Geomechanical constitutive modelling of gas hydrate-bearing sediments by a state-dependent multishear bounding surface model

Natural gas hydrate in marine sediments and permafrost areas is considered as an important potential energy source. Since hydrate dissociation will reduce the stability of gas hydrate-bearing sediments (GHBS) and may cause wellbore failures and geological disasters during gas production, it is necessary to reveal the mechanical behavior of GHBS for the safe exploitation of natural gas hydrate. This paper proposes a geomechanical constitutive model of GHBS within the multishear bounding surface framework. Following the slip theory of plasticity, a constitutive formulation is obtained by splitting the macro constitutive response of sediments into a macro volume response and a series of micro shear responses in spatial distributions related to virtual microshear structures. Each microshear structure describes micro shear and dilatancy responses in three orthogonal orientations. A micro stress-strain relationship and a micro stress-dilatancy relationship are established for each orientation of the microshear structure. The model comprehensively describes the consolidation, hardening, softening, dilatation, collapse, and non-coaxial characteristics of gas hydrate-bearing sediments by introducing the multishear concept, state parameter, evolution law of hydrate bonding and debonding, and collapse strain caused by hydrate dissociation. The effectiveness of the model is confirmed by simulating the available published laboratory tests on the samples of synthetic and natural GHBS under different pore pressures, temperatures, initial void ratios, hydrate saturations, and initial effective confining stresses.