Page 280 - Reservoir Geomechanics

261 Wellbore failure and stress determination in deviated wells

and the orientation will be dependent on the propagation direction. However, the appar-
ent fast direction has to be in the plane perpendicular to the borehole. Therefore, the
apparent fast direction that is observed with the sonic tool will be a line that lies in
both the true fast plane and the plane normal to the borehole, i.e. a line that marks
the intersection of both planes (Figure 8.17c). The goal of this formalism is to allow
one to either determine the true fast direction in the earth given an observed apparent
fast direction or if the formation geometry is known one can predict the apparent fast
direction that will be recorded by the dipole sonic tool for known transverse anisotropy.
Fora borehole with azimuth from north, α, and inclination from the vertical, I, the
vector, B n that defines the axis of the borehole from an arbitrary origin is given by:



2
π

B n =  sin α 1 + sin − I
2


2

π π
× cos α 1 + sin − I − sin − I  (8.9)
2 2
where all angles are in radians. Given the dip, f d , and dip direction, f α ,of the true fast
plane we compute three discrete points, F 1 , F 2 and F 3 ,in the fast plane that has a corner
at the origin used to define the borehole. The normal to the fast plane, F n , may now be
computed using A = F 1 − F 2 and B = F 2 − F 3 , thus giving F n = A × B. The vector
a
defining the apparent fast direction, f ,is then found by computing the vector that is
a
both in the true fast plane and perpendicular to the borehole such that f = B n × F n .
For the arbitrary case of a well with an azimuth of 45 (i.e. northeast) and an incli-
◦
◦
nation of 45 , Figure 8.18 shows the apparent fast direction and dip that will be mea-
sured in the borehole for true fast directions dipping to the north, east, south and west
◦
◦
◦
◦
(i.e. 0 ,90 , 180 and 270 ) overa range of true fast dip angles from horizontal to
◦
vertical (i.e. 0 to 90 ). Typically the azimuth of the fast direction is reported (as a
◦
◦
◦
direction between −90 west and 90 east) but the dip of the fast direction is omitted
as only a vertical T.I. symmetry is considered. However, the dip of the apparent fast
direction can easily be computed given the orientation of the borehole as the observed
azimuth lies in a plane normal to the borehole. For completeness we present both the
◦
◦
azimuth (as an angle between −180 and 180 in the direction of dip) and the dip of
the apparent fast direction. The dip of the fast azimuth provides valuable information
about the true orientation of the fast direction within the formation.
Figure 8.18 illustrates that the apparent fast direction strongly depends on the relative
geometry of the borehole and true fast direction (shown here as a bedding plane). In this
example with a northeast trending borehole, one can see that if the beds dip to the north
the apparent fast direction will be southwest. However, if the beds dip to the east, the
apparent fast direction is southeast. For this borehole trajectory, the dip of the true fast
direction (or bedding planes) has the biggest effect on the apparent fast direction when
the beds are dipping to the south and west, i.e. away from the direction of penetration.