Distinct creep regimes of methane hydrates can be predicted by a monatomic water model

Although both ice and methane hydrates are hydrogen-bonded structures of water molecules, methane hydrates are orders of magnitude more creep resistant than ice. The power law scaling properties of this creep resistance was shown experimentally two decades ago, but a molecular-scale explanation for these exponents has still been lacking. Using molecular dynamics simulations over almost two orders of magnitude of stresses and three orders of magnitude of strain rates, we show that power law creep consistent with the creep experiments by Durham and coauthors in 2003 can emerge from a monatomic water model. A monatomic water model with an angular term resulting in tetrahedral ordering, a spherically symmetric methane model and the concept of a hydrate polycrystal are sufficient conditions for this behavior to emerge. We attribute a low-stress low-power relationship to shear of the amorphous layer on grain boundaries between hydrate grains, and show this by a separate set of simulations only containing amorphous hydrate. Higher power creep of polycrystalline hydrate at higher stresses scales with an exponent about twice that of the low-stress regime, but is slower than expected from the amorphous hydrate simulation results. We therefore attribute this creep to the degradation of hydrate corners that are carrying the compressional loading of the hydrate at stresses that cannot be carried by the grain boundaries.