Page 219 - Reservoir Geomechanics

200 Reservoir geomechanics

a. b.

Figure 6.17. The area in which wellbore breakouts form around a cylindrical well can be modeled
using a total plastic strain criterion rather than a stress criterion. These finite element calculations
indicate the zone of expected breakouts assuming a critical strain level at which failure occurs
(courtesy S. Willson). (a) Strain around a wellbore assuming a strain softening model of rock
deformation (red indicates high strain). (b) Failure zone predicted using a strength of materials
approach and Mohr–Coulomb failure criterion.

through zones of near-horizontal bedding. In the first case, wellbore stability, even
when drilling near-vertical wells, must take into account the presence of weak bedding
planes (Willson, Last et al. 1999). In the second, small changes in wellbore deviation
and azimuth can have a significant effect on wellbore stability depending on whether
slip on weak bedding planes is activated by the stress concentration around the well.
As shown in two case studies presented in Chapter 10, when weak bedding planes are
present, their presence needs to be incorporated into wellbore stability calculations.
With respect to predicting breakout widths using the strength of materials approach
adopted here, it is important to note that a number of relatively comprehensive theories
have been developed to evaluate the formation of breakouts. For example, Vardu-
lakis, et al.(1988)investigated breakout formation in terms of bifurcation theory and
Germanovich and Dyskin (2000)investigated breakout formation in terms of micro-
crack growth utilizing fracture mechanics theory. There is no doubt that such theories
may eventually lead to a more complete and useful understanding of breakout formation
than the relatively simple theory discussed here. Nonetheless, we shall see in future
chapters that even relatively simple theories of rock failure can be quite effective in
predicting wellbore failure with sufficient accuracy to be quite useful for both stress
estimation (Chapter 7) and prediction of wellbore stability (Chapter 10).
One important approach for predicting the zone of failure around a well is to utilize
an elastic–plastic failure criterion and predict the zone of failure around a well in terms
of a total plastic strain failure criterion. In practice, such calculations are performed
using a numerical analysis technique such as the finite element method. An example of
such calculations is shown in Figure 6.17 (courtesy S. Willson). The colors indicate the
plastic strain calculated with both a strain softening model (Figure 6.17a) and a standard
Mohr–Coulomb model (Figure 6.17b). Note that the shapes of the breakouts are quite