Study On Poromechanical Problems Of Hydrate Sediment During Phase Transition Process

Natural gas hydrate has many advantages,such as high energy density,wide distribution,huge reserves,etc.Thus,hydrate is taken as a kind of clean energy with great prospect of future development and utilization.During hydrate recovery process,the dissociation of hydrate leads to the deformation of hydrate sediment,which involves the coupling of multi-physical fields.This dissertation establishes a model which combines unsaturated poromechanics and phase equilibrium theory of hydrate in porous media to describe the deformation of hydrate sediment involved in the hydrate dissociation process.Firstly,the poromechanical constitutive equations of hydrate sediment are derived by thermodynamics approach.Then the capillary effect on the phase equilibrium conditions in fine sediment is obvious,so the phase equilibrium conditions in fine sediment are shown as a zone rather a line in p-T diagram.Based on the established model,this thesis explores the deformation of hydrate sediment when hydrate is exploited by different stimulation methods(depressurization method,and thermal stimulation method in both drained and undrained conditions).In undrained condition where gas and water are released but cannot be expelled from pores,the pore pressure rises significantly during the phase transition.This build-up of pore pressure plays a crucial role in the deformation of the sediment;it also inhibits the dissociation of hydrate,and thus more heat supply is required in order to recover all the hydrate.Finally,the different types of recovery methods are compared from the viewpoint of geomechanical stability: the thermal stimulation method in drained condition leads to least deformation,whereas the thermal stimulation method in undrained condition results in large tensile deformation,which may lead to the instability of hydrate reservoir.