HDR Experiments at Falkenberg/Bavaria

Abstract

A test site for HDR experiments has been set up in granite in the Bohemian massif in east Bavaria. A large fracture was stimulated by hydraulic fracturing in a depth of 250 m. The orientation of the fracture was investigated by seismo-acoustic recording during the frac-experiments and by examining the fracture pattern initiated in the injection interval of the borehole by an impression packer and by acoustic televiewer measurements. Geoelectric measurements for fracture location at the surface were also tested before attempting to drill through the fracture. The fracture has now been intersected by 6 additional boreholes at distances between 2 m and 70 m from the initiation interval. According to the results of seismo-acoustic recording, temperature logging and acoustic televiewer measurements in the intersection boreholes, a single plane artificial fracture has been produced, striking E-W and dipping 50°S. The orientation of the fracture, which should be normal to the least compressive principal stress, and the value of the normal stress on the fracture are not consistent with the direction and the values of the principal stresses determined by wire line hydraulic fracturing stress measurements. Various hydraulic experiments were performed in the different boreholes to investigate the mechanical reaction of the fracture due to varying fluid pressure, the hydraulic properties of the fracture, the fluid losses and the heat exchange within the fracture. The models developed to interpret the results make it possible to translate the results to greater depth and higher flow rates.