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Borehole collapse is predicted by common rock failure criteria despite they give conservative results due to some simplification of rock behavior. It is the source of many difficulties in implementation of oil well, underground structures and foundations. Utilizing damage theory could improve the conservancy of rock failure criterion. Damage theory considers loss of function rather than loss of strength. Unlike criteria which were derived from plasticity that assume yielding point as the limit of functionality of rock, damage theory uses a state between failure and yielding in stress-strain diagram as the limit of the rock applicability. In this study, a new model is proposed to predict minimum borehole pressure based on rock damage theory. The model is verified by actual wellbore data. The proposed model predicts minimum borehole pressure close to actual data and more accurate than Mohr-coulomb and Mogi-Coulomb criteria. [ABSTRACT FROM AUTHOR]

Abstract: In order to provide a physical interpretation of the variation of the mechanical properties of Callovo-Oxfordian argillite with mineral composition, we implement three linear homogenization schemes. The argillite is modeled as a three phase material composed of a clay matrix and inclusions of quartz and calcite. It is shown that, unlike the dilute scheme and the self-consistent scheme, the Mori-Tanaka model describes the in situ experimental data well. The determined properties are finally used in a finite element computation. The aim is to evaluate the effect of mineral composition on the elastic response of the excavation of a vertical shaft in the context of the underground laboratory of Meuse/Haute Marne. [Copyright &y& Elsevier]

The microscopic characteristics greatly influence the nonlinear mechanical behavior and failure process of rock materials. In the present work, the experimental investigations are firstly performed including microstructural observations and macroscopic laboratory tests. Based on various experimental evidences, a micromechanical damage model is proposed according to the microscopic characteristics for the description of the local mechanical features such as mineralogical composition, plastic deformation and failure process caused by induced damage. The microstructure of the studied rocks is artificially reconstructed using Fast Fourier Transforms (FFT) based technique without meshing the complex geometry composed of clay matrix and two randomly distributed mineral inclusions. Comparisons between experimental data and numerical predictions are presented for uniaxial and triaxial compression tests. The characterization of damage evolution is visualized by the direct full field numerical simulations corresponding the macroscopic mechanical response. The effects of induced damage, the confining pressure and the overburden depth on the nonlinear mechanical behavior and failure process are schematically studied. [ABSTRACT FROM AUTHOR]