Page 250 - Reservoir Geomechanics

231 Determination of S 3 from mini-fracs

excess mud weight or cooling of the wellbore wall. This represents an upper bound
value of S Hmax . The uncertainty in the values of S Hmax (such as those shown in Figure
7.13) can be taken into account when the values are used for consideration of wellbore
stability or fault reactivity as described in subsequent chapters.
A point not mentioned above is that for the case study shown in Figure 7.10,itwas
important to have observations of tensile fractures in the well in order to constrain the
magnitude of both S hmin and S Hmax . This is because the least principal stress was so close
to the magnitude of the vertical stress that it was not clear if S 3 corresponded to S v or
S hmin .If the former, it would have indicated a thrust faulting regime and the magnitude
of S hmin would have been unknown. However, because there are drilling-induced tensile
fractures in the well, the stress state had to be a strike-slip/reverse faulting regime with a
stress state corresponding to the upper left corner of the stress polygon. If S hmin had been
appreciably larger than S v , drilling-induced tensile fractures would not have formed.

Estimating rock strength from breakouts when tensile
fractures are present

There is an interesting extension of the discussions above when both breakouts and
drilling-induced tensile fractures occur in a well. As noted above, if there is no infor-
mation on rock strength available, one cannot use observations of breakout width to
constrain S Hmax because breakout width is a function of both S Hmax and rock strength.
When both breakouts and drilling induced tensile fractures are present in a well, S Hmax is
determinable from the occurrence of tensile fractures alone. Thus, the width of break-
outs allows us to estimate the strength of the rock in situ.For example, in the case
illustrated in Figure 7.10, had the strength been appreciably less than 138 MPa, the
breakout width would have been greater. Had the strength been greater than 138 MPa,
the breakouts would have been narrower (or absent altogether). Because the width of
breakouts can be measured accurately (Chapter 6), the occurrence of drilling-induced
tensile fractures not only yields estimates of S Hmax ,in the presence of breakouts, they
provide a means to obtain a direct in situ estimate of rock strength, as well.

Estimating S Hmax from breakout rotations

In areas of active faulting, wells penetrate formations where there are localized stress
perturbations due to slip on faults. These perturbations are manifest as rotations of
breakout (and/or drilling-induced tensile fracture) azimuth along the wellbore as a
function of depth. These have been seen in oil and gas wells in several parts of the
world and this subject is briefly revisited in Chapter 11.
One example of a breakout rotation can be seen in the ultrasonic televiewer in the
left panel of Figure 7.14 from the KTB scientific research well in Germany. As first