Elastoplastic analytical investigation of wellbore stability for drilling in methane hydrate-bearing sediments

Abstract

Methane hydrates (MHs) are popular clean energy resources. The dissociation of MHs near the wellbore wall during drilling in marine methane hydrate-bearing sediments (MHBS) significantly reduces the bearing capacity of the wellbore and increases the risk of wellbore instability. In this study, a new elastoplastic analytical solution for wellbore stability during drilling in MHBS is provided, taking into account the reduction in the stiffness and strength of the formation after the hydrates dissociate, the conduction/diffusion and convection of heat/ion transfer, and the partial coupling among the seepage, temperature, salt concentration and mechanical fields.An axisymmetric plane strain problem is simplified by diving the infinite region into several annular regions with different physical properties. The distribution of pore pressure at a specific time for this multi-region problem is first obtained by introducing an influence radius estimated by the common unsteady flow solution. Accordingly, the conduction/diffusion and convection effects on the temperature and salt concentration fields are provided for balanced, underbalanced and overbalanced drilling conditions, and the dissociated region is finally addressed through the hydrate phase equilibrium equation fitted by numerous experimental data. Then, the mechanical responses of the wellbore in an elastic-brittle-plastic formation are analytically analysed by employing unified strength theory, considering two principal stress orders and three plastic zone locations.The analytical solution agrees very well with the numerical results determined under the same assumptions, and the solutions of the influence radii of the pressure and temperature and distribution of the pressure and effective stress around the wellbore are consistent with the results from complex numerical simulations. A detailed parametric study is carried out to investigate the specific influence of the drilling fluid properties, the size of the dissociated region and the reduction in Young's modulus/cohesion due to hydrate dissociation on wellbore stability during drilling in MHBS.