findings highlight that constraining in-situ leaching rates and hydraulic connectivity is not just a geological challenge, but a critical requirement for de-risking the "lithium-from-brine" industry in the URG.