Temperature Signature of Permeable Fracture Zones in Geothermal Wells of Soultz-sous-Forêts in the Upper Rhine Graben

Identifying fluid circulation in fracture zones is a key challenge to exploit deep geothermal heat from natural reservoirs in the Upper Rhine Graben. The study focuses on permeable fracture zones prior to stimulation operations within the granitic basement penetrated by four deep Soultz wells GPK-1, -2, -3, -4 (France). Old well data were re-interpreted in order to spatially correlate permeability indicators in the granitic basement (post drilling temperature anomalies, mud losses and gas occurrences) and major natural fractures observed in core samples and image logs. Two large-scale clusters of natural fracture zones were observed in the four wells; one between 1.4 km and 2.3 km depth and one between 3.5 and 4.8 km depth. The shallowest cluster is the most permeable. Natural permeability is linked to the degree of hydrothermal alteration around fractures and particularly the occurrence of clay minerals. Temperature anomalies seem to be a reliable indicator to estimate the connection between the fracture zones intersected by the well and the far-well field. The link between the temperature flux and the geothermal fluid flow has to be investigated to support this observation.