Numerical simulation of direct shear tests on gas hydrate-bearing sediments using the discrete element method

Abstract

Natural gas hydrate is a potential unconventional energy resource. Understanding the mechanical behavior of gas hydrate-bearing sediments (GHBS) is important to ensure the safe and efficient exploitation of natural gas hydrate and prevent geohazard problems. This study employs discrete element method to investigate the mechanical properties of GHBS with different hydrate saturations under various vertical stresses, and a series of direct shear tests, which requires less computational consumption than triaxial or biaxial tests. Comprehensive analyses of macroscopic and microscopic mechanical behaviors, including shear strength, internal friction angle, cohesion, shear band, average coordination number, and crack number evolution, are carried out, as an effort for understanding the shear strength breakpoint, which is recognized in laboratory experiments, probably caused by the transition in hydrate morphology in sediment pores. The results obtained reveal that: (1) the way of hydrate in strengthening GHBS is to enhance cohesion; (2) the increasing rate of the strength difference between the sediment matrix with and without hydrate remarkably increases when the hydrate saturation is over certain value at which the shear strength breakpoint is observed, and this rate decreases with the increase in median particle diameter (D50) of host sediments; (3) the transition in hydrate morphology in sediment pores is recognized when the hydrate saturation increases at microscale; and (4) the shear strength breakpoint in the curve of strength difference versus hydrate saturation is explained based on the micromechanical properties of GHBS, including shear bandwidth, average coordination number, average normal force, and crack number evolution.