Methane clathrate hydrate dissociation analyzed with Raman spectroscopy and a thermodynamic mass transfer model considering cage occupancy

Abstract

The objective of this work was to investigate diffusion phenomena in methane clathrate hydrates during depressurization that lead to their dissociation. A thermodynamic mass transfer model is proposed that considers cage occupancy in structure I clathrate hydrates and methane diffusion from both S-cages (dodecahedron: 12 pentagons) and M-cages (tetradecahedron: 12 pentagons, 2 hexagons). Methane occupancies during dissociation were measured in situ with Raman spectroscopy with a newly-constructed optical system and data were estimated with Langmuir constants calculated by assuming a spherically symmetric cell potential. Model results were in agreement with experimentally-measured changes in average occupancy with time, so that dissociation kinetics at the interface were concluded to be strongly related to both methane diffusion and cage occupancy. Dissociation kinetics of methane hydrate can be reliably estimated with the proposed model. The proposed model is applicable to the study of methane hydrate dissociation-limited mechanisms or to larger-scale geological systems being used to estimate gas production rates.