Optical Interpretation of the Chemical Process of CH₄–CO₂ Exchange and Its Application to Gas Hydrate Production

Abstract

The guest-exchange method (or replacement) for methane production from gas hydrates has recently received attention because it can be used for both carbon dioxide sequestration and methane production. The structure of gas hydrates is maintained as a structure I (sI) hydrate while methane molecules are exchanged with carbon dioxide. In this study, CH 4 + CO 2 mixed gas hydrates were examined under terahertz light at various temperatures to simulate CH 4 −CO 2 exchange reactions. Each gas hydrate composition examined was a representative composition at each step of the exchange reaction. The molecular composition was also accurately analyzed by gas chromatography. Refractive indices calculated by the terahertz time-domain spectroscopy (THz-TDS) of gas hydrate samples were correlated to the guest composition, and this novel method was proven to be used to quantify the extent of replacement via optical constant. Furthermore, changes in the water framework from the sI hydrate to ice using THz-TDS were investigated with an increasing temperature. Overall, this study reveals the process of guest exchange and phase transition from a gas hydrate to ice via the optical properties in the terahertz region, and it offers a powerful tool in gas hydrate production.

Direct phase coexistence molecular dynamics study of the phase equilibria of the ternary methane–carbon dioxide–water hydrate system

“…The MD simulations indicate that the presence of carbon dioxide molecules is prevalent in the larger 5 12 6 2 cages. This observation is also in agreement with experimental observations reported by Everett et al, 76 Sum et al, 73 and Kang et al 77…”

“…Studies of the first type performed analysis (e.g., Raman) in order to identify which types of guests are enclathrated inside the different types of cages within a hydrate structure. [73][74][75][76][77] On the other hand, studies of the second type performed analysis of the relative amount of gas that is released from the dissociation of formed hydrates. [78][79][80][81][82][83] Fig.…”

Hydrate Phase Equilibrium of CH4+N2+CO2 Gas Mixtures and Cage Occupancy Behaviors

“…Considerable efforts have been made to develop efficient methods to extract methane from hydrate-bearing sediments. It has been investigated that the injection of CO 2 into CH 4 hydrate reserves could result in both CO 2 sequestration and CH 4 exploitation simultaneously. − The hydrate equilibrium of CH 4 –CO 2 gas mixture formed after CO 2 injection has been extensively explored to confirm the feasibility of methane recovery by CO 2 swapping method. ,− More recently, Park et al proposed that the injection of CO 2 +N 2 gas mixture was more efficient than pure CO 2 for recovering CH 4 from hydrates; a significant increase of CH 4 recovery efficiency up to 85% was observed. Following reports on the CH 4 recovery by the exchange of CO 2 +N 2 mixture appeared later. − The inclusion of N 2 in the gas exchange process between CO 2 and CH 4 in hydrates could offer an additional control on hydrates equilibria. − To analyze and understand this complex thermodynamic phenomenon, the complete hydrate phase behavior of the CH 4 –N 2 –CO 2 mixture must be investigated to predict the exchange conditions and understand the mechanism occurring in the replacement in a gas hydrate reservoir.…”

Diffusion Model for Gas Replacement in an Isostructural CH4–CO2 Hydrate System

“…It should be noted that the above approach does not consider all of the complexity of mixing effects in detail, for example, the cage occupancies, cage diameters, and related physical parameters like lattice constants are assumed to linearly depend on composition. Yet, there are indications that this approximation works reasonably well. , On the other hand, the sometimes assumed full occupancy of SCs finds neither experimental nor theoretical support. , …”

“…Yet, there are indications that this approximation works reasonably well. 30,31 On the other hand, the sometimes assumed full occupancy of SCs 32 finds neither experimental 1 nor theoretical support. 33,34 Although theoretical concepts (1) and (2) are originally constrained to certain assumptions, they definitely capture the basic thermodynamic principles, at least locally, for the given (p, T) conditions with properly tuned Langmuir constants.…”