Numerical evaluation of the deformation behaviour of thick-walled hollow cylinders of shale

The stability of tunnels and boreholes in shales is a major problem. For example in the oil and gas industry, wellbore instability problems cost the industry many millions of dollars annually. In an attempt to minimise instability problems, detailed and careful analyses of the excavation process are often carried out at the planning stage. However, the accuracy of these analyses is highly dependent on the constitutive model adopted for the shale. One important feature of the constitutive behaviour is the dissipation of pore pressure within the shale. In this paper, two FLAG-based models are used to investigate the influence of induced pore water pressure and its dissipation on borehole deformation of a thick-walled hollow cylinder of synthetic shale. The two models are: a time-dependent model that incorporates coupled flow-mechanical interaction and a steady state time-independent analysis that only accounts for mechanical-induced pore pressure. In both models, a linear elastic-plastic constitutive model (Mohr-Coulomb) is used. Non-linear elastic-plastic and strain-softening constitutive models are also investigated in the coupled flow-mechanical analyses. The numerical predictions obtained using linear coupled Mohr-Coulomb. non-linear elastic-plastic and strain-softening constitutive models are assessed against experimental observations. The FLAG predictions use material parameters obtained from conventional laboratory tests. The investigation shows that there are large differences between predictions obtained from the coupled flow-mechanical analysis and the mechanical-induced pore pressure only simulation. The non-linear coupled Mohr-Coulomb numerical model is shown to be in good agreement with the results of the laboratory tests. The investigation also shows that the simple Mohr-Coulomb constitutive model can adequately predict the deformation in thick-walled hollow cylinders of shale. Further work needs to be done before the simple strain-softening constitutive model developed in this paper can be used confidently for shale. Otherwise, more sophisticated strain-softening constitutive models may have to be used. (C) 2000 Elsevier Science Ltd. All rights reserved.