Page 122 - Reservoir Geomechanics

106 Reservoir geomechanics

b.
t
= 0.7
m 0
m = 0.5
w

a.
S 0

2b
b S w 2b w 0
s 3 s 1 min s 1 max s

c.
15

s 1
S w
10
s 3
2
S w
1
5
0

b w
0
30 60 90
b
Figure 4.12. Dependence of rock strength on the angle of weak bedding or foliation planes.
(a) Rock samples can be tested with the orientation of weak planes at different angles, β,tothe
maximum principal stress, σ 1 . (b) The strength can be defined in terms of the intact rock strength
(when the weak planes do not affect failure) and the strength of the weak planes. (c) Prediction of
rock strength (normalized by the cohesion of bedding planes) as function of β. Modified from
Donath (1966) and Jaeger and Cook (1979).

plane and the orientation of the plane with respect to the applied stresses. This is
illustrated in Figure 4.12a, for strength tests with bedding planes whose normal is at an
angle, β,to the applied maximum stress. Intuitively, one can see that when β ∼ 0 or
◦
90 , the bedding planes will have relatively little influence on rock strength. However,
◦
◦
when β ∼ 60 , slip on a weak bedding plane would occur at a markedly lower stress
level than that required to form a new fault. To be more quantitative, one could view
a rock as having two strengths (illustrated in the Mohr diagram in Figure 4.12b). The
intact rock would have its normal strength which would control failure when slip on