Molecular Studies and Advanced Visualization of the Trapping of Methane Nanobubbles during Hydrate Growth

The potential application of gas hydrates in storing clean energy has increased the interest in studying clathrate hydrates of gases like methane, CO2, and hydrogen. In this work, we conduct large-scale molecular studies of methane hydrate growth and visualize the simulation results using mixed reality (MR) headsets and regular two-dimensional snapshots of the simulation domain. The results show the novel molecular observation of the trapping of gas nanobubbles within the growing solid hydrate. Our first-of-a-kind visualization of the internal hydrate structures in mixed reality enabled the length measurements of the simulation domain and nanobubble sizes, which showed that the gas nanobubbles were up to 9 nm in diameter. This is bigger than the simulation domain commonly used in atomistic gas hydrate studies, which explains why this is the first observation of the trapping of methane gas nanobubbles within a growing hydrate. Furthermore, our estimates of the increased storage due to the trapping of the nanobubbles indicate a 37% increase in the weight percentage of methane stored. Although this work focused on nanobubble-enhanced methane storage in hydrates, the idea, methods, and tools developed can be leveraged to enhance the storage of other gases, like hydrogen and CO2. This study also revealed that the presence of gas nanobubbles accelerates the rate of hydrate formation, which is consistent with experimental observations. Finally, we expect our workflow for MR visualization of gas hydrate structures to facilitate other novel observations and insights from molecular dynamics (MD) studies of gas hydrates.

Physical breakdown of CH4 hydrate under stress: a molecular dynamics simulation study

As a solid energy source, CH4 hydrate will inevitably break down physically as the result of geological movement or exploitation. Here, the molecular dynamics method was employed to simulate the uniaxial-deformation behavior of structure I (sI type) CH4 hydrate under stress. The stress increases regardless of whether the hydrate is stretched or squeezed, and other physical parameters also changed, such as hydrate cage numbers, order parameters, and the number of water molecules. A noticeable difference is observed between the two systems. Upon stretching, the stress immediately recovers to 0 GPa once the hydrate is completely stretched apart. During the squeeze process, the stress is ultimately not zero since solid and liquid are always in contact. When the hydrate is stretched apart, about 5% of water molecules change from solid to liquid, about 7.8% of CH4 molecules lose their shelter and become free due to the disintegration of water cages. While in the squeezing process, large cages (51262) are crushed more easily than small cages (512); in the end, about 93.5% of large cages and 73% of small cages are crushed, and approximately 87.5% CH4 is released from the cages. In mining CH4 hydrates, caution must be exercised, as if the hydrates break as a result of stress, a large release of CH4 may pose a security risk.