3D Hydrothermal model at the Upper Rhine Graben scale considering faults
Authors: ArmandineLesLandes Antoine , Annaelle Jacquin , Chassagne Romain , Dezayes Chrystel , Guillon Théophile , Lopez Simon , Beaude Laurence 
Published: March 2024

Any unconventional deep geothermal project involves inherent risks due to uncertainties related to the presence of the geothermal resources at depth. Moreover, the investment in drilling in the early stage is large and the success rate remains low. This requires precise identification of the resource location to enhance the likelihood of successful and economically viable projects. To mitigate risks, it is crucial to gather and compile as much information as possible before initiating the drilling phase. Typically, the selection of exploration well locations in geothermal energy systems relies mainly on observations in structural geology, geophysical measurements, and geochemical analyses. Cross-referencing these observations with findings from additional disciplines contributes to pin down regions for exploration and then geothermal system development.
The Upper Rhine Graben (URG) represents a tectonically active rift system, forming one branch of the European Cenozoic Rift System, with significant geothermal energy potential within its basin. The extensive fault network, shaped by a complex tectonic history and situated beneath sedimentary deposits, facilitates fluid circulation patterns.
Geothermal anomalies are heavily influenced by fluid circulation within permeable structures like fault zones. To improve the predictability of geothermal resource locations, it is essential to understand hydrothermal processes in large-scale fractured media for identifying preferential zones at a finer scale for targeted exploration. Numerical simulations serve as a valuable tool to address fluid circulation challenges within extensive fault networks, facilitating the upward movement of deep and hot fluids.
Based on our previous work published, we developed a new numerical model to investigate hydrothermal processes at the URG scale (150 x 130 km). The numerical model uses the ComPASS geothermal flow simulator, which allows an explicit discretization of faults and fractures as 2D hybrid objects, immersed in a 3D matrix while considering mass and thermal transfers, both diffusive and convective.
The developed approach involves building models with increasing geometric complexity to observe the impact on thermal field of geometric elements (such as lithologies interface and faults) and to identify the main physical processes underlying thermal signatures at the Upper Rhine Graben scale. One of the underlying objectives is to identify the key elements (both geometric and physical) required for a relevant representation of the behavior of the hydrothermal systems at this exploration scale. The simulated temperature fields for successive geometries are compared to temperature distribution at different depths derived from temperature measurements from boreholes in the URG to assess the impact of these different geometries as well as the model fidelity.
Finally, we performed a prior model calibration that tends to converge to the observed temperature field and matches the characteristic temperature distribution with depth in the boreholes of URG. This work is a first stepping-stone toward an automated assimilation of data framework for geothermal exploration and geothermal co-production.