Model study of borehole stability

Hole problems during the drilling phase of operations are often the consequence of mechanical wellbore instability and will dramatically increase the drilling cost. A number of analytical and numerical models are available for diagnosis and prediction of wellbore instability. Firstly, this paper reviews the merits and pitfalls of the models widely used in hollow cylinder tests and field situations. Two main elements, that is, the constitutive relations and the failure criteria of the rocks are stressed in the borehole stability analysis. Some typical models including two linear elastic models, the stress dependent modulus elastic model and the elastoplastic model are especially discussed in the paper. To illustrate the performance and the availability of the typical models, case studies are carried out by analytic solutions or numerical simulations with finite element method. The results show that it is possible to diagnose the borehole instability if one can choose a suitable behavior rule of the rock and an available failure criterion for a given formation.