Modeling the non-standard behaviors observed during the tracer test at Soultz-sous-Forêts (France)

Tracer test is largely used to understand hydraulic processes in geothermal fields. In the framework of the European Enhanced Geothermal System (EGS) project, a deep geothermal exchanger is in progress at Soultz-sous-Forêt (Alsace - northeast of France). The triplet is constituted by an injection well (GPK3) and two production wells (GPK2 and GPK4) drilled at a 5 km depth within the Rhine Graben granitic basement. A tracer test between the three boreholes was performed during a five months circulation test from July to December 2005 (+15 m3/s in GPK3 and -12 m3/s and -3 m3/s respectively out of GPK2 and GPK4). The breakthrough curves (BTC) observed for GPK2 and GPK4 show totally different behaviors. The GPK2 BTC displayed a non-Fickian behavior with a typical curve of a fractured medium, i.e. an early arrival of the tracer return four days after the tracer injection time, a concentration peak of 700 µg/l and a tailing effect. The GPK4 BTC was almost linear with time from the breakthrough time, which is much later than in GPK2 (28 days) and a concentration which only attained a maximum value of 30 µg/l at the end of the experiment. The fluorescein recovery rates in GPK2 and GPK4 were respectively of 23.5% and 1.8%. This in-situ test pointed out a high hydraulic connection between GPK3-GPK2 and a very low conductivity between GPK3-GPK4. In order to simulate the hydraulic behavior described above, a 3D flow and transport model with a DFN approach (Discrete Fracture Network) was built from the knowledge of regional structural context of the Soultz-sous-Forêt reservoir in relation with the tectonic history and from the hydraulic data obtained from the various well tests. We consider that the fluid flow occurs only within the fracture network and the rock matrix is considered as impermeable. Tracer transport is solved by the particle tracking method. The realistic fractures model, based on the analysis of the fractures collected from well imagery, is mainly composed of five statistical fracture sets that constitute the reservoir in itself. Some deterministic fractures are used for conditioning the access from the wells (open holes) to the reservoir. All fractures of a given family have the same hydraulic properties. The investigated numerical simulations reproduced very well the two GPK2 and GPK4 BTCs of the medium-term circulation test performed between the three wells in Fall 2005. However, using this spatially homogeneous model, we can already obtain simultaneously the different forms of the two BTCs of GPK2 and GPK4 independently of the non-symmetry of the production flow rates in GPK2 and GPK4. On a structural point of view, the two ~N-S striking sets create the shortest paths between GPK3-GPK2 and that the two other main fracture families, NE-SW and NW-SE striking are responsible for the tailing of the GPK2 BTC and the long paths between GPK3-GPK4. An appropriate fracture network is a key to reproduce breakthrough curves, in order to be able to perform predictive scenarios of EGS exploitation.