Page 589 - Petrophysics

556 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

ux

1
UZ
Figure 9.2a. Tkee4rnensional stresspeld of a cubic element.

sample in a threedimensional stress field, as shown in Figure 9.2a [5].
To understand the significance of the diagram, a number of important
physical concepts associated with the mechanical behavior of rocks must
be defined.

STRESS ANALYSIS

If a rock body is subjected to an external load or force, internal stresses
are developed. If these stresses are strong enough, the rock deforms.
Deformation refers to changes in shape (distortion) accompanied by
change in volume (dilation). Three basic internal stress conditions are
recognized: compressive, shear, and tensile, as illustrated in Figure 9.2b
[6]. Compressive stresses occur when external forces are directed toward
each other along the same plane. If the external forces are parallel
and directed in opposite directions along the same plane, tensile stress
develops. Shear stress occurs when the external forces are parallel and
directed in opposite directions, but in different planes.
pq-=iqw

Tension Compression Couple
A 6 C
Figure 9.2b. Representation of tension, compression, and shear or couple.