An experimental and theoretical study of a partially saturated collapsible rock

Abstract

Experiments on Pietra Leccese chalk revealed that many of the material parameters describing its mechanical behaviour, such as the Young's modulus, the uniaxial compressive strength and the plastic yield stress, depend strongly on the water saturation and the ensuing capillary suction. The experimental results show that Bishop's generalized effective 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 partially-saturated collapsible rocks is proposed where the capillary suction is assumed to affect primarily the material properties related to the intergranular cohesion, such as the elastic stiffness, the plastic tension 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 variable. In low levels of water saturation, as is often the case in oil reservoir formations, the experimental and theoretical results demonstrate the potential significance of capillary effects on the response of reservoir rocks, a mechanism that has been largely underestimated in the petroleum industry. © 1997 John Wiley & Sons, Ltd.