Model for Pore-Fluid Induced Degradation of Soft Rocks

Abstract

Porous rocks, particularly chalk, are known to behave differently when saturated with different pore fluids. The mechanical behavior of these rocks varies with different pore fluid composition and additional deformation occurs when the pore fluid composition changes. In this article, we review the evidence that behavior of porous rocks is pore fluid dependent, and present a constitutive model for pore fluid dependent porous rocks. Our review indicates that theories of Unsaturated Soil Mechanics (USM) are not fully applicable to the modeling of the effects of pore fluid composition on soft rocks such as chalk. Instead of using USM, the paper proposes a model that is based on chemo-plasticity whereby the material response is dependent on the pore-fluid composition, and the material can degrade with changes in pore-fluid composition. Three degradation matrices are introduced, which are namely the elastic, elastoplastic and viscoplastic degradation matrices, to model, respectively, the reduced elastic stiffness, reduced shear strength, and the lower pore collapse strength and accelerated time-dependent deformation of soft rocks due to changes in pore-fluid composition. Comparisons of model predictions with published experimental data indicate that the model is capable of reproducing observed behavior of chalk under a variety of loading and pore fluid conditions.