Stability and exchange of guest molecules in gas hydrates under the influence of CH4, CO2, N2 and CO2+N2 gases at low-pressures

Abstract

Gas recovery from the natural gas hydrate (NGH) deposits is a current topic of interest. Apart from the conventional thermal stimulation, depressurisation and chemical injection methods, the guest molecular replacement method in hydrates offers dual advantage such as retention of their structural stability, and as a sink for carbon dioxide (CO2). However, the molecular exchangeability shows significant variance and the mechanism is also incomprehensible. This study is aimed at probing the stability of gas (CH4 & CO2) hydrates in the presence of CO2/CH4, N2 and their mixtures at low-pressure (~1.0 MPa) conditions. The selected experimental conditions help gain insight into the guest-guest exchange. Evaluation of gas pressure during the dissociation process indicates the presence of mixed hydrates. The micro-Raman investigations elucidate the occurrence of mixed hydrate seed crystals, encasing both CH4/CO2 molecules of initial hydrates, and also the molecules from the injected gas. The characteristic signatures of CH4 & CO2 molecules in hydrate systems broadly agree with the literature data, while that of N2 pointedly closer to the gas phase signature, indicating its role as a help-gas to promote replacement/rearrangement of guests in hydrate lattice.