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VSP Orthorhombic Anisotropy Inversion for Fractured Reservoir Characterization
- Publisher: European Association of Geoscientists & Engineers
- Source: Conference Proceedings, 3rd EAGE Workshop on Borehole Geophysics, Apr 2015, cp-447-00011
- ISBN: 978-94-6282-144-6
Abstract
The slowness-polarization method was developed to infer VTI/TTI anisotropy local to a walkaway receiver array, and successful case studies have been previously reported (Leaney and Hornby, 2007). We consider vertical orthorhombic anisotropy as the next target because a VTI formation under stress could present orthorhombic anisotropy with a vertical symmetry axis (VOR) due to stress-induced parallel vertical fractures in a background VTI medium. Usually, estimating lower symmetry anisotropy than VTI requires a 9C 3D VSP survey because quasi-P (qP) and two quasi-S waves are needed (Rusmanugroho and McMechan, 2012). However, if other measurements (walkaround VSP, fast and slow dipole sonic, microseismic, etc.) already constrain fracture azimuth, walkaway VSPs parallel and normal to the fracture strike will provide slowness – polarization evidence of vertical parallel fracture sets in VTI formations. We present VOR anisotropy slowness-polarization inversion using walkaways parallel and normal to the fracture strike acquired from a conventional P-wave source. Test results are presented.