Molecular Dynamics Study on Mechanical Properties of CO2–N2 Heteroclathrate Hydrates

One of the key issues in the exploitation of natural gas hydrates is the geo-mechanical stability of hydrate-bearing sediments. Herein, based on classical molecular dynamics simulations, we have investigated the mechanical stability of CO2-N2 heteroclathrate hydrates (CNHHs) under uniaxial loading. The results show that the ratio of CO2 to N2 in the large and small water cages has a crucial effect on the mechanical properties and fracture behaviors of CNHHs. In most cases, CNHHs show brittle fracture behavior, and their ultimate tensile strength, failure strain, and Young's modulus decrease significantly with the increase of CO2 ratio in small cages. However, when the small cages are all occupied by CO2, CNHHs display ductile fracture behavior. In particular, the CNHHs with a CO2 fraction of 0.75 have their hydrated cages broken by amorphization with increasing strain. The effect of CO2 molecules occupying the different types of water cages on fracture behavior has been discussed in detail. The results indicate that the occupancy of small cages by CO2 plays a decisive role in the fracture position. In the case of the nearest-neighbor large cages, the fracture positions are consistent with the positions of CO2 molecules with a high probability. The lattice distortion caused by the occupancy of the water cage by CO2 is the key factor affecting fracture behavior. These results should be helpful in understanding the deformation and fracture mechanisms of heteroclathrate hydrates as well as in assessing the impact of CH4-CO2/N2 replacement and CO2 sequestration on the geological stability of gas hydrate reservoirs.