AN EXPERIMENTAL-STUDY AND THEORETICAL-STUDY OF A PARTIALLY SATURATED COLLAPSIBLE ROCK

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.