Abstract:In view of the wellbore stability of drilling in methane hydrate-bearing sediments， an analytical model in unsteady state is established considering hydrate dissociation， heat conduction， and fully fluid-solid coupling.And analytical solutions for seepage， temperature and mechanical fields with time and polar radius during overbalanced and underbalanced drilling are obtained. The analytical solutions are in good agreement with the numerical results under the same conditions and compared with the partial hydraulic-mechanical coupling results. The key parameters affecting wellbore stability， such as drilling fluid pressure， and the elastic modulus reduction caused by hydrate dissociation， are analyzed. The results show that compared with partial hydraulic-mechanical coupling results， if the influence of volume deformation of solid on seepage is considered， the pore pressure decreases （increases） while stresses increase （decrease） and radial displacement decreases during overbalanced （underbalanced） drilling. The most dangerous position for the formation is at the wellbore wall and a too high or too low drilling fluid pressure will lead to wellbore instability. The deterioration of formation in mechanical properties caused by hydrate dissociation will reduce the safest drilling fluid pressure. The reduction of formation stiffness caused by hydrate dissociation is very easy to induce wellbore instability and the wellbore instability can usually be caused by reducing the elastic modulus in the dissociated region by 50% during overbalanced drilling.

Abstract:When drilling in methane hydrate-bearing sediments， the variation of temperature and seepage pressure around the wellbore likely leads to the hydrate dissociation， reducing the safety of wellbore. The analytical solution for wellbore mechanical analysis is derived accounting for the ideal elastic-plastic behavior of the formation， the Mohr-Coulomb yield criterion and non-associated flow rule. Based on a steady-state axisymmetric plane strain model considering the effect of hydrate dissociation on temperature， seepage flow and mechanical property， the stress and displacement solutions are presented under unbalanced and overbalanced drilling conditions. The analytical solutions agree well with the results by numerical methods under the same assumptions， as well as the complex conditions. The results show that the increase of the size of dissociated region leads to the expanding of plastic zone obviously within a certain range. The size of plastic zone is decreased with the reduction of Youngs modulus of dissociated region under unbalanced drilling condition， while the result is inverse under overbalanced drilling condition.