Page 184 - Reservoir Geomechanics

6 Compressive and tensile failures

in vertical wells

The principal topics I address in this chapter are the relationships among in situ stress
magnitudes, rock strength and the nature of compressive and tensile failures that can
result from the concentration stress around a wellbore. To establish the principles
of wellbore failure with relatively simple mathematics, I consider in this chapter only
vertical wells drilled parallel to the vertical principal stress, S v .In Chapter 8 I generalize
this discussion and consider deviated wells of arbitrary orientation in an arbitrarily
oriented stress field.
In some ways every well that is drilled can be thought of as a rock mechanics exper-
iment. The formation surrounding the wellbore wall is subject to a stress concentration
that varies strongly with the position around the well and the distance from the well-
bore wall. The way in which this formation responds to the stress concentration is a
function of both the stress field and rock strength. As discussed in Chapter 7, detailed
knowledge of the nature of wellbore failure (especially as revealed by wellbore imag-
ing, as discussed in Chapter 5) allows one to estimate (or constrain) the magnitude and
orientation of in situ stresses at depth. In some cases it also allows one to obtain direct
information about rock strength in situ.
The concentration of stress around wellbores can lead to compressive failures known
as stress-induced breakouts and/or tensile failure of the wellbore wall that we will refer
to as drilling-induced tensile wall fractures. Breakouts are quite common in many wells
and yield important information about both stress orientation and magnitude. How-
ever, excessive wellbore breakouts can lead to problematic (potentially catastrophic)
wellbore instabilities. I address methods to analyze and mitigate such problems in
Chapter 10.
In vertical wells, the occurrence of tensile fractures in a well usually implies (i) that
S hmin is the minimum principal stress and (ii) there are large differences between the
two horizontal principal stresses, S Hmax and S hmin .As discussed below, the occurrence
of tensile fractures is also influenced by high mud weight and cooling of the wellbore
wall. The processes that control the initiation of tensile wall fractures are important for
understanding the initiation of hydraulic fractures (Hubbert and Willis 1957; Haimson
and Fairhurst 1967). However, hydrofracs are distinguished from tensile wall fractures
in that they propagate from the wellbore into the far field,away from the wellbore
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