Destabilization of Marine Gas Hydrate-Bearing Sediments Induced by a Hot Wellbore: A Numerical Approach

Abstract

This study addresses a numerical approach for exploring how thermal change destabilizes marine gas hydrate-bearing sediments. The underlying physical processes of hydrate-bearing sediments, such as hydrate dissociation, self-preservation, pore pressure evolution, gas dissolution, and sediment volume expansion, are incorporated with the thermal conduction, pore fluid flow, and mechanical response of sediments. Two-dimensional numerical modeling is conducted using a verified finite difference method, in which a steady-state hot wellbore transfers heat to the surrounding hydrate-bearing sediments, resulting in dissociation of methane hydrate. During gas hydrate dissociation, excess pore fluid pressure is generated such that the sediments undergo plastic deformation in the dissociation region and uplift at the seafloor. Sediment stability in the early stage of heat transfer is governed by the intensity of the heat source and the thermal conductivity of the sediments with gas hydrates in place. Later on, excess pore fluid pressure diffusing from the dissociation region destabilizes the shallower overlying sediments. Case studies show that the stability of sediments experiencing thermal change is worsened by an increase in the intensity of the heat source and the initial hydrate saturation. In addition, a decrease in the permeability, initial free gas saturation, and sediment strength also decreases the stability of sediments. A considerable uplifting deformation of the overlying sediments and a sediment failure in a cylindrical or conical shape around a wellbore are observed when the factor-of-safety becomes less than one.