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

TL;DR

This study uses molecular dynamics to simulate how methane hydrate breaks down under stress, revealing significant differences in behavior during stretching and squeezing.

Contribution

The novel contribution is the detailed simulation of methane hydrate breakdown under stress, revealing distinct behaviors during stretching and squeezing.

Findings

During stretching, hydrate stress recovers to zero once fully stretched apart.

Squeezing results in non-zero stress due to persistent solid-liquid contact.

Squeezing crushes large cages more than small ones, releasing up to 87.5% of methane.

Abstract

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.

The online version contains supplementary material available at 10.1186/s13065-024-01191-6.

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