Page 156 - Reservoir Geomechanics

139 Rock failure in compression, tension and shear

Figure 4.31bagain illustrates the fact that elevated pore pressure reduces the differ-
ence between principal stresses at depth as shown previously in Figure 4.30. When pore
pressure is elevated, all three principal stresses are close in magnitude to the vertical
stress and relatively small changes in the stress field can cause a transition from one
style of faulting to another. Moos and Zoback (1993)hypothesize that because of ele-
vated pore pressure at depth in the vicinity of Long Valley caldera, the style of faulting
goes from NF/SS faulting on one side of the caldera to RF/SS faulting on the other side
as the direction of the horizontal principal stresses change.
The stress polygon shown in Figure 4.31a permits a very wide range of stress values
at depth and would not seem to be of much practical use in limiting stress magnitudes.
However, as extended leak-off tests or hydraulic fracturing tests are often available to
provide a good estimate of the least principal stress (Chapter 6), the polygon is useful
for estimating the possible range of values of S Hmax .As noted above, we will illustrate in
Chapters 7 and 8 that if one also has information about the existence of either compres-
sive or tensile wellbore failures, one can often put relatively narrow (and hence, useful)
bounds on possible stress states at depth. In other words, by combining the constraints
on stress magnitudes obtained from the frictional strength of the crust, measurements
of the least principal stress from leak-off tests and observations of wellbore failure
place strong constraints on the in situ stress state (Chapters 6–8) which can be used
to address the range of problems encountered in reservoir geomechanics addressed in
Chapters 10–12.