The Shear Strength Characteristics and Weakening Effect of Methane Hydrate-Bearing Sediments under Partial Hydrate Dissociation

During natural gas hydrate exploitation, hydrate dissociation can deteriorate the shear strength of the hydrate-bearing sediment (HBS). Thus far, however, the hydrate dissociation-induced weakening effect of HBS has not been properly characterized either experimentally or theoretically. In this study, a series of triaxial compression and nuclear magnetic resonance tests are conducted on the HBS specimens with partial hydrate dissociation. It is shown that the geomechanical properties of HBS such as stress–strain response, shear strength, and stiffness are influenced by both hydrate saturation and unhydrated water content. It is also revealed that pore hydrate preferentially dissociates in small pores or in the vicinity of the contact regions of soil particles, where the released unhydrated water accumulates, resulting in the degradation of hydrate cementation and the decrease of soil suction. The conventional methods for determining the shear strength of HBS fail to address the effect of unhydrated water content and thus may yield erroneous results. A shear strength model, which enforces the phase equilibrium condition of pore hydrate, is proposed for HBS with hydrate dissociation by incorporating the combining effect of hydrate saturation and unhydrated water content. The proposed model significantly unifies the influences of temperature, pore pressure, confining pressure, and experimental conditions, which can also describe the evolution of the shear strength of HBS with progressive hydrate dissociation very well.