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7th Middle East Geosciences Conference and Exhibition
Manama, Bahrain
March 27-29, 2006
1 ENAGEO, PO Box 140, Hassi Messaoud, 30500, Algeria, phone: +213.29.73.77.00, fax: +213.29.73.72.12,
[email protected]
2 ENAGEO
Natural micro-fractures are very important in the control of production in the hydrocarbon reservoirs. The presence of the
vertical fractures in the rock mass causes the split of the incident shear wave into two approximately orthogonal
components with different velocities. Splitting shear wave analysis
permits the estimation of fracture orientation.
In the offset VSP experiment, converted SV waves are generated with varying strengths at particularly all depths.
Consequently, the converted Sv waveforms partially overlap with direct P waveforms makes the separate event
analysis
difficult and inaccurate.
In this paper, an automatic picking technique was used to accurately compute travel time of P and Sv down -wave. The
polarization angles are determined from particles motion analysis
. The interval velocities Vp and Vs were than computed
using
the travel time inversion technique.
In this study, an attempt was made to determine the orientation of natural fractures by two analysis
methods; shear wave
splitting and P wave
velocity
anisotropy based on an anisotropic ratio computed from four offset VSP data acquired with
different azimuths in the same well
using
the following formula:
Tau = (Vmax - Vmin) / Vmax
Where Tau is the anisotropy ratio, Vmax is the maximum velocity
assuming equal to oblique
velocity
and Vmin is the
vertical
velocity
calculated from zero offset VSP.
In the results, we have found that offsets B, C and D are practically equal (similar angles of incidence), but the corresponding anisotropy ratio are different.
Consequently, the velocity
variations are rather related to the azimuth and the fracture orientation direction which
corresponds to the smallest anisotropy ratio is about 78°.