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The knowledge of the physical and geo-mechanical properties of hydrate-bearing sediments as a function of hydrate saturation is crucial for the estimation of the mechanical behavior during production. Since the formation of methane hydrate in sediments is a complex and time consuming process, tetrahydrofuran hydrate is commonly used as a substitute for methane hydrates in laboratory experiments. Tetrahydrofuran (C4H8O, hereafter THF) forms structure II hydrates at ambient pressure and temperatures below 277.5 K. In addition, THF is completely miscible in water and thus the formation of THF hydrate in sediments is fast and homogenous compared to the formation of methane hydrates. Depending on the THF concentration in water a certain hydrate volume fraction forms in coexistence with an aqueous phase. At equilibrium, the chemical potential has to be the same in both the THF hydrate phase and the coexisting aqueous phase. Thus, not all THF dissolved in the original aqueous phase can be transferred into THF hydrate. In this study we determined the formed hydrate volume fraction depending on the THF concentration in water, the remaining THF concentration in the coexisting aqueous phase and the lowest concentration of THF in water necessary for hydrate formation using Raman spectroscopy, calorimetry and X-ray diffraction.
The results of Raman spectroscopic measurements indicate that about 5.5 wt% of THF remain in the aqueous phase coexisting with the hydrate phase. There was no clear coexistence of a hydrate phase and an aqueous phase observed for THF-H2O mixtures containing less THF in the aqueous phase. This is in agreement with calorimetric data. On the contrary, X-ray diffraction measurements indicate a coexistence of ice and THF hydrate for the complete THF-H2O solution series.
