Kinetic mechanisms of methane hydrate replacement and carbon dioxide hydrate reorganization

Abstract

The carbon dioxide replacement is an ideal way to mine natural gas hydrates. However, current challenges of low replacement efficiency and immature technology compromise its application spectrum. This study aims to simulate the displacement of methane hydrate by carbon dioxide in a porous media system and verify the effect of water saturation on methane hydrate through experimental tests. By constructing a molecular dynamics model to investigate the replacement process of methane hydrate with carbon dioxide, the role of free water in this process was examined. Additionally, the study elucidated the factors contributing to suboptimal methane storage rate and low carbon dioxide recovery rate from a heat exchange perspective. The results show that methane replacement from methane hydrate can be divided into four stages. When the water saturation approach to hydrate saturation, and the temperature and pressure are between the phase curves of methane hydrate and carbon dioxide hydrate, the carbon dioxide storage rate and methane recovery rate can be significantly improved. The participation of free water can make the replacement more complete. The primary factor determining the replacement effect is the area of contact between carbon dioxide and methane hydrate, which is followed by the newly created carbon dioxide hydrate, which will impact the subsequent mass transfer. Although the heat released by the formation of carbon dioxide hydrate favor the replacement reaction proceed, most of the heat may get lost with the volatilization of carbon dioxide, resulting in the inability or incomplete decomposition of methane hydrate far away from free water. This research will surely have a profound impact on how hydrate replacement mining is monitored and managed as well as the ecosystem around it.