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 ex ample in the oil and gas industry, wellbore instability problems cost the i ndustry many millions of dollars annually. In an attempt to minimise instab ility problems, detailed and careful analyses of the excavation process are often carried out at the planning stage. However, the accuracy of these an alyses 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 us ed to investigate the influence of induced pore water pressure and its diss ipation on borehole deformation of a thick-walled hollow cylinder of synthe tic shale. The two models are: a time-dependent model that incorporates cou pled flow-mechanical interaction and a steady state time-independent analys is that only accounts for mechanical-induced pore pressure. In both models, a linear elastic-plastic constitutive model (Mohr-Coulomb) is used. Non-li near elastic-plastic and strain-softening constitutive models are also inve stigated 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 obs ervations. The FLAG predictions use material parameters obtained from conve ntional laboratory tests. The investigation shows that there are large differences between prediction s obtained from the coupled flow-mechanical analysis and the mechanical-ind uced pore pressure only simulation. The non-linear coupled Mohr-Coulomb num erical model is shown to be in good agreement with the results of the labor atory tests. The investigation also shows that the simple Mohr-Coulomb cons titutive model can adequately predict the deformation in thick-walled hollo w cylinders of shale. Further work needs to be done before the simple strai n-softening constitutive model developed in this paper can be used confiden tly for shale. Otherwise, more sophisticated strain-softening constitutive models may have to be used. (C) 2000 Elsevier Science Ltd. All rights reser ved.