Unsteady Analytical Investigation of Wellbore Stability in Methane Hydrate-Bearing Sediments Based on Fluid-Solid Coupling

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.