Development and Characterization of a HDR Heat Exchanger at the HDR Test Site at Soultz-Sous-forÍts: Flow Logs, Joint Systems and Hydraulic Active Fractures

Abstract

In addition to previous work of fracture evaluation of Ultrasonic Borehole Imager (UBI) logs (in cooperation with BRGM, OrlÈans) a more detailed evaluation of fracture orientation was carried out. During reprocessing procedure, the distribution of fracture network versus depth and the distribution of different classes of apparent apertures of fractures were amended. A total number of 1827 natural fractures and 872 stress induced en-echelon type fractures were individually evaluated from UBI-logs within the depth interval from 1400 m to 3800 m. The frequency distribution of strike orientation were evaluated by plotting data into rose diagrams. In the granitic section of GPK2, highly dipping natural fractures mainly strike approximately N-S. The orientation of the major horizontal stress field direction can be deducted from strike direction of stress related vertical or subvertical induced fractures and en-echelon type structures. As a whole the very small and small en-echelon type fractures are organized in a principal fracture set striking mainly NNW-SSE. Large apparent aperture fracture classes strike predominantly NNWSSE, whereas small apparent apertures strike N-S. In the hydraulic active part of the lower bore hole section of GPK2, seven joints show a hydraulic active behaviour at depths between 3244 m and 3562 m with a strike orientation between N 120? E and N 198? E. Flow logs in GPK1 during the hydraulic tests in 1997, show water flow in different joints below casing shoe. The flow regime shows significant changes during various test conditions and flow rates in GPK1. The orientation and the apparent aperture of hydraulic active joints were determined. Totally 14 different hydraulic active joints with mainly medium to large apparent aperture size could be detected between 2866 m and 3497 m depth. The major strike direction of medium size apparent aperture is NNW-SSE and large size aperture NNE-SSW. Hydraulic active fractures determined on FMI-logs dip in general steep and few moderately steep. In combination with results of seismic investigations and the orientation of horizontal stress direction flow paths were estimated.