Volumetric strain analysis model for gas hydrate-bearing sediment considering effects of hydrate dissociation

In the exploitation of gas hydrate, recovering methane from gas hydrate breaks the phase equilibrium state of hydrate and produces water and gas, which reduces the quality of the solid phase in the gas hydrate-bearing sediment (GHBS). Based on the triaxial tests as well as the mechanical properties of GHBS, the solid skeleton is divided into indecomposable soil skeleton and decomposable solid hydrate. The compression parameters of GHBS varying with hydrate saturation are introduced to establish an analysis model that can describe the coupling effects of stress, hydrate decomposition and variation of volumetric strain of GHBS with time during hydrate dissociation process. The proposed model can describe the effects of depressurization rate, pore pressure reduction and hydrate dissociation rate on deformation of GHBS. The numerical results show that with the increase of the depressurization rate, the volumetric strain rate increases in depressurization stage and the time to reach phase equilibrium decreases. The hydrate dissociation rate that has an obvious effect on the deformation rate of reservoir is different in sediments with different particle sizes. The stable pore pressure affects the final settlement of the reservoir, and reducing it can improve the efficiency of gas hydrate exploitation, however, the larger the reduction of pore pressure, the larger the volumetric strain.