Simplified Two-stage Approach for Estimating Geohazard-induced Wellbore Deformation in Marine Methane Hydrate Exploitation

Abstract Because of their high energy density, lack of pollution and prevalence in the most carbon-rich reserves in the world, methane hydrates (MHs) have potential economic and environmental significance and are currently recognized as an alternative energy source. However, there are still many challenges in MH exploitation, such as casing safety as affected by geohazards or other disturbances. A modified simple two-stage approach is proposed in this study to quickly estimate wellbore deformation and the associated casing safety in hydrate exploitation when nearby geohazards/disturbances occur. The nonuniform reduction in the elastic modulus around the wellbore due to hydrate dissociation, as well as various forms of free-field displacement, are fully considered. In this study, a wellbore with a casing that is equivalent to a beam interacts with a formation by series springs and is subjected to the free-field lateral displacement induced by nearby geohazards and axial forces, which probably increases the risk of wellbore instability. The free-field displacement is modified by multiplying a correction coefficient related to hydrate dissociation. The lateral displacement of the casing is then determined by simply solving a differential equation. The results from the modified two-stage approach agree with the numerical results under the same conditions. A detailed parametric study is carried out to analyze the influence of the boundary displacement, elastic modulus ratios and size of the dissociated region. According to the results, the elastic modulus ratios are key factors affecting casing safety.