Experimental study on direct shear properties and shear surface morphologies of hydrate-bearing sediments

Abstract

The safe production of methane gas requires a comprehensive understanding of the geomechanical behavior for hydrate-bearing sediments (HBS). Currently, there is a lack of detailed exploration of the shear surface failure characteristics of specimens. To address this, direct shear tests were performed to observe shear surfaces at the microscopic level. This study mainly focuses on the mechanical parameters of HBS and the morphological characteristics of shear surfaces, aiming to elucidate the deformation modes and failure mechanisms. Results reveal that shear load-displacement curves illustrate three stages: elastic deformation, strain softening, and stability stage. The shear strength increases from 12.65 KPa to 153.86 KPa with hydrate saturation rising from 12% to 72%, while decreases versus shear rate. Shear surfaces exhibit concave, convex, and S-shape patterns with remarkable differences in crack number and orientation. The shear load-displacement curves and shear surface morphology are closely correlated with shear rates, especially shear strength and shear surface roughness. Furthermore, the shear surface roughness varies between 2.02 mm and 3.61 mm with hydrate saturation increasing from 12% to 72%, which are also influenced by the shear rate. Empirical formulas were established based on test data to predict the shear strength and surface roughness. The failure mechanism of the specimens mainly depended on the movement modes of the sediment particles, hydrate strength, and hydrate-sediment cementation strength. Findings of this study provide a theoretical reference for evaluating geo-mechanical stability and predicting the destabilization of natural gas hydrate reservoirs.