Physical chemistry and technological applications of gas hydrates: topical aspects

The review considers the research and engineering works in selected fields of the physical chemistry of gas hydrates and gas hydrate technologies, mainly performed over the last 10–15 years. Data on the hydrate structures are given, including the structures formed during phase transitions at low temperatures, and on new hydrate structures that are formed under ultrahigh pressures. The dynamics of guest and host subsystems is considered in brief. Phase diagrams of systems involving hydrate formation (in particular, in some porous media) and methods for calculating phase equilibria and compositions of hydrates in these systems are analyzed. Data on the hydrates formed by ‘non-classical’ hydrate formers (alcohols, amines, ammonia, etc.) studied in the last decade and on some of their properties are presented. The Section devoted to hydrate formation kinetics addresses the effect of compounds dissolved in water (surfactants, low-dosage inhibitors) and catalysts on this process. The hydrate growth dynamics on the water surface and hydrate decomposition-formation processes in disperse systems are discussed. Among technological applications, the attention is focused on gas storage and transportation in the hydrate form. Methods for the preparation of hydrates are considered. The bibliography includes 618 references.</jats:p

Clathrate Hydrates

Water is a mysterious substance; being hydrophilic (by definition), it can capture hydrophobic or hydrophobic-hydrophilic species forming different kinds of hydrate inclusion compounds. Hydrate inclusion compounds can be formed by a variety of molecules and molecular associates, which are commonly referred to as guests. Common to all these compounds is the presence of a more or less complex framework built of hydrogen-bonded water molecules. This framework is commonly referred to as the host framework. The chapter deals with two major issues associated with clathration of natural methane by water: (1) possible uses and state of the art in methane hydrate exploitation and (2) disaster risks associated with possible eruptions of methane from submarine deposits being the major component of natural methane stored on Earth. </jats:p

Acoustic properties of hydrate-bearing coal samples depending on temperature and water saturation type

We have developed the first experimental acoustic properties measurement during gas-hydrate formation and dissociation in crushed bituminous coal samples. We also compare the results with acoustic properties measurements during freezing and thawing water in the same samples. The results show a more complicated behavior that differs from similar experiments with sand. For the samples with adsorbed water, it does not freeze at anticipated temperatures, but acoustic velocities gradually increase with the decreasing negative temperatures. It is caused by complicated pore surface structures causing partial formation of ice/hydrate from bound water at different temperatures. We also observe that, for the same samples, the acoustic properties change significantly, becoming stronger during gas-hydrate formation than during freezing. We explain it by competitive sorption of methane and water in the coal pore space; methane under pressure replaces part of the adsorbed water from micropores so that this water can easily form hydrate. The difference in the acoustic properties of frozen and hydrate-bearing coal samples is important for developing seismic methods for geophysical characterization of coal seams. </jats:p

Nucleation rates of methane hydrate from water in oil emulsions

Nucleation of methane hydrate from water emulsions in five different kinds of crude oil and in decane have been studied with the use of isothermal methods. The experiments were conducted at a temperature of –5 °C and pressure of 12 MPa. It is shown that the nucleation rates tend to decrease with the increase in the density of the organic liquid used to make the emulsion. It is most likely that the observed regularities are related to the rate of methane diffusion to water surface. </jats:p