reservoir geomecanics

308 Reservoir geomechanics
a. b.
c.
31 31.5 32
32 34 36 38 40 44 46 48 50 52
Required P m Required P m
Required P m
Figure 10.4. The effects of wellbore trajectory and stress state on wellbore stability. The parameters
used in this figure are the same as those used for the calculations shown in Figure 8.2. The figure
shows the mud pressure (in ppg) required to drill a stable well (maximum breakout width 30 ◦
for a relatively strong rock (UCS ∼50 MPa) as a function of well orientation at a depth of 3 km
for hydrostatic pore pressure: (a) normal faulting, (b) strike-slip faulting and (c) reverse
faulting.
was used for all calculations, recognizing that in practice, this would be a very conservative
approach. As for Figure 8.2, hydrostatic pore pressure (32 MPa) was used.
For the case of normal faulting (S v = 70 MPa, S Hmax = 67 MPa, S hmin = 45 MPa)
shown in Figure 10.4a, a mud weight as low as ∼30 MPa (slightly underbalanced) is
sufficient to drill as stable well that is near vertical, but higher mud weights are needed
to achieve well stability for deviated wells depending on orientation. However, the most
unstable orientations (horizontal wells drilled parallel to either S Hmax or S hmin ) require
a mud weight that balances the pore pressure (32 MPa). In this case, the relatively
high rock strength (50 MPa) and relatively low stresses associated with normal faulting
environments combine to make drilling stable wells at almost any orientation easily
achievable. For the case of strike-slip faulting (S Hmax = 105 MPa, S v = 70 MPa, S hmin =
45 MPa) shown in Figure 10.4b, it is clear that for all orientations except highly deviated
wells parallel to S Hmax , mud weights of 40–42 MPa (corresponding to ∼1.28 sg
or ∼10.7 ppg) are required to achieve the desired degree of stability. As the stresses
are both larger in magnitude and more anisotropic than for the normal faulting case,
the well trajectory has a more important effect on wellbore stability. For the case of
reverse faulting (S Hmax = 145 MPa, S hmin = 125 MPa, S v = 70 MPa) shown in Figure
10.4c, still higher mud weights are needed at all wellbore orientations because of the
very high stress magnitudes. The most unstable wells (i.e. those requiring the highest
mud weights) are near vertical (deviations