Decomposition Path Dependency of Shear Strength in Hydrate-Bearing Sediments Under Phase Transition Conditions

The exploitation of natural gas hydrate can lead to a significant phase change within the reservoir, whereby the solid hydrate transforms into gas hydrate and water. The mechanical and shear strength characteristics of hydrate-bearing sediment in a reservoir are crucial for assessing the exploitation and stability of that hydrate reservoir. To recover the evolutionary mechanism governing the deformation and strength behavior of hydrate-bearing sediment under phase transition conditions, a series of triaxial shear tests were conducted on sediments with varying initial hydrate saturations under depressurization-induced decomposition. The data indicates that undecomposed hydrate-bearing specimens initially exhibit strain-softening stress-strain curves, which gradually shift to strain-hardening curves as the decomposition ratio increases. There is a pronounced decrease in shear strength during the initial phase of decomposition. Upon complete decomposition of the hydrate, the reductions in shear strength are 44.1%, 56.1%, and 61.7% for specimens with three different initial hydrate saturations, respectively. Moreover, the decomposition path dependence of shear strength obviously exists. Despite identical hydrate saturation, the shear strength of sediment varies across different decomposition pathways. Introducing the general phase equilibrium equation, a novel shear strength equation was proposed, incorporating the path dependence of shear strength. Utilizing the effective stress framework, the shear strength of hydrate-bearing sediment can be determined using the shear strength of hydrate-free sediment at a full saturation state. The validity of the equation was subsequently validated through a comparison with experimental data and published literature. These results contribute fundamental support for quantitative evaluations pertaining to the exploitation and stability of hydrate reservoirs.