A simple analytical model of wellbore stability considering methane hydrate saturation-dependent elastoplastic mechanical properties

Abstract

Methane hydrates (MHs) have received widespread attention as a new clean energy resource. However, because of the sharp reduction/change in the mechanical/physical properties of methane hydrate bearing sediments (MHBS) after hydrate dissociation, as well as the strong multi-field coupling, wellbore instability is more likely to occur during drilling in MHBS under the seafloor. To fast predict the wellbore stability in MHBS, an elastoplastic analytical model is proposed in this study that fully considers the saturation-dependent mechanical properties and water/heat production/absorption due to hydrate dissociation. Axisymmetric problems at the horizontal cross-sections of the vertical wellbore are concerned. Based on an analytical model of pore pressure, temperature and salt concentration for drilling in MHBS that considers both conduction/diffusion and convection effects, the produced/absorbed water/heat at the dissociation front related to the MH dissociation rate are additional considered in the compatibility conditions in the modified models. Compared with the results of previous study, the improved solutions show a much lower temperature and slightly higher pressure around the dissociation front. Furthermore, the saturation-dependent mechanical properties of MHBS are simulated by an elastic-brittle-plastic constitutive model with unified strength criterion, where Young's modulus and cohesion are functions of hydrate saturation. The analytical solutions for the stresses, displacement and plastic radius are finally addressed under various locations of the plastic zone and principal stress orders. The analytical models are verified by their good agreement with the results from the FEM model under the same assumptions, and they are consistent with the results from complex numerical models. Through numerous calculations, the influences of the drilling fluid properties and initial conditions on the plastic radius and displacement at the wellbore wall are investigated in detail, and the analytical formula for the safest drilling fluid pressure is presented as a function of the initial pore pressure and in situ stress.