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[MIS24-P11] Formation of sII gas hydrate during dissociation of sI mixed-gas hydrate composed of methane and ethane
Keywords:gas hydrate, methane, ethane, Lake Baikal
Double structure gas hydrate, composed of the cubic structure I and II, has been discovered in a same sediment core retrieved from the Kukuy K-2 mud volcano at Lake Baikal (e.g., Kida et al., 2006). The structure II gas hydrate contained 13-15% of ethane, on the contrary, the structure I has only several % of ethane. It has been reported that a structure II gas hydrate appears in appropriate gas composition of methane and ethane (Subramanian et al., 2000a; 2000b). Some models tried to explain how the double structure formed (Hachikubo et al., 2009; Manakov et al., 2013), however, which structure first formed has been still unknown.
Synthetic mixed-gas (methane and ethane) gas hydrates were formed and their dissociation process was investigated by using a calorimeter (Hachikubo et al., 2008). In most cases, two peaks of heat flow appeared and the dissociation process was divided into two parts. This can be understood in the following explanation that (1) the sample contained both crystal structures (I and II), and/or (2) ethane-rich gas hydrate formed simultaneously from dissociated gas and showed the second peak of heat flow. However, Raman spectra of these samples showed that the crystallographic structure of the samples was originally unique in most cases. In this study we tried to check the latter process, namely a secondary formation of ethane-rich gas hydrate.
We made a methane and ethane mixed-gas hydrate (3.62%C2) in a pressure chamber (volume: 20mL), and stored in liquid nitrogen. A part of the sample was put into a calorimeter (Setaram BT2.15) and dissociated under the temperature gradient of 0.15 K min-1. We observed double peaks in the thermograph, indicating that a new gas hydrate formed from the dissociation gas of the original hydrate, concentrated ethane, and then dissociated. In the next experiment, we put the rest of the sample into the calorimeter, heated in the same condition, and recovered the sample at the end of the first peak of dissociation. The raman spectra of the sample revealed that a structure II ethane-concentrated gas hydrate formed secondarily in the sample. The composition of ethane was 23.4%, corresponded to the field of structure II (Subramanian et al., 2000a; 2000b). Stable isotopes (δ13C and δD) of methane and ethane were also measured, and confirmed the existence of several ‰ difference in ethane δD between the first and second peaks, corresponded to the field data of the double structure observed at the Kukuy K-2 mud volcano (Hachikubo et al., 2009).
Hachikubo et al. (2008) Dissociation heat of mixed-gas hydrate composed of methane and ethane. Proceedings of the 6th International Conference on Gas Hydrates, 6-10 July, 2008, Vancouver, Canada. http://hdl.handle.net/2429/2694
Hachikubo et al. (2009) Model of formation of double structure gas hydrates in Lake Baikal based on isotopic data. Geophys Res Lett 36: L18504. doi:10.1029/2009GL039805
Kida et al. (2006) Coexistence of structure I and II gas hydrates in Lake Baikal suggesting gas sources from microbial and thermogenic origin. Geophys Res Lett 33: L24603. doi:10.1029/2006GL028296
Manakov et al. (2013) A physicochemical model for the formation of gas hydrates of different structural types in K-2 mud volcano (Kukui Canyon, Lake Baikal). Russian Geology and Geophysics, 54, 475-482. doi:10.1016/j.rgg.2013.03.009
Subramanian et al. (2000a) Evidence of structure II hydrate formation from methane + ethane mixtures. Chem Eng Sci 55: 1981-1999. doi:10.1016/S0009-2509(99)00389-9
Subramanian et al. (2000b) Structural transitions in methane + ethane gas hydrates - part I: upper transition point and applications. Chem Eng Sci 55: 5763-5771. doi:10.1016/S0009-2509(00)00162-7
Synthetic mixed-gas (methane and ethane) gas hydrates were formed and their dissociation process was investigated by using a calorimeter (Hachikubo et al., 2008). In most cases, two peaks of heat flow appeared and the dissociation process was divided into two parts. This can be understood in the following explanation that (1) the sample contained both crystal structures (I and II), and/or (2) ethane-rich gas hydrate formed simultaneously from dissociated gas and showed the second peak of heat flow. However, Raman spectra of these samples showed that the crystallographic structure of the samples was originally unique in most cases. In this study we tried to check the latter process, namely a secondary formation of ethane-rich gas hydrate.
We made a methane and ethane mixed-gas hydrate (3.62%C2) in a pressure chamber (volume: 20mL), and stored in liquid nitrogen. A part of the sample was put into a calorimeter (Setaram BT2.15) and dissociated under the temperature gradient of 0.15 K min-1. We observed double peaks in the thermograph, indicating that a new gas hydrate formed from the dissociation gas of the original hydrate, concentrated ethane, and then dissociated. In the next experiment, we put the rest of the sample into the calorimeter, heated in the same condition, and recovered the sample at the end of the first peak of dissociation. The raman spectra of the sample revealed that a structure II ethane-concentrated gas hydrate formed secondarily in the sample. The composition of ethane was 23.4%, corresponded to the field of structure II (Subramanian et al., 2000a; 2000b). Stable isotopes (δ13C and δD) of methane and ethane were also measured, and confirmed the existence of several ‰ difference in ethane δD between the first and second peaks, corresponded to the field data of the double structure observed at the Kukuy K-2 mud volcano (Hachikubo et al., 2009).
Hachikubo et al. (2008) Dissociation heat of mixed-gas hydrate composed of methane and ethane. Proceedings of the 6th International Conference on Gas Hydrates, 6-10 July, 2008, Vancouver, Canada. http://hdl.handle.net/2429/2694
Hachikubo et al. (2009) Model of formation of double structure gas hydrates in Lake Baikal based on isotopic data. Geophys Res Lett 36: L18504. doi:10.1029/2009GL039805
Kida et al. (2006) Coexistence of structure I and II gas hydrates in Lake Baikal suggesting gas sources from microbial and thermogenic origin. Geophys Res Lett 33: L24603. doi:10.1029/2006GL028296
Manakov et al. (2013) A physicochemical model for the formation of gas hydrates of different structural types in K-2 mud volcano (Kukui Canyon, Lake Baikal). Russian Geology and Geophysics, 54, 475-482. doi:10.1016/j.rgg.2013.03.009
Subramanian et al. (2000a) Evidence of structure II hydrate formation from methane + ethane mixtures. Chem Eng Sci 55: 1981-1999. doi:10.1016/S0009-2509(99)00389-9
Subramanian et al. (2000b) Structural transitions in methane + ethane gas hydrates - part I: upper transition point and applications. Chem Eng Sci 55: 5763-5771. doi:10.1016/S0009-2509(00)00162-7