Good Agreement of Fracture Sets Detected by Acoustic Televiewer with Fracture Clusters Revealed by Microseismicity Within the Basel EGS Reservoir

Abstract

The high precision hypocenter locations and focal mechanism solutions of the micro-seismicity during the Basel EGS stimulation were compared to the fracture sets and high fracture frequency zones observed with the acoustic televiewer at the borehole. The focal mechanisms provide orientations for all micro-seismic clusters that match one of the three most common fracture sets determined by borehole televiewer data within the stimulated depth between 4 km and 5 km. Additionally, these sets share the same rake. Clusters in set 2 are characterized by a strong normal component, while set 1 and set 3 are pure strike-slip (right lateral and left lateral respectively). The calculated intersection of each cluster with the borehole also show a match with the observed areas of high fracture frequency at the borehole and coincide with the dominant fracture orientation at that interval. The arrangement of the three sets is concordant with the theoretical Reidel shear fractures where set 1 and set 3 would correspond to the antithetic R' and synthetic R components, while set 2 would be the T component (extension) parallel to the maximum regional horizontal stress. The consistency of the two independent data sets adds confidence on their good representation and understanding of the main geo-mechanical characteristics of the Basel EGS stimulation. Furthermore, we conjecture that good acoustic televiewer data could be utilized for other projects to predict EGS reservoir creation before stimulation.