Microseismic reflection imaging and its application to the Basel geothermal reservoir

Abstract

We have developed a new approach for reflection seismic imaging using microseismic waveform data. We took the specific character of microseismic downhole data into account and were able to image regions in the direct vicinity or inside of a seismically active zone. We used a directional migration method that reduced imaging artifacts caused by the low aperture of the data, but required estimation of the wavefield polarization. We also used hodogram analysis to identify parts of data that provided reliable polarization estimates. Only these parts were then used for imaging, whereas the rest of the data are excluded. The approach was successfully applied to microseismic waveforms recorded during and after the stimulation of the enhanced geothermal system in Basel, Switzerland. This data set included more than 2000 events recorded at five sparsely distributed borehole sensors. The reflection images revealed a distinct network of reflectors in the vicinity of the open hole section within the injection well. The results were compared with the injection borehole ultrasonic image revealing a quite good agreement. Combined analysis of imaged reflectors, event locations, and event focal mechanisms indicated that the approach resolved mostly aseismic highly permeable fluid pathways, whereas most of the seismicity occurred in the second-order fractures.