Self-Organized Colloids Thermodynamically Weaken the Effect of Salt on Methane Hydrate Formation

Abstract

Natural gas hydrate deposits are typically rich in organic matter; they together with the host sediments play a crucial role in the nucleation and accumulation of gas hydrates, yet their potential thermodynamic effect on hydrate formation is rarely studied. In the present work, it was found that the deprotonated organic compounds could self-organize with the cations in the solution, forming organic colloids via coordination bonds. Elemental mapping showed that these organic colloids were filled with Ca 2+ and Na + ions as well as more concentrated organic matter surrounded by thin rings of Mg 2+ and Cl − ions. This was found to result in a concentration decrease of Ca 2+ and Na + ions in the solution with the presence of the colloids; the resulting decreased electrostatic effect of salt ions on water molecules would subsequently relieve the energy barrier of hydrate formation, showing a shift of the phase equilibrium conditions to a milder situation. This finding would be more nontrivial in low-permeability hydrate layers where mass exchange with the abundant ions in the seawater could be rather sluggish. The results could be of help in determining the local thickness and location of the gas hydrate stability zone and thereby the estimated reserves.

Mechanical Destabilization and Cage Transformations in Water Vacancy-Contained CO2 Hydrates

“…As is known, clathrate hydrate is a kind of nonstoichiometric crystalline structure consisting of water and small molecules, where the small guest molecules are physically stored in the polyhedral cages formed by the hydrogen-bonded water molecules under low temperatures and high pressures. ,,− To date, it is identified that there are a variety of organic molecules that are molecularly sized and appropriate for being guests in the polyhedral cages of clathrate hydrates, varying from small molecules like H 2 , N 2 , CH 4 , CO 2 , C 2 H 6 , C 2 H 8 , and C 4 H 10 to heavier compounds such as cyclopentane and cyclobutanone. ,, Among these molecules, methane is the most common guest molecular species in nature; ,,, thereby, its corresponding clathrate hydrates are named natural gas hydrates (NGHs) that widely exist in the submarine continental margin and permafrost area. ,,,,, As a result, many studies on the mechanical stability of methane hydrates have been reported. ,− For example, Cao et al reported via classic molecular dynamics (MD) simulations that factors such as strain rate, temperature, and occupancy of guest molecules in 5 12 6 2 cages have a great effect on the mechanical properties and failure strain of monocrystalline structural I (sI) methane hydrate. In contrast, the crystal orientation has a negligible influence on the tensile response of monocrystalline methane hydrate, for example, along the [110] and [100] directions.…”

“…11,15,17−28 To date, it is identified that there are a variety of organic molecules that are molecularly sized and appropriate for being guests in the polyhedral cages of clathrate hydrates, varying from small molecules like H 2 , N 2 , CH 4 , CO 2 , C 2 H 6 , C 2 H 8 , and C 4 H 10 to heavier compounds such as cyclopentane and cyclobutanone. 17,22,25 Among these molecules, methane is the most common guest molecular species in nature; 18,22,29,30 thereby, its corresponding clathrate hydrates are named natural gas hydrates (NGHs) that widely exist in the submarine continental margin and permafrost area. 10,12,18,22,29,30 As a result, many studies on the mechanical stability of methane hydrates have been reported.…”

Environmentally Sustainable Large-Scale CO2 Sequestration through Hydrates in Offshore Basins: Ab Initio Comprehensive Analysis of Subsea Parameters and Economic Perspective

“…Researchers observed that sediments with strong water affinity require higher water saturation for hydrate formation than sediments with low water affinity. , The salinity of sedimentary water also inhibits the hydrate-formation process. Moreover, there is a strong electrostatic interaction between dissolved cations and anions in aqueous solutions containing salt . During hydrate formation, guest molecules will compete with salt ions for molecules of water, and fewer water molecules will be available to construct water cages through hydrogen bonding. , The salinity also increases in the surrounding region of the hydrate when the hydrate molecules are formed.…”

Progress on Laboratory-Scale Reactors for Simulating Gas Production from Hydrate Reservoir

“…Over the years, a series of methods for gas production have been developed, such as the depressurization, heat injection, chemical inhibitor, huff and puff, and CO 2 replacement. Carbon dioxide was promoted by the clays in the seabed; methane gas is produced while carbon dioxide is sequestered. − However, these methods have certain limitations. The working principle of the depressurization method is to promote the decomposition of the hydrate by reducing the environmental pressure around the hydrate.…”