Comparative study of Basel EGS reservoir faults inferred from analysis of microseismic cluster datasets with fracture zones obtained from well log analysis

Abstract

Petrothermal systems seek to extract energy from hot, low-permeability rocks that in northern Switzerland lie at 4–6 km depth. Permeability is enhanced by performing large ‘hydraulic stimulation’ injections, such as occurred in the 5 km deep well at Basel, Switzerland. Knowledge of the discontinuity properties and distribution in target reservoirs can help design such stimulations, but characterisation is challenging because of limited rock mass exposure. Rock mass fractures below Basel were investigated in previous studies by analyses of acoustic televiewer data of the Basel-1 well and by fault plane solutions. In addition, seismic events with high similarity of seismic waveforms were grouped into clusters and interpreted to have originated from slip of patches of a common fault or fault zone. In this study we investigate the orientations of fault patches from seismic clusters, considering relative location errors of cluster events with Monte-Carlo simulations, and use common events in clusters ranging in size from few 10s to many 100s of metres to explore the internal architecture of larger fault zones at Basel. We find that the orientations of microseismically-inferred faults and borehole fractures are similar and that fracture zones consist of sub-parallel or ‘anastomosing’ fractures.