Strength analysis of hydrate-bearing sandy sediments in excess gas and excess water based on drained triaxial compression tests

The safe and efficient production of methane hydrate (MH) has attracted much attention, and the engineering geological risks in MH production are highly valued. Safety evaluation of the MH reservoirs is necessary, and the strength characteristics are the key. In this study, the mechanical behaviors of MH-bearing sandy sediments in excess gas and excess water were studied by conducting drained triaxial compression tests, so as to provide reference for the mechanical evaluation of MH reservoirs in natural gas-rich and water-rich environments. The results show that: (1) the effects of MH saturation and effective confining stress on the mechanical properties of sediments are coupled. The hydrate cementation effect is restricted under high effective confining stress; (2) strain softening behavior is prone to occur under higher MH saturation and is hindered by the high effective confining stress; (3) the MH-bearing samples exhibit higher stiffness, failure strength and strain softening tendency in excess gas than in excess water; (4) in excess gas, the MH contribution to the failure strength of MH-bearing samples starts from the cementation component, while in excess water, it starts from the friction component; (5) theoretical analysis based on the Lade-Duncan criterion suggests that the sandy sediments containing MH in excess water can be considered cohesionless until the MH saturation exceeds 40%. This study provides a theoretical reference for geological safety evaluation during MH development. •Hydrate contribution to the failure strength of sediments is cohesive in excess gas, and frictional in excess water.•Hydrate occurrence modes are suggested to be responsible for the different hydrate contributions to sediments.•Sandy sediments in excess water can be considered cohesionless until hydrate saturation exceeds 40%.