Elastoplastic deformation of a porous rock and water interaction

Abstract

In this paper, a new constitutive model is proposed for the description of elastoplastic behaviour of a porous chalk, including creep deformation and effect of water saturation. The chalk is represented by a two-phase porous composite, a homogenized equivalent matrix including solid grains, different contacts between grains, and a connected porosity. Two plastic flow mechanisms are identified: inelastic pore collapse at high confining pressure and plastic shearing at low confining pressure. The plastic yield stress of the equivalent solid matrix and the porosity are used as the fundamental parameters in the macroscopic yield function for pore collapse mechanism. The plastic shearing mechanism describes plastic distortion and failure by the formation of shear bands. Influences of water saturation on the mechanical behaviour are decomposed into a short term and a long term effect. Water saturation induces the diminution of capillary pressure in liquid contact, leading to an instantaneous plastic deformation. As a long term effect, water saturation enhances the pressure dissolution of cemented contacts, resulting in progressive weakening of pore collapse yield stress and failure strength. Comparisons between numerical predictions and experimental data are presented in different loading conditions to verify the predictive capacity of the proposed model.