A state-dependent constitutive model for gas hydrate-bearing sediment incorporating phase change

Abstract

Extracting energy from hydrate-bearing sediment involves phase change, during which the strength of sediment is reduced and the risk of geohazards may subsequently increase. However, existing mechanical constitutive models cannot capture the responses of hydrate-bearing sediment during phase change since kinetics of phase change is ignored. Besides, most models are not suitable for hydrate-bearing sandy sediment since the dilatancy of sand is not considered well. In the current study, a state-dependent mechanical constitutive model for hydrate-bearing sediment considering phase change of hydrate is developed. The bounding surface plasticity is adopted. Fugacity is incorporated into the model to account for phase change of hydrate and the effects of thermodynamic states on mechanical behaviour of hydrate-bearing sediment. Additionally, a non-linear critical state line and a non-associated flow rule are adopted for hydrate-bearing sandy sediment, which is the most favourable for exploitation due to high hydrate content and permeability. These unique features can improve model performance regarding sediment responses during phase change of hydrate. The model is verified by comparing measured results of hydrate dissociation tests, drained triaxial tests and dissociation tests of hydrate-bearing sand. It is evident that stress-strain and volumetric responses of hydrate-bearing sandy sediments can be well captured by this model over a broad range of effective stresses from 0.1 MPa to 5 MPa. Additionally, the responses of hydrate-bearing sediment during dissociation under different stress states have been successfully reproduced. With an initial axial strain of 5%, dissociation of hydrate results in the development of over 20% axial strain and causes the specimen to collapse.