Page 261 - Reservoir Geomechanics

242 Reservoir geomechanics

discussed for the case of vertical wells in Chapters 6 and 7. Conversely, in normal and
reverse faulting environments, drilling-induced tensile fractures are expected to occur
at mud weights close to the pore pressure only in highly deviated wells. In normal
faulting areas, such fractures are expected in sub-horizontal wells drilled parallel to
S Hmax , whereas in reverse faulting environments the same is true for sub-horizontal
wells drilled parallel to S hmin .
As mentioned above, the orientation of the wellbore breakouts in deviated wells
depends on the orientation of the well with respect to the stress field and in situ stress
magnitudes. Figure 8.4a shows the orientation of breakouts for deviated wells in a
strike-slip faulting regime with S Hmax acting in the NW–SE direction. The orientations
of breakouts (if they were to occur) are shown in a looking down the well reference frame
(see inset). Thus, wells deviated to the northeast or southwest would have breakouts
on the top and bottom of the well whereas deviated wells drilled to the southeast or
northwest would have breakouts on the sides. The orientations of tensile fractures (if
they were to occur) are shown in Figure 8.4b. The two lines indicate the position of
the tensile fractures around the well and the angle with respect to the wellbore axis
(see Peska and Zoback 1995 and the inset). As noted by Brudy and Zoback (1993) and
Peska and Zoback (1995), drilling-induced tensile fractures in deviated wells generally
occur as en echelon pairs of fractures which are inclined to the wellbore wall at the
angle ω, referred to above. In Chapter 10,we discuss how hydraulic fractures that form
at the wellbore wall as en echelon tensile fractures propagate away from a well must
coalesce (link-up)as they turn and become perpendicular to the least principal stress.
A similar situation is discussed by Baumg¨artner, Carvalho et al.(1989) for the case of
hydraulically fracturing a vertical well when the least principal stress is vertical (reverse
faulting regime). Axial tensile fractures form at the wellbore wall when the σ θθ goes to
zero (as a result of borehole pressurization), but the fractures roll-over into a horizontal
plane as they propagate away from the well.
To make some of the previous calculations more relevant to wellbore stability,
Figure 8.5 was calculated using the stress state used in the construction of Figure
6.3. This is a strike-slip stress state with S Hmax acting in an E–W orientation. However,
the colors in Figure 8.5anow indicate the width of breakouts for wellbores of any arbi-
trary orientation at the depth of interest in the prescribed stress state assuming a uniaxial
compressive strength of 45 MPa, a coefficient of internal friction of 1.0 and a Mohr–
Coulomb failure criterion. As can be seen by comparison with Figure 6.3,vertical wells
are expected to have breakout widths of about 90 .Wellbore deviations up to about 30 ◦
◦
(independentofazimuth)haveasimilardegreeofinstability,asdowellsofanydeviation
drilled approximately east–west. Breakout orientations in east–west striking wells are
expected on the sides of the hole whereas those trending north–south would be expected
to have breakouts on the top and bottom (Figure 8.5b). Note that highly deviated wells
drilled in the north–south direction are much more unstable as breakouts with much
greater width would be expected to occur. In fact, such wells would undoubtedly be