Page 114 - Applied Petroleum Geomechanics

106 Applied Petroleum Geomechanics

Figure 3.15 Compressive strength (or s 1 s 3 ) of the anisotropic rocks as function of
the angle a between the maximum principal stress (s 1 ) and weak planes under the
conventional triaxial compression (Donath, 1964).

The effect of the stress states on failure behavior in anisotropic rocks has
been studied by many researchers (e.g., Donath, 1964; McLamore and
Gray, 1967; Niandou et al., 1997; Brady and Brown, 2004; Mogi, 2007)
using triaxial compressive tests (s 1 > s 2 s 3 ). Donath extensively studied
the strong influence of planar anisotropy on rock strength. Fig. 3.16 shows
schematically his experimental method and the observed results (Donath,
1964). The right graph in Fig. 3.15 shows the differential stress (s 1 s 3 )at
failure as a function of angle a for different confining pressure
(P c ¼ s 2 ¼ s 3 ). Here a is the angle between the maximum stress (s 1 ) and
the orientation of the weak planes (or bedding planes). As can be seen in
this figure, the compressive strength of the anisotropic rock is the lowest at

around a ¼ 30 . Fig. 3.16 presents laboratory test results of the compressive
strength of Tournemire shale versus the plane of weakness. Again, it shows
that the compressive strength of the anisotropic rocks is the lowest at

a z 30 . These results show that failure of anisotropic rocks is likely to
occur when the angle a is nearly equal to the shear failure angle of isotropic