ACQUISITION, ANALYSIS, AND INTEGRATION OF WELLBORE LOGS TO CHARACTERIZE A FRACTURED GEOTHERMAL RESERVOIR: THE CASE OF RITTERSHOFFEN, FRANCE

The Exploitation de la Chaleur d’Origine Géothermique pour l’Industrie (ECOGI) site lies in a deep geothermal doublet in Rittershoffen (Alsace, Eastern France), producing heat for an industrial plant. The two wells, GRT-1 and GRT-2, targeted local natural fracture zones in the vicinity of a large normal fault across the Buntsandstein sediments and the granitic basement at a depth of 2,000 m below surface. An extensive measurement campaign was carried out in both wells by means of wireline logging. Pressure and temperature logs, nuclear logs (density and porosity), resistivity logs, dipole sonic logs, and wellbore image logs were acquired in the open hole over the target fractured aquifer and partially across the overburden. These logs were processed and interpreted to build an integrated model of the site, describing its geological properties, notably the fracture network, its dynamic behavior in terms of fluid and heat flow, and its geomechanical properties. Wellbore imaging results from acoustics imagers were interpreted to understand the geometry of the natural fracture network, which acts as the main fluid pathway in the Rittershoffen geothermal system. The results were integrated with temperature logging to understand which fractures were open and therefore cooling down when invaded by the drilling mud. Density and sonic logs were used to derive the mechanical properties of the near-wellbore rock and the stress magnitudes; the interpretation of drilling-induced features in the wellbore images allowed determining the orientation of the local stress acting on the wellbore. The geomechanical model obtained was used to predict the occurrence of mechanical or hydraulic instability along the well and compare the prediction with the events actually observed in the well, providing a validation of the geomechanical model. The results of well-centric fracture and geomechanical analysis were integrated in a 3D reservoir model and used to understand the performance and the risks associated with geothermal operations at the site.