Natural gas hydrates with locally different cage occupancies and hydration numbers in Lake Baikal

Abstract

[1] Knowledge of cage occupancies and hydration numbers (n) of naturally occurring gas hydrate in a local environment is important for the improvement in global estimates of hydrate-bound natural gas. We report on local differences in cage occupancies and hydration number of gas hydrates from Lake Baikal. Natural gas hydrates of both structures I and II (sI and sII) and ranging in composition from pure CH 4 to mixed gas hydrate containing up to 15% C 2 H 6 are compared. The average hydration numbers are n = 6.1 for the sI CH 4 hydrates recovered from the Malenky and Bolshoy mud volcanoes, n = 6.2 for the sI hydrates, containing 3-4% C 2 H 6 recovered from the K-2 mud volcano, and n = 6.9 for the sII hydrate containing about 15% C 2 H 6 recovered from the K-2 mud volcano. The differences in hydration number are due to the differences in the small cage occupancy of CH 4 among the samples studied.

Chemical and crystallographic characterizations of natural gas hydrates recovered from a production test site in the eastern Nankai Trough

“…The 13 C single-pulse NMR spectrum of the hydrate-bearing sediment sample is portrayed in Figure 4. The sediment sample showed two 13 C NMR signals at À4.0 and À6.4 ppm, assigned to methane molecules in the small and large cages, respectively, in sI hydrate crystals distributed in the bulk sample, in accordance with earlier reports (Kida et al, , 2009a(Kida et al, , 2009bKim et al, 2005;Lu et al, 2005). No signals from other guest molecules such as ethane, propane, or butane are observed.…”

“…However, the sI hydrate structure can encage ethane molecules into the large cages; the occupation of ethane molecules has been confirmed for naturally occurring hydrates (Kida et al, , 2009a. The coexistence of ethane and methane molecules in the sI hydrate lattice shows expansion of up to approximately 0.006 nm against the sI lattice of pure methane hydrate at same temperature (Takeya et al, 2005).…”

“…The hydration number, defined as the number of water molecules per guest gas molecule, affects the gas capacity of gas hydrate (Moridis and Sloan, 2007;Sloan, 2003). Hydration numbers of naturallyoccurring gas hydrate vary depending on the natural gas components that are present and their compositions (Kida et al, 2009a). Natural gas components vary according to their origin (Bernard et al, 1976) and affect the thermodynamic stability of gas hydrates (Sloan, 2003).…”

Methane Hydrate in Confined Spaces: An Alternative Storage System

“…Experimental hydration number was calculated and reported in several works for natural and synthetic sI methane hydrate, either in bulk or confined conditions. The reported values range from about 5.8 to 6.3 when full large cavity occupancy is assumed ,…”

Recoverable gas from hydrate-bearing sediments: Pore network model simulation and macroscale analyses

“…The theoretical hydration number χ in structure I methane hydrate is χ = 5.75 (= 46/8) [ Sloan and Koh , 2008]. Slightly higher values (typically between 6.0 and 6.3) are found in natural and laboratory‐made gas hydrates [ Kida et al , 2009a, 2009b; Kim et al , 2005; Lu et al , 2005; Uchida et al , 1999; Ripmeester et al , 2005; Ripmeester and Ratcliffe , 1988; Sum et al , 1997; Seo et al , 2002]. The mass density of hydrates ρ h = 0.92g/cm 3 used in this study assumes full cage occupancy χ = 5.75 (8CH 4 ·· 46H 2 O) in the unit lattice whose volume is (∼12Å) 3 .…”