Analysis of wellbore stability for overbalanced drilling in marine methane hydrate-bearing sediments under non-hydrostatic stresses

Abstract

New challenges have emerged in drilling marine methane hydrate-bearing sediments (MHBSs); these challenges are closely related to methane hydrate (MH) dissociation and complex multi-physics coupling. This study investigates wellbore stability in overbalanced drilling by proposing exact solutions for multi-physics fields, taking into account the more general non-hydrostatic far-field stresses, partial multi-field coupling and the influences of hydrate dissociation, i.e., the water/heat production/absorption and the change in the physical properties of reservoirs due to hydrate dissociation.Wellbore drilling is simplified as a non-axisymmetric plane strain problem for the mechanical field and an axisymmetric problem for the pressure/temperature/salt concentration fields, in which the drilling domain includes four annular regions with different physical properties. The closed-form solutions for the temperature, pore pressure and salt concentration fields are obtained by considering the influence of water/heat production/absorption in the compatibility conditions of the steady-state governing equations. The solutions can explain the secondary formation of MHs around the dissociation front. The stress and drilling-induced displacement are then derived for the multi-region elastic problem subjected to non-hydrostatic far-field stresses considering seepage flow, with which the initial failure zone can be addressed by considering a failure criterion. The analytical solutions are verified by the good agreement between analytical and numerical results based on the same assumptions. Furthermore, the comparison between the analytical solutions and the results from complex numerical models suggests the applicability of the proposed solutions. Based on the conditions in marine hydrate field trials in the Shenhu area of the South China Sea, the influence of key parameters on wellbore stability in drilling in MHBSs is investigated in detail by the proposed solutions. Some useful dimensionless charts are provided.