DEM simulation of bonded granular material. Part II: Extension to grain-coating type methane hydrate bearing sand

Abstract

In free gas environment, methane hydrate (MH) tends to grow on sand grain surfaces and forms cementations (bonds) at contacts where the MH coating layers overlap. This grain-coating type growth habit of hydrate endows methane hydrate bearing sediment (MHBS) with special mechanical behavior. This study first examines the mechanics of grain-coating type MHBS based on previously reported experimental results. Then, three particle-scale mechanisms are proposed, i.e. contact cementation (MH acting as bonds at contacts), particle enlargement (MH coating grain surface and thus increasing effective particle size) and particle angulation (the presence of rough fracture surface when a MH bond is broken). The three mechanisms are implemented into a bonded contact model for the discrete element simulations of MHBS. Grain angularity, MH growth habit and mechanical properties of MH are incorporated in the bonded contact model in a consistent way. The simulation results show that the experimentally observed effects of MH saturation and test conditions (temperature and pore pressure) on MHBS behavior can be well reproduced, thus validating the three proposed mechanisms. Cementation effect can be fully seen under very low stress state, whereas under in-situ stress conditions for MHBS, cementation only influences the small-strain behavior. Particle enlargement and angulation are dominant to control strength and dilation of MHBS.