Online Journal for E&P Geoscientists

Planchart, Carlos ^*1
(1) Aramco, Dhahran, Saudi Arabia.

Shear waves propagating through an anisotropic medium split into approximately two orthogonal phases with different velocities. The faster shear wave is polarized along the fracture planes, while the slower shear wave is polarized in a direction orthogonal to the fracture planes. The effect of shear-wave splitting can be observed in radial and transverse components of walkaround vertical seismic profile (VSP) data. If azimuthal anisotropy is detected, both the radial and the transverse components show a sinusoidal variation of arrival times with azimuth. In the transverse component, the coherent energy may go to zero along the symmetry axes of the fracture planes and the polarity may reverse along these axes.

Theoretical models and observations suggest that in fractured reservoirs, dispersion varies azimuthally and may be correlated to saturating fluid and fracture size. For this reason, azimuthal dispersion analysis has been proposed as a method to discriminate between open and closed fractures (Chapman, 2009).

In this paper, shear-wave splitting analysis for walkaround VSP data was applied for the characterization of fractures in the Marrat and Hith formations in Saudi Arabia. The downgoing and upgoing shear waves were separated in the horizontal components and used to identify shear-wave splitting intervals. Also, the transverse-component common receiver gathers were analyzed to investigate polarity reversals and shear-wave traveltime variations to identify the symmetry axes and fast shear direction. Dispersion of the reflected P waves was also investigated.

The walkaround VSP confirmed the presence of near vertical fractures within the Hith and Marrat formations. Also, shear-wave splitting identified the presence of relatively strong anisotropy within the interval considered hydrocarbon source rock. For the salt interval (Hith formation), a non-orthogonal set of fractures is predicted with the difference of azimuth being larger than 30 degrees. The dominant E-W fractures are presumably cemented while the less dominant N-S fractures are presumed open based on the P-wave dispersion. For the Marrat formation, a N-S non-orthogonal set of fractures, with the difference of azimuths being small (less than 30 degrees) is predicted.