Page 276 - Reservoir Geomechanics

257 Wellbore failure and stress determination in deviated wells

N

Figure 8.14. Map of S Hmax orientations in vertical wells of the Scott Field of the North Sea utilizing
both wellbore breakouts (solid arrows) and the fast shear direction in dipole sonic logs (dashed
arrows) (after Yale 2003).

(or aligned fractures) on velocity anisotropy needs to be taken into account because
aligned features might be encountered at a wide range of orientations to that of the
wellbore and result in a fast direction that is difficult to discriminate from that induced
by stress. As mentioned above, the shear waves generated and received by the dipole
sonic tools are recorded in the planes normal to the axis of the borehole. Thus, the
minimum and maximum shear velocities observed (and used to compute the amount
of anisotropy) are not necessarily the absolute minimum and maximum velocities in
the earth, which may exist in planes that are not perpendicular to the borehole axis.
We define the true fast direction as the orientation in the earth with the absolute fastest
shear velocity (a series of parallel planes described by a dip and dip direction) and the
apparent fast direction as the fastest direction in a plane perpendicular to the borehole.