Local Structure and Dynamic Studies of mixed CH4-CO2 Gas Hydrates via Computational Simulation and Neutron Scattering

Permeated throughout the ocean floor and arctic permafrost, natural gas hydrates contain an estimated 3000 trillion cubic meters, over three times that of traditional shale deposits, of CH₄ that is accessible for extraction. Gas hydrates are a crystal structure in which water molecules form a cage network, the host, through hydrogen bonds while trapping a guest molecule such as CH₄ in the cavities. These compounds form naturally where the appropriate low temperature and high pressure conditions occur. A promising and tested method of methane recovery is through exchange with CO₂, which energetically takes place of the methane when pressurized into hydrate deposits. When CH₄ is replaced with CO₂ in the hydrate structure, the stability temperature is increased. Currently, hydrate deposits are at risk of releasing CH_4,, and potent greenhouse gas, into the oceans and atmosphere. Recovery of CH₄ via CO₂ exchange presents natural gas hydrates as a potential fuel source and carbon sequestration medium while mitigating the risk of CH₄ release. This work studies the molecular level structure and properties of gas hydrates with CH₄, CO₂, and mixed CH₄ and CO₂ occupying the cage structure in order to better understand how CO₂ stabilizes hydrates, the effectiveness of altering a deposit with a mixed CH₄-CO₂ result, and how each guest molecule type affects interactions in the hydrate framework. A combined approach of computational simulations and neutron scattering is used to characterize how altering the guest molecule composition with CH₄ and CO₂ impacts the guest-host, host-host, and guest-guest interactions in hydrates. Carried out over temperature ranges, this work provides insight to show that in mixed CH₄-CO₂ and pure CO₂ hydrate structures the CO₂ guest interacts strongly with the surrounding cages and guest molecules to stabilize the hydrate.