Piezo-elasticity and stability limits of monocrystal methane gas hydrates: Atomistic-continuum characterization

Gas hydrates are guest-host crystalline materials formed by water cages and guest gases such as methane and carbon dioxide under simultaneously relative high-pressure and low-temperature conditions. With this unique guest-host structural feature, gas hydrates can be used for gas storage and carbon dioxide (CO₂) sequestration, creating challenges such as flow assurance and geological stability. Some of these challenges are related to material instabilities caused by changing external conditions. Thus, this paper aims to determine the theoretical pressure stability limits of monocrystal defect-free sI methane gas hydrates at 0 K using accurate density functional theory to simulate the hydrate's thermodynamic and elastic responses under varying pressures. The pressure stability limits are determined by Born stability criteria and piezo sensitivity factors. The important brittle-to-ductile transitions of gas hydrates are established. Also, polycrystalline mechanical properties, including the Poisson ratio and Young modulus, are calculated from the second-order elastic constants obtained from the monocrystal sI methane hydrates, which provide the upper bounds. Taken together, the piezo-sensitivity of a complete set of elastic properties of sI methane gas hydrates and material stability limits determined by atomistic calculations provide new data and fundamental understanding for technological applications.