Page 74 - Reservoir Geomechanics

58 Reservoir geomechanics

60
s
1

50 FAILURE
s
3
ONSET OF
INELASTIC BEHAVIOR
40

UNLOADING
Stress (MPa) 30 ELASTIC BEHAVIOR YOUNG’S MODULUS
(DYNAMIC)

E
20
LOADING
(STATIC)
YOUNG’S MODULUS

10

CRACK CLOSING

0
0 4 8 12 16 20
x 10 −5
Strain
Figure 3.2. Typical laboratory stress–strain data for a well-cemented rock being deformed
uniaxially. There is a small degree of crack closure upon initial application of stress followed by
linear elastic behavior over a significant range of stresses. Inelastic deformation is seen again just
before failure due to damage in the rock.

in Figure 3.2 to illustrate how a relatively well-cemented sandstone exhibits nearly ideal
elastic behavior over a considerable range of applied stresses. As axial stress is applied
to this rock, there is some curvature in the stress–deformation curve upon initial loading
due to the closure of micro-cracks. Once these cracks are closed (at a stress of about
9MPa),therockexhibitslinearelasticbehavioruntilastressofabout45MPaisreached.
At this pressure, the stress applied to the rock is so large that it begins to damage
the rock such that permanent, or plastic, deformation is observed prior to eventual
failure of the sample at a stress of about 50 MPa (rock failure is discussed at length in
Chapter 4).