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The effect of self-preservation of methane hydrate particles with a characteristic size of a few tens of micrometers was found in suspensions of the hydrate in four crude oils. For example, an ice−hydrate suspension in one of the oils at atmospheric pressure and a temperature of −20°C loses only 4% of its gas in 5 h. No self-preservation occurs in suspensions of the hydrate in decane with similarly sized hydrate particles. We consider that the observed self-preservation of such small particles is mainly due to the formation of insulating ice shell on the surface of the hydrate particles. Our results show that the adsorbed medium influences the self-preservation of gas hydrates and allows for this phenomenon to be managed.

In this work, physicochemical and structural studies have been carried out for semiclathrate hydrates of linear (un-cross-linked) and cross-linked tetrabutylammonium polyacrylates with different degrees of cross-linking of the polymeric guest molecules (n = 0.5, 1, 2, 3%) and different degrees of substitution of proton ions of carboxylic groups in poly(acrylic acid) for TBA cations (x = 1, 0.8, 0.6). The changes in the hydrates' stability and composition depending on the outlined parameters were examined in the course of phase diagram studies of the binary systems water-tetrabutylammonium polyacrylates using differential thermal analysis method and calorimetric measurements of fusion enthalpies of the hydrates. Phase diagram studies of the binary system water-linear tetrabutylammonium polyacrylate revealed the formation of four hydrates. Based on the data of chemical analysis of hydrate crystals the compositions of all hydrates have been determined. Single-crystal X-ray diffraction studies revealed a tetragonal structure, space group 4/m, and unit cell parameters are close for different hydrates and lie in the ranges a = 23.4289-23.4713 Å and c = 12.3280-12.3651 Å (150 K). The structure can be related to tetragonal structure I typical for the clathrate hydrates of tetraalkylammonium salts with monomeric anions. Powder X-ray diffraction analyses confirmed the identity of the above crystal structure to that of the hydrates with cross-linked tetrabutylammonium polyacrylates. The behavior of TBA polyacrylate hydrates under the pressure of methane was studied and quantitative assessment of the gas content in the hydrates was made using volumetric analysis method.

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The temperature of methane hydrate dissociation in silica mesopores has been monitored within a wide range of pressures from 10 MPa to 1 GPa. Because the determination of pore size appears to be crucial for the studied phenomenon, several methods of calculation have been applied. According to our findings, the size that corresponds to the mean size of the most representative pores is to be considered as the most reliable. It was concluded that the shape of hydrate particles replicates a host space of pores and may have a complex (e.g., fractal) shape. An attempt to simulate the curvature of hydrate particles by globular (quasi-spherical), elongated (quasi-cylindrical), or any intermediate models has been done. The quasi-spherical model seems to be more adequate for hydrate particles in small pores (<8 nm), while the quasi-cylindrical model fits better the experimental data for hydrate particles in larger pores. According to our experimental results, the hydrate can be formed in pores only by capillary condensate, without involving the water layers tightly bound by the surface, and pressure has an insignificant effect on the decrease of the dissociation temperature of the confined hydrate. A new effect of the formation of hydrates at a temperature higher than the bulk hydrate dissociation temperature has been observed for silica gels with the narrowest pores studied.

The solubility of helium in ice Ih has been examined both experimentally and theoretically. It has been demonstrated that the calculations are in good accord with the experimental data. The tested calculation method has been used for deriving the helium solubility in ice Ih at pressures up to 2000 bar and at temperatures of 0-50 °C. Obtained data may be useful in some practical applications (storage of enriched with helium natural gas in permafrost, extraction of helium from natural gas).