Analytical investigation of wellbore stability during drilling in marine methane hydrate-bearing sediments

Abstract

Methane hydrates (MHs) have attracted increasing interest as potential alternative clean energy resources. However, drilling and MH exploitation in marine methane hydrate-bearing sediments (MHBS) face new challenges associated with MH dissociation and complex multi-physics coupling. This study mainly focuses on wellbore stability during drilling in marine MHBS by employing a properly simplified multi-field theoretical model. The new solutions are derived analytically for the temperature, seepage pressure and stress/displacement field, considering the changes in the mechanical and physical properties of reservoirs induced by MH dissociation, as well as the partial coupling of multi-fields.The axisymmetric plane strain wellbore model is simplified, where the infinite ground is divided into three annular regions with different physical/mechanical properties. The closed-form solutions for temperature and seepage pressure fields are addressed by assuming steady-state heat transfer and fluid flow, by which we find that the temperature and pressure distribution, as well as the size of dissociated region are closely related to the relative thermal conductivity and permeability of the regions. An additional infinite region is introduced to obtain the decayed solutions of displacement. The analytical solutions agree very well with the results from finite element method (FEM) in that the conditions are completely consistent, and the analytical results qualitatively agree with experimental data and the results from numerical simulation under complex conditions. A parametric study is finally performed to investigate the influence of the relative thermal conductivity and permeability of the dissociated region on the size of the dissociated region, as well as the influence of the drilling fluid temperature/pressure and the reduction in the elastic modulus/cohesion of the dissociated region on wellbore stability.