Page 259 - Reservoir Geomechanics

240 Reservoir geomechanics

a. b. c.
Normal Strike-Slip Reverse

S S
S Hmax Hmax Hmax
Sv Sv Sv

S hmin S hmin S hmin
75 80 85 90 95 100 100 120 140 160 180 220 240 260 280 300
Required C 0 Required C 0 Required C 0
S Hmax = 67 MPa S Hmax = 105 MPa S Hmax = 145 MPa
S hmin = 45 MPa S hmin = 55 MPa S hmin = 125 MPa
S = 70 MPa S v = 70 MPa S v = 70 MPa
v
P = 32 MPa P p = 32 MPa P p = 32 MPa
p
P mud = 32 MPa P mud = 32 MPa P mud = 32 MPa
Figure 8.2. The tendency for the initiation of wellbore breakouts in wells of different orientation
for normal, strike-slip and reverse faulting stress regimes. Similar to the figures in Peska and
Zoback (1995). The magnitudes of the stresses, pore pressure and mud weight assumed for each
case is shown. The color indicates the rock strength required to prevent failure, hence red indicates
a relatively unstable well as it would take high rock strength to prevent failure whereas blue
indicates the opposite. The strength scale is different for each figure as the stress magnitudes are
progressively higher from normal to strike-slip to reverse faulting. Note that because these
calculations represent the initiation of breakouts, they are not directly applicable to considerations
of wellbore stability (see Chapter 10).

Figures 8.2a, b and are quite different. Hence, no universal rule-of-thumb defines the
relative stability of deviated wells with respect to the principal stress directions. In the
case of strike-slip faulting, vertical wells are most likely to fail whereas horizontal wells
drilled parallel to S Hmax are most stable. In the case of reverse faulting environments,
sub-horizontal wells drilled parallel to S hmin are most unstable. Again, for vertical and
horizontal wells, these general patterns are somewhat intuitive in terms of magnitudes
of the principal stresses acting normal to the wellbore trajectory.
There are two important additional points to note about these figures. First, the
strength scale is different for each figure. At a given depth, stress magnitudes are
more compressive for strike-slip faulting regimes than for normal faulting regimes and
more compressive still for reverse faulting regimes. Therefore, it takes considerably
higher strengths to prevent breakout initiation in strike-slip regimes than normal fault-
ing regimes and still higher strengths in reverse faulting regimes. Thus, for a given
value of rock strength, wellbores are least stable in reverse faulting regimes and most