Page 121 - Reservoir Geomechanics

105 Rock failure in compression, tension and shear

a. 700 b. 600

600 BEREA SANDSTONE BEREA SANDSTONE
500
500
400
S 1 (MPa) 400 S 1 − S 3 (MPa) 300

300
S = 62.8 + 2.82 S 200
200 1 3
100
100
0 50 100 150 200 S = 500 MPa
3
0 0
0 50 100 150 200 250 0 100 200 300 400 500 600
S 3 (MPa) P (MPa)
p
c. 700 d. 600

600 MARIANNA LIMESTONE MARIANNA LIMESTONE
500
500
400
S 1 (MPa) 400 S 1 − S 3 (MPa) 300
300
S = 40.8 + 3.01 S 200
200 1 3
100
100
0 50 100 200 S = 500 MPa
0 0 3
0 50 100 150 200 250 0 100 200 300 400 500 600
S (MPa) P (MPa)
p
3
Figure 4.11. (a) Dependence of rock strength on confining pressure in the absence of pore pressure
for Berea sandstone. (b) Dependence of strength on confining pressure and pore pressure assuming
the simple Terzaghi effective stress law (equation 3.8)isvalid (straight diagonal lines). (c) and
(d) show similar data for Marianna limestone. Data derived from Handin, Hager et al.(1963).

Rock strength anisotropy

The presence of weak bedding planes in shaley rocks (or finely laminated sandstones
or foliation planes in metamorphic rocks) can sometimes have a marked effect on rock
strength. In Chapter 10 we will investigate several cases that illustrate the importance of
slip on weak bedding planes on wellbore stability when wells are drilled at an oblique
angle to the bedding planes.
The influence of weak bedding planes on rock strength is referred to as strength
anisotropy. The importance of this depends both on the relative weakness of the bedding