3D Flow Modelling of the Medium-Term Circulation Test Performed in the Deep Geothermal Site of Soultz-Sous-forêts (France)

Sylvie Gentier (1) , Xavier Rachez (1) , Tien Dung Tran Ngoc (1) , Mariane Peter-Borie (1) , Christine Souque (1)

1 BRGM - Bureau de Recherches Géologiques et Minières (BRGM)

Three boreholes have been drilled at 5-km depth within the fractured granite of the Soultz-sous-Forêts geothermal site (France). A five-month circulation test, joint with a tracer test, have been performed in 2005 between the injection well GPK3 and the production wells GPK2 and GPK4. After an analysis by a method of dispersive transfer model of the experimental breakthrough curves (BTC), three main circulation loops within the heat exchanger have been obtained. The aim of our work is to build a flow model of the Soultzsous- Forêts fractured reservoir, based on a realistic fracture network from a hydraulic point of view and consistent with the knowledge of the regional tectonic history. In this paper, the first stages of this model are presented. The proposed conceptual fracture model is based on five main fracture sets that constitute the reservoir in itself, and on some deterministic fault zones that condition the access from the well to the statistical fracture network to the three open holes. On the basis of fracture data collected from well imagery, new fracture density distributions based on Fisher distribution have been calculated. Four main fracture sets corresponding to main statistical modes are individualised: two ~N-S striking sets respectively W-dipping and Edipping; and two NE-SW and NW-SE striking sets respectively NW-dipping and SW-dipping. Those four main sets are consistent with the strain history. Indeed, two ~N-S striking sets respectively W-dipping and E-dipping are widely observed in the Rhine Graben and are commonly interpreted as the result of Oligocene deformation, but could also be related to lower Carboniferous to Permian NNE-SSW sinistral faulting. In addition, the two NE-SW and NW-SE striking sets are consistent with the orientation of the Hercynian structures observed regionally. A fifth fracture set is introduced in order to incorporate the “background noise”. Following this, a Discrete Fracture Network (DFN) model has been built. The rock matrix is considered as impermeable, and the fluid flow takes place only within the fractures. Tracer transport is solved by the particle tracking method. At this stage of progress, the fracture model tends to reproduce the GPK2 breakthrough curve obtained during the medium-term circulation test performed between the three wells. The GPK2 breakthrough peak is mainly due to the two ~N-S striking sets which constitute the shortest paths between GPK3 and GPK2 while its tail is rather due to the two other main fracture sets, NE-SW and NW-SE striking which create longer paths between GPK3 and GPK2 than the two ~N-S striking sets. However, it seems impossible to define a unique homogeneous statistical fracture model leading to a simultaneous fit of the two wells GPK2 and GPK4. The work in progress lets assume that a regional evolution of the basic pattern could be a solution. At this stage, it appears that the NE-SW and NW-SE striking sets could play a role more important between GPK3 and GPK4 than between GPK3 and GPK2.

Mots clés: DFN; flow; fractures; Soultz-sous-Forêts; transport model

Domaines: Sciences de la Terre

Sylvie Gentier, Xavier Rachez, Tien Dung Tran Ngoc, Mariane Peter-Borie, Christine Souque. 3D Flow Modelling of the Medium-Term Circulation Test Performed in the Deep Geothermal Site of Soultz-Sous-forêts (France). World Geothermal Congress 2010, Apr 2010, Bali, Indonesia. 13 p. ⟨hal-00493679⟩