reservoir geomecanics

175 Compressive and tensile failures in vertical wells
considered. In the general region of the maximum stress concentration around the well
(θ = 0 ◦ , 180 ◦ ), wherever the stress concentration exceeds the strength of the rock,
failure is expected. Thus, the zone of compressive failure (initial breakout formation)
within the contour line in Figure 6.3c indicates the region of initial breakout formation
using the strength of materials concept first introduced in Chapter 3. The growth of
breakouts after their initial formation is discussed later in this chapter.
The most reliable way to observe wellbore breakouts is through the use of ultrasonic
image logs that were described in Chapter 5. Asshown in Figure 6.4a, a standard
unwrapped televiewer images breakouts as dark bands of low reflectance on opposite
sides of the well. Interactive digital processing allows cross-sections of a well (such as
that shown in Figure 6.4c) to be easily displayed (Barton, Tessler et al. 1991), which
makes it straightforward to determine both the orientation and opening angle, w BO ,
of the breakouts. Breakouts form symmetrically on both sides of the well, but during
routine data analysis, the orientations of the breakouts are documented independently
(e.g. Shamir and Zoback 1992). The two out-of-focus zones on opposites sides of
the well in the electrical image shown in Figure 6.4b also correspond to breakouts.
These result from poor contact between the wellbore wall and the pad upon which the
electrode array is mounted. At any given depth, the azimuth of maximum horizontal
stress is 90 ◦ from the mean of the azimuths of the breakouts on either side of the
well. As illustrated below, comprehensive analysis of breakouts in wellbores can yield
thousands of observations, thus enabling one to make profiles of stress orientation (and
sometimes magnitude) along the length of a well.
It is easily seen in the equations above that if we raise mud weight, σ θθ decreases
(and σ rr increases) at all positions around the wellbore. This is shown in Figure 6.5a for
P = 10 MPa (compared to Figure 6.3a). As a point of reference, at a depth of 3213 m,
this is equivalent to about a 10% increase in excess of hydrostatic pressure. Two phenomena
are important to note. First, with respect to compressive failures, by increasing
the mud weight, the zone of failure is much smaller in terms of both w BO and breakout
depth (the dashed lines indicate w BO in Figure 6.3). This is shown in Figure 6.5b which
was calculated with exactly the same stresses and rock strength as Figure 6.3c, except
for the change in P m . This is because as P increases, σ θθ decreases and σ rr increases
such that the size of the Mohr circle (Figure 6.3b) decreases markedly in the area of the
wellbore wall subjected to most compressive stress. This demonstrates why increasing
mud weight can be used to stabilize wellbores, a subject to be considered at length in
Chapter 10.
Introduction to drilling-induced tensile fractures
The second point to note about wellbore failure is that as P increases and σ θθ
decreases, the wellbore wall can locally go into tension at θ = 90 ◦ , 270 ◦ and contribute
to the occurrence of drilling-induced tensile fractures. This is illustrated in Figure 6.5a.