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 3-D 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 13x11x5 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 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.

This study presents a probabilistic analysis of 3D Navier‐Stokes (NS) fluid flow through 30 randomly generated sheared fractures with equal roughness properties (Hurst exponent = 0.8). The results of numerous 3D NS realizations are compared with the highly simplified local cubic law (LCL) solutions regarding flow orientations and regimes. The transition between linear and nonlinear flow conditions cannot be described with a generally valid critical Reynolds number (italicRecrit), but rather depends on the individual fracture's void geometry. Over 10% reduction in flow is observed for increased global Re (>100) due to the increasing impact of nonlinear conditions. Furthermore, the fracture geometry promotes flow anisotropy and the formation of channels. Flow perpendicular to the shearing leads to increased channeling and fluid flow (∼40% higher) compared to flow parallel to the shearing. In the latter case, dispersed flow and irregular flow paths cause a reduction of LCL validity.

HT-ATES (high-temperature aquifer thermal energy storage) systems are a future option to shift large amounts of high-temperature excess heat from summer to winter using the deep underground. Among others, water-bearing reservoirs in former hydrocarbon formations show favorable storage conditions for HT-ATES locations. This study characterizes these reservoirs in the Upper Rhine Graben (URG) and quantifies their heat storage potential numerically. Assuming a doublet system with seasonal injection and production cycles, injection at 140 °C in a typical 70 °C reservoir leads to an annual storage capacity of up to 12 GWh and significant recovery efficiencies increasing up to 82% after ten years of operation. Our numerical modeling-based sensitivity analysis of operational conditions identifies the specific underground conditions as well as drilling configuration (horizontal/vertical) as the most influencing parameters. With about 90% of the investigated reservoirs in the URG transferable into HT-ATES, our analyses reveal a large storage potential of these well-explored oil fields. In summary, it points to a total storage capacity in depleted oil reservoirs of approximately 10 TWh a−1, which is a considerable portion of the thermal energy needs in this area.

Background The determination of the subsurface conditions and reservoir properties is indispensable for exploration and exploitation of a geothermal field. It includes the expected geological structures as well as the hydraulic, thermal, chemical and mechanical parameters of the target horizon which should be favorable for geothermal utilization. Most essential is a high permeability to supply sufficient flow rates and high temperatures (Stober and Bucher 2012). An area in Germany which fulfills the criteria is the Upper Rhine Graben with a unique geothermal potential (Agemar et al. 2014). Local temperature anomalies with elevated temperatures up to 165 °C are expected at a depth of 2500 m, while the average geothermal gradient of the URG is above 40K km −1 (Agemar et al. 2013). Especially the sedimentary formation of the Triassic, Buntsandstein and Muschelkalk shows high permeabilities, which are necessary to provide high flow rates (Stober and Bucher 2014).

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