Temperature, pressure, and compositional effects on anomalous or "self" preservation of gas hydrates

Abstract

We previously reported on a thermal regime where pure, polycrystalline methane hydrate is preserved metastably in bulk at up to 75 K above its nominal temperature stability limit of 193 K at 0.1 MPa, following rapid release of the sample pore pressure. Large fractions (>50 vol.% ) of methane hydrate can be preserved for 2–3 weeks by this method, reflecting the greatly suppressed rates of dissociation that characterize this "anomalous preservation" regime. This behavior contrasts that exhibited by methane hydrate at both colder (193–240 K) and warmer (272–290 K) isothermal test conditions, where dissociation rates increase monotonically with increasing temperature. Here, we report on recent experiments that further investigate the effects of temperature, pressure, and composition on anomalous preservation behavior. All tests conducted on sI methane hydrate yielded self-consistent results that confirm the highly temperature-sensitive but reproducible nature of anomalous preservation behavior. Temperature-stepping experiments conducted between 250 and 268 K corroborate the relative rates measured previously in isothermal preservation tests, and elevated pore-pressure tests showed that, as expected, dissociation rates are further reduced with increasing pressure. Surprisingly, sII methane–ethane hydrate was found to exhibit no comparable preservation effect when rapidly depressurized at 268 K, even though it is thermodynamically stable at higher temperatures and lower pressures than sI methane hydrate. These results, coupled with SEM imaging of quenched sample material from a variety of dissociation tests, strongly support our earlier arguments that ice-"shielding" effects provided by partial dissociation along hydrate grain surfaces do not serve as the primary mechanism for anomalous preservation. The underlying physical-chemistry mechanism(s) of anomalous preservation remains elusive, but appears to be based more on textural or morphological changes within the hydrate material itself, rather than on compositional zoning or ice-rind development. PACS Nos.: 82.30Lp, 81.40Gh, 81.40Vw, 68.37Hk, 83.80Nb

Modeling the Self-Preservation Effect in Gas Hydrate/Ice Systems

“…4,5,10,13) Large-scale nanosecond classical molecular dynamics (MD) calculations support this assumption (the formation of an ice layer inhibits the decomposition of gas hydrate). 14) However, as mention about, similar self-preservation effect was never observed in the experimental studies of the mixed methaneethane hydrate, 12) which does not support the ''kinetic'' approach of self-preservation effect. Moreover, as it was shown in 9,12,15) the macroscopic ice-shielding model does not explain anomalous preservation up to 93% of methane hydrate.…”

“…3) This term describes the ability of methane hydrate to resist dissociation at temperatures higher than the equilibrium temperature of decomposition. The effect of incomplete dissociation of methane hydrate has been the subject of many experimental studies in the last years (see, for example [4][5][6][7][8][9][10][11][12][13] ). The experiments show the anomalous preservation of methane hydrate at temperatures below 273 K (ice Ih melting point) under ambient pressure with simultaneous formation of ice phase at temperatures above 242 K (beginning of methane hydrate dissociation).…”

“…9,12) The most popular explanation of the self-preservation effect assumes a kinetic barrier for methane diffusion through the ice film of the surface of gas hydrate particles. 4,5,10,13) Large-scale nanosecond classical molecular dynamics (MD) calculations support this assumption (the formation of an ice layer inhibits the decomposition of gas hydrate).…”

Formation of the steam phase in superheated liquids in the state of metastable equilibrium

“…As a result of the surfactant action, the interphase tension at the interface between the dispersion medium (water) and the dispersed phase (oil) decreases [11,12]. For example, in a water-oil type emulsion, interfacial tension is σ wo ≈16 mN/m, and tension at the water-vapor interface σ wv ≈73 mN/m [13].…”

A study of phase transition processes features in liquid-gas systems

“…As an example, we should mention the following technologies: degassing of water, distillation of petroleum products, cleaning of surfaces by cavitation, homogenization of fuel, mixing of colloidal solutions, foam formation in the food industry, etc. Modern technologies for the insulating materials production [1], desalination of sea water with the help of isobutane hydrate [2][3][4], and obtaining of natural gas hydrate [5,6] for transportation and storage are also based on heat and mass transfer processes in the gas-liquid systems.…”