Interpretation of Reservoir Structure from Super-Resolution Mapping of Microseismic Multiplets from Stimulation at Basel, Switzerland in 2006

Abstract

The authors used the multiplet method to re-locate approximately 2000 microseismic events collected during a hydraulic stimulation in the Deep Heat Mining Project at Basel, Switzerland in December 2006. The microseismic events were clustered into multiplet groups by quantitatively evaluating the waveform similarity using the magnitude square coherence function in the frequency domain. This made it possible for us to cluster the events by different criteria which correlate to different physical phenomena associated with shear slip in the fracture system. We then interpreted the behavior of each multiplet cluster from the orientation and size of the multiplet seismic structure, fault plane solution, source radius, and hydraulic record. It has been revealed that the multiplets identified in this study correlate to (a) identical direction of shear slip on a macroscopic single fracture or thin fracture network, (b) repeating slip within a small-scale fracture, and (c) extension of rupture zone on a small-scale fracture or an asperity. From the multiplet analysis, it can be said that several sub-vertical fractures or thin fracture networks with an extension of 200-400m were mainly stimulated at Basel. Strike-slip was the dominant rupture mechanism and improvement of vertical permeability can be expected by break through of the asperities.