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Technical Committee Earth Sciences

Localisation of induced microseismicity in anisotropic media

In this study new methods for the effective localisation of acoustic emissions were developed and applied on two different data sets. The usually applied "straight ray approximation" was relaxed since it is inappropriate for reservoir environments. The newly developed tools enable to localise acoustic emissions in heterogeneous and anisotropic media. Using numerical case studies the localisation error for different acquisition geometries was investigated and quantified assuming realistic errors for event picking. These case studies also allowed to investigate the planned one hole configuration in detail and to optimise it for the Völkersen fracture experiment. The complete dataset recorded during Völkersen test was analysed and different events were identified. These events, however, are not linked to the generation of a hydraulic fracture but have other reasons (strokes at the rig, cable vibrations, resonances). Since a sufficient sensitivity for the used configuration was proved by an airgun experiment, we conclude that no new fracture was produced. The gas production rate increased after the experiment which indicates that an existing fracture generated in 2001 was widened during this test.
In an appendix it is proven that significant errors occur in the localisation of acoustic emission at the German Continental Borehole (KTB) if a present anisotropy of the rocks is not considered.

D. Gajewski, R. Patzig, K. Sommer
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