Raman spectroscopy of ocean 
and laboratory hydrates

Keith Hester and E. Dendy Sloan
Colorado School of Mines

Thursday, June 5, 2003
12:00 p.m. – Pacific Forum

Raman spectroscopy can determine hydrate structure and single-guest hydrate cage occupancies. Methane hydrate non-stoichiometry causes cage occupancies to change, depending on whether hydrate formed in a laboratory, gas-rich environment or an ocean floor, gas-lean environment. Hydrates formed in nature could, therefore, have different properties than laboratory hydrates. 

A conceptual understanding has been developed for sII structural transitions with mixtures of simple sI binary guests such as CH₄ + C₂H₆. An sII transition should occur when one simple sI-forming guest is near the lower size limit of sI formation, and the other simple sI-forming guest is near the upper size limit. 

As recently as ODP Leg 204, ethane was measured from cores in sufficient quantity to induce sII formation. From all diffraction/spectroscopy measurements on Leg 204 recovered samples, only sI has been found. The structure, along with the hydrate cage occupation, aid understanding of mechanisms for in-situ hydrates. 

Understanding in-situ hydrate properties via Raman spectroscopy is crucial for energy recovery, CO₂ sequestration, and seafloor stability.