E. Papamichos et al., AN EXPERIMENTAL-STUDY AND THEORETICAL-STUDY OF A PARTIALLY SATURATED COLLAPSIBLE ROCK, MECHANICS OF COHESIVE-FRICTIONAL MATERIALS, 2(3), 1997, pp. 251-278
Experiments on Pietra Leccese chalk revealed that many of the material
parameters describing its mechanical behaviour, such as the Young's m
odulus, the uniaxial compressive strength and the plastic yield stress
, depend strongly on the water saturation and the ensuing capillary su
ction. The experimental results show that Bishop's generalized effecti
ve stress is not by itself adequate to explain the observed behaviour
and in particular the spontaneous collapse which may occur upon water
injection under stress. An elastoplastic constitutive model for partia
lly-saturated collapsible rocks is proposed where the capillary suctio
n is assumed to affect primarily the material properties related to th
e intergranular cohesion, such as the elastic stiffness, the plastic t
ension cut-off and pressure cap parameters. The model builds upon the
well-established Mohr-Coulomb theory of elastoplasticity by including
a pressure cap and considering the water saturation as an internal var
iable. In low levels of water saturation, as is often the case in oil
reservoir formations, the experimental and theoretical results demonst
rate the potential significance of capillary effects on the response o
f reservoir rocks, a mechanism that has been largely underestimated in
the petroleum industry. (C) 1997 by John Wiley & Sons, Ltd.