Abstract
The formation of methane hydrate in two significantly different media was investigated, using silica gel as an artificial medium and loess as a natural medium. The methane hydrate formation was observed through the depletion of water in the matrix, measured via the matrix potential and the relationship between the matrix potential and the water content was determined using established equations. The velocity of methane hydrate nucleation slowed over the course of the reaction, as it relied on water transfer to the hydrate surfaces with lower Gibbs free energy after nucleation. Significant differences in the reactions in the two types of media arose from differences in the water retention capacity and lithology of media due to the internal surface area and pore size distributions. Compared with methane hydrate formation in silica gel, the reaction in loess was much slower and formed far less methane hydrate. The results of this study will advance the understanding of how the properties of the environment affect the formation of gas hydrates in nature.
Highlights
Natural gas hydrates are non-stoichiometric compounds consisting of small gas molecules fitted into polyhedral water cavities [1] and have been found throughout the World in a variety of locations, including on land under permafrost, offshore under the sea floor, and in sediments of deep lakes [2].Natural gas hydrates are potential energy sources because of the naturally present enormous amount and the growing demand for natural gas [3,4]
When the system reached 9.5 °C at about 5.1h, the temperature and the matrix potential as measured in all three positions rose substantially, indicating that methane hydrate began nucleating at that time and releasing a significant amount of heat, depleting the water present in the medium (Figure 2)
The matrix potential continuously rose until the system cooled to 0.5 °C, indicating that the hydrate continued to form throughout the matrix during that period
Summary
Natural gas hydrates are non-stoichiometric compounds consisting of small gas molecules fitted into polyhedral water cavities [1] and have been found throughout the World in a variety of locations, including on land under permafrost, offshore under the sea floor, and in sediments of deep lakes [2].Natural gas hydrates are potential energy sources because of the naturally present enormous amount and the growing demand for natural gas [3,4]. A review of the literature revealed that much attention had been given to the conditions under which natural gas hydrates are present in sediments, but that the relevant formation and dissociation processes had been largely ignored. On this basis, we undertook the characterization of the hydrate formation process as observed through the water conversion ratios. Several reports in the literature [23,24,25,26]
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