Implications on large-scale flow of the fractured EGS reservoir Soultz inferred from hydraulic data and tracer experiments

The Enhanced Geothermal System in Soultz-sous-Forêts, located in the geothermal favorable Upper Rhine
Graben, is a fracture-controlled reservoir that was highly investigated in the last decades generating a huge
geoscientific database. Numerical reservoir models use this database to simulate the operation of the subsurface
heat exchanger, yet suffer from simplifications regarding the transfer of experimental into model data, dimensional
extension, and computational power and efficiency. The new extensive transient 3D simulations, based on
geophysical, geological and hydraulic data, highlight the hydraulic and transport feedback of the Soultz EGS due
to convective and advective fluid flow. Developed with the goal of simulating the vast tracer test data during the
reservoir-testing phase in 2005, the finite element model is focusing on the main fractured zones, which connect
the wells in the deep reservoir. It comprises 13 major hydraulically active faults and fractures in a
13×11 x 5 km extending model domain, as well as open-hole sections of the wells GPK1 to GPK4 and their
casing leakages. The simulation of the tracer experiment confirms the strong heterogeneity of the reservoir and
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highlights the importance of a potential fractured zone, hydraulically separating the reservoir in a northern
(GPK1 to 3) and southern section (GPK4). This zone tends to connect the reservoir to the main fault system by
hydraulically separating GPK4 from the other wells. The calibration and sensitivity analyses provide a unique,
broad understanding of the reservoir flow zones providing information on the extension of the Soultz reservoir in
the future and on the fluid pathways in the deep subsurface of the Upper Rhine Graben.