Advanced Failure Analysis in Geomaterials: Application to Reservoir Geomechanics

The manifestation of failure in geomaterials and its proper analysis are constitutive aspects that geotechnical engineers are faced with routinely in design. In most instances, geostructures are examined at the ultimate plastic state where failure is deemed to occur along a slip surface where plastic deformations localize. This plasticity condition is classically analyzed with the Mohr-Coulomb failure criterion. However, other forms of failure also exist where the localization of deformations is totally absent such as in the case of static liquefaction. This distinct mode has been coined as ‘diffuse failure’ which has the peculiarity of occurring at stress levels well below the plastic limit, thus rendering a classic Mohr-Coulomb analysis insufficient. Hence, the signature of failure in geomaterials seems to be directly related to two principal modes by which it is manifested: one with localized slips, and another variant where deformations are diffused without any localization phenomena. In order to address the many subtle features of failure, a clear mathematical representation of the underlying physical phenomena is needed. In this thesis, failure is considered as an instability of homogeneous deformations, and as such the observed failuremode is a direct result of the underlying constitutive equations admitting bifurcations in solutions for the material response. Different failure criteria are derived, serving as failure indicators which signal the various modes that emerge during loading history following a certain hierarchy. To translate theory into engineering practice, the thesis endeavors to apply the above mathematical aspects of failure in the study of geomaterials undergoing multiphasic flow and thermal transport such as in the extraction of heavy oil from an oilsand reservoir in Alberta, Canada. Governing equations describing the physics of all phases (solid, water, gas and oil) involved are formulated within mixture theory using continuum mechanics principles. A special computational strategy is adopted to solve efficiently the coupled system of equations using both finite elements and finite differences. Finally, the developed computational model is tested in the context of an actual oil field case study implicating steam injection and oil production in an oilsand reservoir in Alberta, Canada. To close the loop, attention is obviously focused on material failure concepts developed in the first part of the thesis. Geomechanical properties that enter the computational model are obtained from a separate comprehensive laboratory testing of shales and oilsands at high temperature and pressure.