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

Abstract The global transition to low‐carbon energy systems is driving demand for lithium, a critical raw material for battery storage technologies. Geothermal brines, particularly in the Upper Rhine Graben (URG) in western Europe, offer a potential lithium source co‐produced with heat, providing economic and strategic benefits. A numerical model was calibrated against comprehensive field data (pressure transients, tracer tests, thermal profiles) from the Rittershoffen geothermal doublet system in the URG. The model uses a control volume finite element method with discrete discontinuities to simulate coupled fluid flow, heat, and solute transport in the fault‐controlled system. Results reveal a highly heterogeneous flow field: a rapid primary flow path through the major fault and its damage zone, and slower secondary paths through the surrounding reservoir. The interplay between these paths, modulated by damage zone permeability, controls hydraulic short‐circuiting. Simulations of 50 years of production show that while thermal output remains stable, produced lithium concentration declines early as lithium‐depleted injected brine recirculates to the producer. Lithium release from the host rock, modeled here as a volumetric leaching source term and treated as a sensitivity parameter in the absence of long‐term field data, can counteract this dilution; sustained leaching rates as low as 0.3 g Li per of bulk rock per year maintain concentrations at about 100 ppm or more. This research emphasizes that constraining in situ leaching rates and hydraulic connectivity is essential for de‐risking long‐term geothermal‐lithium projects in fault‐hosted settings such as the URG.