Decomposition behaviors of CO2 hydrate sandwiched in double-symmetrical flat system:A molecular dynamics simulation study

Abstract

Sequestration of carbon dioxide (CO2) in the form of hydrate is considered to be an effective way to achieve carbon emission reduction. The replacement method of nature gas hydrates with CO2 has been proved to be a very promising technique as well. A precise acknowledge of the decomposition characteristics of CO2 hydrate in porous media, which will help further implement these applications. In this work, molecular dynamics simulation method is employed to investigate the whole decomposition process of CO2 hydrate sandwiched in the double-symmetrical flat that is the approximate imitation system of hydrate located in veined distribution or throat space of porous media. The results indicate that the decomposition of the hydrate phase follows a somewhat relaxed shrinking-core mechanism from outer to inner, with this deviation being reflected in the differential collapse rates of the symmetrical sections of the hydrate layer. The difference of the collapse rate of hydrate layers is not only reflected in the distance from the interface, but also closely related to the surrounding crystal state. During the first 200 ps, the average diffusion coefficient of CO2 molecules is 0.659 × 10−9 m/s which is 0.53 times than that of the pure hydrate system with 1.241 × 10−9 m/s. Both stages of initial cage structure breakdown and CO2 nanobubbles formation evolution are clearly observed, and the phenomenon of molecular diffusion gradient is also appeared in last stage. Additionally, with the four spacing of 29.08 Å, 41.08 Å, 53.08 Å and 101.08 Å, an interesting decomposition mechanism was found, that is, the destabilization of hydrates occurred preferentially at the geometric center not just the interface of hydrate-media in the symmetric narrow confined space. The relevant result is very helpful to further understand the stable geological storage mechanism of CO2 gas in the form of hydrate in sediments.